Proteins are important building blocks of all cells and tissues. They are important for body growth, development, and health. They form the structural part of most organs and make up enzymes and hormones that regulate body functions. This test measures the amount of protein in your blood.

Two classes of proteins are found in the blood, albumin and globulin.

Albumin is made by the liver and makes up about 60% of the total protein. Albumin keeps fluid from leaking out of blood vessels, nourishes tissues, and transports hormones, vitamins, drugs, and substances like calcium throughout the body.
Globulins make up the remaining 40% of proteins in the blood. The globulins are a varied group of proteins, some produced by the liver and some by the immune system. They help fight infection and transport nutrients.
The test also compares the amount of albumin with globulin and calculates what is called the A/G ratio. A change in this ratio can provide your healthcare practitioner with a clue as to the cause of the change in protein levels.

Total protein levels in the blood may increase or decrease, to a greater or lesser degree, with various conditions.

Total protein levels may decrease in conditions that:

Interfere with production of albumin or globulin proteins, such as malnutrition or severe liver disease
Increase the breakdown or loss of protein, such as kidney disease (nephrotic syndrome)
Increase or expand the volume of plasma, the liquid part of blood (diluting the blood), such as congestive heart failure
Total protein levels may increase with conditions that cause:

Abnormally high production of protein (e.g., inflammatory disorders, multiple myeloma)
Dehydration

Hemoglobin A1c, also called A1c or glycated hemoglobin, is hemoglobin with glucose attached. The A1c test evaluates the average amount of glucose in the blood over the last 2 to 3 months by measuring the percentage of glycated hemoglobin in the blood.

Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). There are several types of normal hemoglobin, but the predominant form – about 95-96% – is hemoglobin A. As glucose circulates in the blood, some of it spontaneously binds to hemoglobin A.

The higher the level of glucose in the blood, the more glycated hemoglobin is formed. Once the glucose binds to the hemoglobin, it remains there for the life of the red blood cell – normally about 120 days. The predominant form of glycated hemoglobin is referred to as A1c. A1c is produced on a daily basis and slowly cleared from the blood as older RBCs die and younger RBCs (with non-glycated hemoglobin) take their place.

An A1c test may be used to screen for and diagnose diabetes or risk of developing diabetes. Standards of medical care in diabetes from the American Diabetes Association (ADA) state that diabetes may be diagnosed based on A1c criteria or blood glucose criteria (e.g., the fasting blood glucose (FBG) or the 2-hour glucose tolerance test).

A1c is also used to monitor treatment for individuals diagnosed with diabetes. It helps to evaluate how well your glucose levels have been controlled by treatment over time. For monitoring purposes, an A1c of less than 7% indicates good glucose control and a lower risk of diabetic complications for the majority of people with diabetes.

However, the ADA and the European Association for the Study of Diabetes (EASD) recommend that the management of glucose control in people with type 2 diabetes be more “patient-centered.” It is recommend that people work closely with their healthcare practitioner to select a goal that reflects each person’s individual health status and that balances risks and benefits.

Acetaminophen is one of the most common pain relievers (analgesics) and fever reducers (antipyretics) available over the counter. It is generally regarded as safe. However, it is also the most common cause of toxic hepatitis in North America and Europe and one of the most common poisonings from either accidental or intentional overdose.

Once entering your body, like many other medications, acetaminophen is processed in the liver into harmless substances and removed in your urine. However, about 5 to 10% of the absorbed acetaminophen is converted to a toxic, highly-reactive byproduct. Fortunately, your liver makes a antioxidant called glutathione that binds to the byproduct and minimizes its toxic effects. When a large dose of acetaminophen is ingested or that exceeds the recommended amount over a period of time, the liver is no longer able to produce enough protective glutathione; as a result, the toxic byproduct builds up in the liver and may cause severe liver damage if timely treatment is not given.

For this reason, acetaminophen can be harmful or even fatal if not taken correctly and children in particular are at risk if caregivers do not follow dosing instructions carefully. Often, people do not realize that acetaminophen is one of the ingredients in many combination medications such as cold and flu preparations. If two or more of these medications are taken together, levels of acetaminophen may exceed safe limits.

Acetaminophen preparations come in varying strengths and several different forms, including tablets, capsules and liquid.

• For adults, the typical maximum daily limit for acetaminophen is 4000 milligrams (mg). Consuming more than 4000 mg in a 24-hour period is considered an overdose, while ingesting more than 7000 mg can lead to a severe overdose reaction unless treated promptly.
• For children, the amount that is considered an overdose depends on their age and body weight. (For more on this, see the MayoClinic webpage Acetaminophen and children: Why dose matters.)

If it is known or suspected that someone has ingested an overdose of acetaminophen, it is recommended to take the person to the emergency room. If a health practitioner determines that an overdose has occurred, treatment may include an antidote, N-acetylcysteine (NAC), which can help minimize damage to the liver, especially if given within 8 to 12 hours after an overdose. Though NAC is ideally administered within this timeframe, people who seek treatment more than 12 hours after ingestion may still be given the antidote.

Until recently, NAC for people who visit healthcare providers later than 24 hours after acetaminophen ingestion was not the standard of care for acetaminophen overdose management in the United States. However, study data from England suggest that NAC may be beneficial for acetaminophen-induced liver failure more than 24 hours after ingestion.

Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the healthcare practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person’s sample and help identify an infection caused by AFB.

There are several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.

Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help diagnose, track, and minimize the spread of TB in these populations and help determine the effectiveness of treatment.

Another group of mycobacteria referred to as nontuberculous mycobacteria (NTM) can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause lung infection and disseminated disease in people with weakened immune systems. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.

See “How is it used?” under Common Questions below for details on AFB tests.

How is the sample collected for testing?

Sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Preferably, three early morning samples obtained by deep cough are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.

If a person is unable to produce sputum, a healthcare practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the healthcare practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient’s upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the healthcare practitioner will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the healthcare practitioners suspect TB is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may be used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed, except to rinse the mouth with water before collecting the sputum sample.

Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the healthcare practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person’s sample and help identify an infection caused by AFB.

There are several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.

Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help diagnose, track, and minimize the spread of TB in these populations and help determine the effectiveness of treatment.

Another group of mycobacteria referred to as nontuberculous mycobacteria (NTM) can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause lung infection and disseminated disease in people with weakened immune systems. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.

See “How is it used?” under Common Questions below for details on AFB tests.

How is the sample collected for testing?

Sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Preferably, three early morning samples obtained by deep cough are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.

If a person is unable to produce sputum, a healthcare practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the healthcare practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient’s upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the healthcare practitioner will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the healthcare practitioners suspect TB is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may be used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed, except to rinse the mouth with water before collecting the sputum sample.

The activated clotting time (ACT) is a test that is used primarily to monitor high doses of unfractionated (standard) heparin therapy. Heparin is a drug that inhibits blood clotting (anticoagulant) and is usually given through a vein (intravenously, IV), by injection or continuous infusion. High doses of heparin may be given during medical or surgical procedures that require that blood be prevented from clotting, such as heart bypass surgery.

In moderate doses, heparin is used to help prevent and treat inappropriate blood clot formation (thrombosis or thromboembolism) and is monitored using the partial thromboplastin time (PTT) or the heparin anti-factor Xa test. Monitoring is a vital part of the anticoagulation therapy because the blood thinning (anticoagulant) effect of heparin can affect each person a little bit differently. If the amount of heparin administered is not enough to inhibit the body’s clotting system, blood clots may form in blood vessels throughout the body. If there is too much heparin, excessive, even life-threatening, bleeding can occur.

High doses of heparin are given, for example, before, during, and for a short time after, open heart surgeries. During these operations, the patient’s heart and lungs are often bypassed. This means their blood is filtered and oxygenated outside of the body using mechanical devices. The blood’s contact with artificial surfaces activates platelets and coagulation, initiating a sequence of steps that results in blood clot formation. A high dose of heparin prevents clot formation but leaves the body in a delicate dynamic balance between clotting and bleeding. At this level of anticoagulation, the PTT is no longer clinically useful as a monitoring tool. The PTT test involves an in vitro clotting reaction and at high levels of heparin, it will not clot. In these cases, the ACT must be used for monitoring.

The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) is a screening test that helps evaluate a person’s ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets stick to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. This forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.

With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered “prolonged.”

When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A healthcare practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways.

The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).
For more on this, see the article on the Coagulation Cascade.

A bacterial wound culture is a test that detects and identifies bacteria that cause infections (pathogenic) in a wound. Any wound may become infected with a variety of bacteria. A culture helps to determine whether a wound has become infected, which type(s) of bacteria are causing the infection, and which antibiotic would best treat the infection and help heal the wound.

Wounds may be superficial breaks in the skin such as scrapes, cuts and scratches or may involve deeper tissues such as incisions, bites, punctures or burns. (Read the article on Wound and Skin Infections.) A culture is performed by collecting a sample of fluid, cells or tissue from the wound and placing it on or in appropriate nutrient media. The media encourages the growth of bacteria that may be present, allowing for further testing and identification.

Typically, only one kind of pathogenic bacteria is causing the infection in a wound. However, there may be several types of normal skin bacteria present in the culture. Separating the various types of bacteria and identifying the pathogenic bacteria requires one or more days to perform.

A Gram stain is usually performed to help determine the type of bacteria present and provide a rapid result to the healthcare practitioner. The shape and color (morphology and staining characteristics) also help determine what other tests may need to be performed to definitively identify the cause of infection.

Because the results of the stain read under the microscope are not definitive, further tests such as biochemical reactions or mass spectrometry must be performed to identify the bacteria. Mass spectrometry using matrix assisted laser desorption ionization time of flight (MALDI-TOF) can provide an identification to the genus and species level in less than an hour after the bacterial colony is grown on the culture media. This technique significantly decreases the time needed to identify bacteria from traditional biochemical reactions that require overnight incubation.

For many of the pathogens identified in wound cultures, testing is done to determine which antibiotics will be effective in inhibiting the growth of the bacteria (see Antibiotic Susceptibility Testing). The Gram stain of the wound, the culture, and susceptibility testing all contribute to inform the healthcare practitioner which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.

Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the healthcare practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person’s sample and help identify an infection caused by AFB.

There are several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.

Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help diagnose, track, and minimize the spread of TB in these populations and help determine the effectiveness of treatment.

Another group of mycobacteria referred to as nontuberculous mycobacteria (NTM) can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause lung infection and disseminated disease in people with weakened immune systems. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.

See “How is it used?” under Common Questions below for details on AFB tests.

How is the sample collected for testing?

Sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Preferably, three early morning samples obtained by deep cough are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.

If a person is unable to produce sputum, a healthcare practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the healthcare practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient’s upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the healthcare practitioner will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the healthcare practitioners suspect TB is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may be used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed, except to rinse the mouth with water before collecting the sputum sample.

Alpha-fetoprotein (AFP) is a protein produced primarily by the liver in a developing baby (fetus). AFP levels are normally elevated when a baby is born and then decline rapidly. Outside of pregnancy and birth, liver damage and certain cancers can increase AFP levels significantly. This test measures the level of AFP in your blood.

AFP is produced whenever liver cells are regenerating. With chronic liver diseases, such as hepatitis and cirrhosis, AFP may be chronically elevated. Very high concentrations of AFP may be produced by certain tumors. This characteristic makes the AFP test useful as a tumor marker. Increased amounts of AFP are found in many people with the most common type of liver cancer called hepatocellular carcinoma and in a rare type of liver cancer that most commonly occurs in infants called hepatoblastoma. They are also found in some people with cancers of the testicles or ovaries.

AFP exists in several different forms. The standard AFP test is for a total AFP, one that measures all of the AFP forms together. This is the primary AFP test used in the United States.

One of the AFP forms is called L3. The AFP-L3% test is a relatively new test that compares the amount of AFP-L3 to the total amount of AFP. An increase in the percentage of L3 is associated with increased risk of developing hepatocellular carcinoma in the near future and of having a poorer prognosis, as the L3-related cancers tend to be more aggressive.

Among people with low total AFP, AFP-L3 can be higher in those with hepatocellular carcinoma than in people with benign liver diseases. Tumor markers including total AFP and AFP-L3 are used in addition to ultrasound for surveillance of hepatocellular carcinoma in Japan. This practice is different from that in the U.S. and Europe, but the two tests are occasionally ordered by healthcare practitioners in the U.S.

Human immunodeficiency virus (HIV) is the virus that can cause acquired immunodeficiency syndrome (AIDS). HIV screening tests detect the HIV antigen (p24, a viral protein) and/or HIV antibodies produced by the body in response to an HIV infection in the blood. Some tests detect HIV antibodies in saliva.

An HIV infection may initially cause no symptoms or cause flu-like symptoms that resolve after a week or two. The only way to determine whether you have been infected is through HIV testing.

If left untreated, an HIV infection can progressively destroy the body’s ability to fight infections and certain cancers. HIV weakens the immune system by infecting lymphocytes (T-cells), a type of white blood cell, that normally help the body fight infections.

During the first few weeks following infection with HIV, the virus infects T-cells, making numerous copies of itself and continuing to infect more T-cells. The amount of virus (viral load) and the p24 antigen level in blood can be quite high. HIV tests that detect the p24 antigen can generally identify infections in the first weeks after infection, even before antibodies develop.

About 2-8 weeks after exposure to the virus, the immune system responds by producing antibodies directed against the virus that can be detected in the blood. As the initial infection resolves and the level of HIV antibody increases, both virus and p24 antigen levels decrease in the blood. HIV tests that detect HIV antibodies can identify HIV infections about 2 to 8 weeks after infection.

If you become infected with HIV and it is not detected early and treated, it may become a simmering infection that may cause few symptoms for a decade or more. If your infection is still not treated, eventually symptoms of AIDS emerge and begin to progressively worsen. Over time and without treatment, HIV destroys the immune system and leaves your body vulnerable to other debilitating infections.

For additional details on HIV signs and symptoms, risks, treatment, etc., see the article on HIV Infection and AIDS.

Detecting and diagnosing HIV early in the course of infection is important because:

• It allows for early treatment that slows or even prevents progression to AIDS.
• You can learn of your status and modify behavior to prevent spreading the disease to others.
• A pregnant woman can undergo treatment that would help prevent passing the disease to her child.

There are two types of HIV, called HIV-1 and HIV-2. HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.

Human immunodeficiency virus (HIV) is the virus that can cause acquired immunodeficiency syndrome (AIDS). HIV screening tests detect the HIV antigen (p24, a viral protein) and/or HIV antibodies produced by the body in response to an HIV infection in the blood. Some tests detect HIV antibodies in saliva.

An HIV infection may initially cause no symptoms or cause flu-like symptoms that resolve after a week or two. The only way to determine whether you have been infected is through HIV testing.

If left untreated, an HIV infection can progressively destroy the body’s ability to fight infections and certain cancers. HIV weakens the immune system by infecting lymphocytes (T-cells), a type of white blood cell, that normally help the body fight infections.

During the first few weeks following infection with HIV, the virus infects T-cells, making numerous copies of itself and continuing to infect more T-cells. The amount of virus (viral load) and the p24 antigen level in blood can be quite high. HIV tests that detect the p24 antigen can generally identify infections in the first weeks after infection, even before antibodies develop.

About 2-8 weeks after exposure to the virus, the immune system responds by producing antibodies directed against the virus that can be detected in the blood. As the initial infection resolves and the level of HIV antibody increases, both virus and p24 antigen levels decrease in the blood. HIV tests that detect HIV antibodies can identify HIV infections about 2 to 8 weeks after infection.

If you become infected with HIV and it is not detected early and treated, it may become a simmering infection that may cause few symptoms for a decade or more. If your infection is still not treated, eventually symptoms of AIDS emerge and begin to progressively worsen. Over time and without treatment, HIV destroys the immune system and leaves your body vulnerable to other debilitating infections.

For additional details on HIV signs and symptoms, risks, treatment, etc., see the article on HIV Infection and AIDS.

Detecting and diagnosing HIV early in the course of infection is important because:

• It allows for early treatment that slows or even prevents progression to AIDS.
• You can learn of your status and modify behavior to prevent spreading the disease to others.
• A pregnant woman can undergo treatment that would help prevent passing the disease to her child.

There are two types of HIV, called HIV-1 and HIV-2. HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.

Alanine aminotransferase (ALT) is an enzyme found mostly in the cells of the liver and kidney. Much smaller amounts of it are also found in the heart and muscles. Normally, ALT levels in blood are low, but when the liver is damaged, ALT is released into the blood and the level increases. This test measures the level of ALT in the blood and is useful for early detection of liver disease.

The function of ALT is to convert alanine, an amino acid found in proteins, into pyruvate, an important intermediate in cellular energy production. In healthy individuals, ALT levels in the blood are low. When the liver is damaged, ALT is released into the blood, usually before more obvious signs of liver damage occur, such as jaundice.

The liver is a vital organ located in the upper right side of the abdomen, just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body’s nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors and albumin, and breaks down potentially toxic substances into harmless ones that the body can use or eliminate.

A number of conditions can cause damage to liver cells, resulting in an increase in ALT. The test is most useful in detecting damage due to hepatitis (inflammation of the liver) or as a result of drugs or other substances that are toxic to the liver.

ALT is commonly tested in conjunction with aspartate aminotransferase (AST), another liver enzyme, as part of a liver panel. Both ALT and AST levels usually rise whenever the liver is being damaged, although ALT is more specific for the liver and, in some cases, may be the only one of the two to be increased. An AST/ALT ratio may be calculated to aid in distinguishing between different causes and severity of liver injury and to help distinguish liver injury from damage to heart or muscles.

Alanine aminotransferase (ALT) is an enzyme found mostly in the cells of the liver and kidney. Much smaller amounts of it are also found in the heart and muscles. Normally, ALT levels in blood are low, but when the liver is damaged, ALT is released into the blood and the level increases. This test measures the level of ALT in the blood and is useful for early detection of liver disease.

The function of ALT is to convert alanine, an amino acid found in proteins, into pyruvate, an important intermediate in cellular energy production. In healthy individuals, ALT levels in the blood are low. When the liver is damaged, ALT is released into the blood, usually before more obvious signs of liver damage occur, such as jaundice.

The liver is a vital organ located in the upper right side of the abdomen, just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body’s nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors and albumin, and breaks down potentially toxic substances into harmless ones that the body can use or eliminate.

A number of conditions can cause damage to liver cells, resulting in an increase in ALT. The test is most useful in detecting damage due to hepatitis (inflammation of the liver) or as a result of drugs or other substances that are toxic to the liver.

ALT is commonly tested in conjunction with aspartate aminotransferase (AST), another liver enzyme, as part of a liver panel. Both ALT and AST levels usually rise whenever the liver is being damaged, although ALT is more specific for the liver and, in some cases, may be the only one of the two to be increased. An AST/ALT ratio may be calculated to aid in distinguishing between different causes and severity of liver injury and to help distinguish liver injury from damage to heart or muscles.

Albumin is a protein made by the liver. It makes up about 60% of the total protein in the blood and plays many roles. This test measures the level of albumin in the blood.

Albumin keeps fluid from leaking out of blood vessels, nourishes tissues, and transports hormones, vitamins, drugs, and substances like calcium throughout the body. Levels of albumin may decrease, to a greater or lesser degree, when conditions interfere with its production by the liver, increase protein breakdown, increase protein loss via the kidneys, and/or expand the volume of plasma, the liquid portion of blood (diluting the blood).

Two important causes of low blood albumin include:

• Severe liver disease—since albumin is produced by the liver, its level can decrease with loss of liver function; however, this typically occurs only when the liver has been severely affected.
• Kidney disease—one of the many functions of the kidneys is to conserve plasma proteins such as albumin so that they are not released along with waste products when urine is produced. Albumin is present in high concentrations in the blood, and when the kidneys are functioning properly, virtually no albumin is lost in the urine. However, if a person’s kidneys become damaged or diseased, they begin to lose their ability to conserve albumin and other proteins. This is frequently seen in chronic diseases, such as diabetes and hypertension. In nephrotic syndrome, very high amounts of albumin are lost through the kidneys.

Albumin is a major protein normally present in blood, but virtually no albumin is present in the urine when the kidneys are functioning properly. However, albumin may be detected in the urine even in the early stages of kidney disease. The urine albumin test (formerly called microalbumin) detects and measures the amount of albumin in the urine to screen for kidney disease.

Most of the time, tests for albumin and creatinine are done on a urine sample collected randomly (not timed) and an albumin-to-creatinine ratio (ACR) is calculated. This is done to provide a more accurate indication of the how much albumin is being released into the urine. Creatinine, a byproduct of muscle metabolism, is normally released into the urine at a constant rate and its level in the urine is an indication of the urine concentration. This property of creatinine allows its measurement to be used to correct for urine concentration when measuring albumin in a random urine sample.

The presence of a small amount of albumin in the urine may be an early indicator of kidney disease. A small amount of albumin in the urine is sometimes referred to as urine microalbumin or microalbuminuria. “Microalbuminuria” is slowly being replaced with the term “albuminuria,” which refers to any elevation of albumin in the urine.

Plasma, the liquid portion of blood, contains many different proteins, including albumin. One of the many functions of the kidneys is to conserve plasma proteins so that they are not released along with waste products when urine is produced. There are two mechanisms that normally prevent protein from passing into urine:

1. Specialized structures in the kidney called glomeruli are composed of loops of specialized capillaries that filter the blood, allowing small substances to pass through towards the urine, but provide a barrier that keeps most large plasma proteins inside the blood vessels.
2. The smaller proteins that do get through are almost entirely reabsorbed by tubes (tubules) that have a number of sections that collect the fluid and molecules that pass through the glomeruli.

Protein in the urine (proteinuria) most often occurs when either the glomeruli or tubules in the kidney are damaged. Inflammation and/or scarring of the glomeruli can allow increasing amounts of protein to leak into the urine. Damage to the tubules can prevent protein from being reabsorbed.

If a person’s kidneys become damaged or diseased, they begin to lose their ability to conserve albumin and other proteins. This is frequently seen in chronic diseases, such as diabetes and hypertension, with increasing amounts of protein in the urine reflecting increasing kidney dysfunction.

Albumin is one of the first proteins to be detected in the urine with kidney damage. People who have consistently detectable small amounts of albumin in their urine (albuminuria) have an increased risk of developing progressive kidney failure and cardiovascular disease in the future.

Albumin is a major protein normally present in blood, but virtually no albumin is present in the urine when the kidneys are functioning properly. However, albumin may be detected in the urine even in the early stages of kidney disease. The urine albumin test (formerly called microalbumin) detects and measures the amount of albumin in the urine to screen for kidney disease.

Most of the time, tests for albumin and creatinine are done on a urine sample collected randomly (not timed) and an albumin-to-creatinine ratio (ACR) is calculated. This is done to provide a more accurate indication of the how much albumin is being released into the urine. Creatinine, a byproduct of muscle metabolism, is normally released into the urine at a constant rate and its level in the urine is an indication of the urine concentration. This property of creatinine allows its measurement to be used to correct for urine concentration when measuring albumin in a random urine sample.

The presence of a small amount of albumin in the urine may be an early indicator of kidney disease. A small amount of albumin in the urine is sometimes referred to as urine microalbumin or microalbuminuria. “Microalbuminuria” is slowly being replaced with the term “albuminuria,” which refers to any elevation of albumin in the urine.

Plasma, the liquid portion of blood, contains many different proteins, including albumin. One of the many functions of the kidneys is to conserve plasma proteins so that they are not released along with waste products when urine is produced. There are two mechanisms that normally prevent protein from passing into urine:

1. Specialized structures in the kidney called glomeruli are composed of loops of specialized capillaries that filter the blood, allowing small substances to pass through towards the urine, but provide a barrier that keeps most large plasma proteins inside the blood vessels.
2. The smaller proteins that do get through are almost entirely reabsorbed by tubes (tubules) that have a number of sections that collect the fluid and molecules that pass through the glomeruli.

Protein in the urine (proteinuria) most often occurs when either the glomeruli or tubules in the kidney are damaged. Inflammation and/or scarring of the glomeruli can allow increasing amounts of protein to leak into the urine. Damage to the tubules can prevent protein from being reabsorbed.

If a person’s kidneys become damaged or diseased, they begin to lose their ability to conserve albumin and other proteins. This is frequently seen in chronic diseases, such as diabetes and hypertension, with increasing amounts of protein in the urine reflecting increasing kidney dysfunction.

Albumin is one of the first proteins to be detected in the urine with kidney damage. People who have consistently detectable small amounts of albumin in their urine (albuminuria) have an increased risk of developing progressive kidney failure and cardiovascular disease in the future.

Proteins are important building blocks of all cells and tissues. They are important for body growth, development, and health. They form the structural part of most organs and make up enzymes and hormones that regulate body functions. This test measures the amount of protein in your blood.

Two classes of proteins are found in the blood, albumin and globulin.

• Albumin is made by the liver and makes up about 60% of the total protein. Albumin keeps fluid from leaking out of blood vessels, nourishes tissues, and transports hormones, vitamins, drugs, and substances like calcium throughout the body.
• Globulins make up the remaining 40% of proteins in the blood. The globulins are a varied group of proteins, some produced by the liver and some by the immune system. They help fight infection and transport nutrients.

The test also compares the amount of albumin with globulin and calculates what is called the A/G ratio. A change in this ratio can provide your healthcare practitioner with a clue as to the cause of the change in protein levels.

Total protein levels in the blood may increase or decrease, to a greater or lesser degree, with various conditions.

Total protein levels may decrease in conditions that:

• Interfere with production of albumin or globulin proteins, such as malnutrition or severe liver disease
• Increase the breakdown or loss of protein, such as kidney disease (nephrotic syndrome)
• Increase or expand the volume of plasma, the liquid part of blood (diluting the blood), such as congestive heart failure

Total protein levels may increase with conditions that cause:

• Abnormally high production of protein (e.g., inflammatory disorders, multiple myeloma)
• Dehydration

Alkaline phosphatase (ALP) is an enzyme found in several tissues throughout the body. The ALP in blood samples of healthy adults comes mainly from the liver, with most of the rest coming from bones (skeleton). Elevated levels of ALP in the blood are most commonly caused by liver disease, bile duct obstruction, gallbladder disease, or bone disorders. This test measures the level of ALP in the blood.

In the liver, ALP is found on the edges of cells that join to form bile ducts, tiny tubes that drain bile from the liver to the bowels, where it is needed to help digest fat in the diet. ALP in bone is produced by special cells called osteoblasts that are involved in the formation of bone. Each of the various tissue types produces distinct forms of ALP called isoenzymes.

ALP blood levels can be greatly increased, for example, in cases where one or more bile ducts are blocked. This can occur as a result of inflammation of the gallbladder (cholecystitis) or gallstones. Smaller increases of blood ALP are seen in liver cancer and cirrhosis, with use of drugs toxic to the liver, and in hepatitis.

Any condition causing excessive bone formation, including bone disorders such as Paget’s disease, can cause increased ALP levels. Children and adolescents typically have higher blood ALP levels because their bones are still growing. As a result, the ALP test must be interpreted with different reference (normal) values for children and for adults.

It is possible to distinguish between the different forms (isoenzymes) of ALP produced by different types of tissues in the body. If it is not apparent from clinical signs and symptoms whether the source of a high ALP test result is from liver or bone disease, then a test may be performed to determine which isoenzyme is increased in the blood.

Alkaline phosphatase (ALP) is an enzyme found in several tissues throughout the body. The ALP in blood samples of healthy adults comes mainly from the liver, with most of the rest coming from bones (skeleton). Elevated levels of ALP in the blood are most commonly caused by liver disease, bile duct obstruction, gallbladder disease, or bone disorders. This test measures the level of ALP in the blood.

In the liver, ALP is found on the edges of cells that join to form bile ducts, tiny tubes that drain bile from the liver to the bowels, where it is needed to help digest fat in the diet. ALP in bone is produced by special cells called osteoblasts that are involved in the formation of bone. Each of the various tissue types produces distinct forms of ALP called isoenzymes.

ALP blood levels can be greatly increased, for example, in cases where one or more bile ducts are blocked. This can occur as a result of inflammation of the gallbladder (cholecystitis) or gallstones. Smaller increases of blood ALP are seen in liver cancer and cirrhosis, with use of drugs toxic to the liver, and in hepatitis.

Any condition causing excessive bone formation, including bone disorders such as Paget’s disease, can cause increased ALP levels. Children and adolescents typically have higher blood ALP levels because their bones are still growing. As a result, the ALP test must be interpreted with different reference (normal) values for children and for adults.

It is possible to distinguish between the different forms (isoenzymes) of ALP produced by different types of tissues in the body. If it is not apparent from clinical signs and symptoms whether the source of a high ALP test result is from liver or bone disease, then a test may be performed to determine which isoenzyme is increased in the blood.

Alpha-fetoprotein (AFP) is a protein produced primarily by the liver in a developing baby (fetus). AFP levels are normally elevated when a baby is born and then decline rapidly. Outside of pregnancy and birth, liver damage and certain cancers can increase AFP levels significantly. This test measures the level of AFP in your blood.

AFP is produced whenever liver cells are regenerating. With chronic liver diseases, such as hepatitis and cirrhosis, AFP may be chronically elevated. Very high concentrations of AFP may be produced by certain tumors. This characteristic makes the AFP test useful as a tumor marker. Increased amounts of AFP are found in many people with the most common type of liver cancer called hepatocellular carcinoma and in a rare type of liver cancer that most commonly occurs in infants called hepatoblastoma. They are also found in some people with cancers of the testicles or ovaries.

AFP exists in several different forms. The standard AFP test is for a total AFP, one that measures all of the AFP forms together. This is the primary AFP test used in the United States.

One of the AFP forms is called L3. The AFP-L3% test is a relatively new test that compares the amount of AFP-L3 to the total amount of AFP. An increase in the percentage of L3 is associated with increased risk of developing hepatocellular carcinoma in the near future and of having a poorer prognosis, as the L3-related cancers tend to be more aggressive.

Among people with low total AFP, AFP-L3 can be higher in those with hepatocellular carcinoma than in people with benign liver diseases. Tumor markers including total AFP and AFP-L3 are used in addition to ultrasound for surveillance of hepatocellular carcinoma in Japan. This practice is different from that in the U.S. and Europe, but the two tests are occasionally ordered by healthcare practitioners in the U.S.

Aminoglycosides are a group of antibiotics that are used to treat serious bacterial infections. The level of the prescribed aminoglycoside in the blood is measured in order to adjust doses as necessary and ensure effective treatment while avoiding toxic side effects. (For more information about this, see the article on Therapeutic Drug Monitoring.)

Gentamicin, tobramycin, and amikacin are the most commonly prescribed aminoglycosides, and they are used to treat infections caused by certain types of Gram-negative bacteria as well as a few Gram-positive bacteria. (For more on these, see the article on the Gram stain).

It is important to monitor the concentration of aminoglycosides because their effectiveness depends on having an adequate level in the blood. Aminoglycosides are associated with serious toxic side effects, including damage to hearing and/or balance (ototoxicity) and acute kidney damage (nephrotoxicity). Though kidney damage caused by aminoglycosides is usually reversible, hearing and/or balance loss is frequently permanent. These side effects can occur at any time, but the risk is greater with elevated blood levels and when the drugs are given for an extended period of time. The risk of side effects is lower with some of the aminoglycosides that have been developed recently.

Aminoglycosides are not well absorbed by the digestive system, so they are typically be administered either through a needle into a vein (intravenously, IV) or by injection into a muscle (intramuscularly, IM). Aminoglycosides can be given:

• Using dosing intervals (such as every 8-12 hours), or
• As a large single dose once every 24 to 48 hours (also called extended-interval or pulse dosing).

The amount of an aminoglycoside given per dose depends on a variety of factors, including kidney function, other drugs you may be taking, your age and weight.

When a dose of aminoglycosides is given, the level typically rises in the blood to a peak concentration and then falls over time to a lower (trough) concentration. Sometimes these drugs are prescribed using interval dosing, in which the subsequent dose is timed to be given in anticipation of the falling level. The goal is to dose a sufficient amount of drug to maintain a therapeutic level that will kill the bacteria causing the infection. The dose and the dosing interval are optimized to give the body enough time to clear most of the drug from the previous dose before the next dose is given. This minimizes the risk of complications and helps ensure that an adequate drug level is always maintained in the blood.

• For interval dosing, drug monitoring typically involves assessing the maximum concentration soon after a dose is given (called a peak level) and the minimum concentration just before the next dose is given (called a trough level). Depending on the measured concentration, the next dose of drug may be adjusted up or down. For example, you may not be able to clear the drug out of your system efficiently if you have kidney disease, resulting in an increased concentration in the blood, so the dose may be adjusted lower or the drug may be given less frequently. On the other hand, if you are given too little drug and have an insufficient level in the blood, it is unlikely that treatment will be effective.
• For extended-interval dosing, testing may be performed similarly to interval dosing, using a peak sample and a sample taken 6-12 hours later, or testing can be performed on a single sample taken 6-14 hours after the first dose of antibiotic.

Aminoglycosides are sometimes prescribed alone but are often combined with other antibiotics. Monitoring the antibiotic blood level is especially important in the presence of other medications, as they can affect the ability of the body to process (metabolize) and clear the drug.

Ammonia is a waste product formed primarily by bacteria in the intestines during the digestion of protein. If not processed and cleared from the body appropriately, excess ammonia can accumulate in the blood. This test measures the amount of ammonia in the blood.

Ammonia is normally transported in the blood to the liver, where it is converted into two substances called urea and glutamine. The urea is then carried to the kidneys, where it is eliminated in the urine. If this “urea cycle” does not complete the breakdown of ammonia, ammonia builds up in the blood and can pass from the blood into the brain.

Ammonia is toxic to the brain. For example, when liver function is significantly reduced due to disorders such as cirrhosis or hepatitis, ammonia and other compounds processed by the liver can accumulate in the brain and cause a condition called hepatic encephalopathy.

Hepatic encephalopathy causes mental and neurological changes that can lead to confusion, disorientation, sleepiness, and eventually to coma and even death.

Infants and children with increased ammonia levels may vomit frequently, be irritable, and be increasingly lethargic. Left untreated, they may experience seizures, have difficulty breathing, and may lapse into a coma.

Problems with ammonia processing can arise from conditions such as:

• Severe liver disease – damage limits the ability of the liver to process ammonia; spikes in ammonia blood levels may be seen in people with stable liver disease, especially following a triggering event such as gastrointestinal bleeding or an electrolyte imbalance.
• Decreased blood flow to the liver – ammonia is less able to get to the liver to be processed.
• Reye syndrome – a rare condition that affects the blood, brain, and liver; it typically causes a rise in ammonia levels and a fall in glucose. It affects primarily children and young adults. In most cases, it follows and appears to be triggered by a viral infection, such as the flu or chickenpox. Children who are given aspirin are at an increased risk.
• Kidney (renal) failure – the kidneys are unable to effectively rid the body of urea, leading to a build-up of ammonia in the blood.
• Rare genetic disorder such as a defect in the urea cycle – a deficiency in one of the enzymes necessary to complete the conversion of ammonia to urea.

Amylase is an enzyme produced primarily by the pancreas and the salivary glands to help digest carbohydrates. This test measures the amount of amylase in the blood or urine or sometimes in peritoneal fluid, which is fluid found between the membranes that cover the abdominal cavity and the outside of abdominal organs.

The pancreas is a narrow, flat organ about six inches long located deep within the abdominal cavity, below the liver and between the stomach and the spine. Its head section connects to the duodenum, the first part of the small intestine. Inside the pancreas, small ducts (tubes) feed digestive enzymes produced by the pancreas into the pancreatic duct. The pancreas releases amylase through the pancreatic duct into the first part of the small intestine, where it helps break down dietary carbohydrates.

Amylase is usually present in the blood and urine in small quantities. When cells in the pancreas are injured, increased amounts of amylase are released into the blood. This also increases concentrations of amylase in the urine because amylase is eliminated from the blood through the urine. Increased amylase levels can occur with pancreatitis or when the pancreatic duct is blocked by a gallstone or, in rare cases, with a pancreatic tumor.

Antinuclear antibodies (ANA) are a group of autoantibodies produced by a person’s immune system when it fails to adequately distinguish between “self” and “nonself.” The ANA test detects these autoantibodies in the blood.

ANA react with components of the body’s own healthy cells and cause signs and symptoms such as tissue and organ inflammation, joint and muscle pain, and fatigue. ANA specifically target substances found in the nucleus of a cell, hence the name “antinuclear.” They probably do not damage living cells because they cannot access their nuclei. However, ANA can cause damage to tissue by reacting with nuclear substances when they are released from injured or dying cells.

The ANA test is one of the primary tests for helping to diagnose a suspected autoimmune disorder or rule out other conditions with similar signs and symptoms. The ANA test may be positive with several autoimmune disorders. Patients with the autoimmune disorder systemic lupus erythematosus (SLE) are almost always positive for ANA, but the percentage of patients with other autoimmune disorders who have positive ANA results varies. Also, a significant number of patients with a variety of other types of disorders (and even some healthy people) may be positive for ANA, especially at low levels.

A bacterial wound culture is a test that detects and identifies bacteria that cause infections (pathogenic) in a wound. Any wound may become infected with a variety of bacteria. A culture helps to determine whether a wound has become infected, which type(s) of bacteria are causing the infection, and which antibiotic would best treat the infection and help heal the wound.

Wounds may be superficial breaks in the skin such as scrapes, cuts and scratches or may involve deeper tissues such as incisions, bites, punctures or burns. (Read the article on Wound and Skin Infections.) A culture is performed by collecting a sample of fluid, cells or tissue from the wound and placing it on or in appropriate nutrient media. The media encourages the growth of bacteria that may be present, allowing for further testing and identification.

Typically, only one kind of pathogenic bacteria is causing the infection in a wound. However, there may be several types of normal skin bacteria present in the culture. Separating the various types of bacteria and identifying the pathogenic bacteria requires one or more days to perform.

A Gram stain is usually performed to help determine the type of bacteria present and provide a rapid result to the healthcare practitioner. The shape and color (morphology and staining characteristics) also help determine what other tests may need to be performed to definitively identify the cause of infection.

Because the results of the stain read under the microscope are not definitive, further tests such as biochemical reactions or mass spectrometry must be performed to identify the bacteria. Mass spectrometry using matrix assisted laser desorption ionization time of flight (MALDI-TOF) can provide an identification to the genus and species level in less than an hour after the bacterial colony is grown on the culture media. This technique significantly decreases the time needed to identify bacteria from traditional biochemical reactions that require overnight incubation.

For many of the pathogens identified in wound cultures, testing is done to determine which antibiotics will be effective in inhibiting the growth of the bacteria (see Antibiotic Susceptibility Testing). The Gram stain of the wound, the culture, and susceptibility testing all contribute to inform the healthcare practitioner which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.

Angiotensin-converting enzyme (ACE) is an enzyme that helps regulate blood pressure. An increased blood level of ACE is sometimes found in sarcoidosis, a systemic disorder of unknown cause that often affects the lungs but may also affect many other body organs, including the eyes, skin, nerves, liver, and heart., This test measures the amount of ACE in the blood.

A classic feature of sarcoidosis is the development of granulomas, small tumor-like masses of immune and inflammatory cells and fibrous tissue that form nodules under the skin and in organs throughout the body. Granulomas change the structure of the tissues around them and, in sufficient numbers, they can cause damage and inflammation and may interfere with normal functions. The cells found at the outside borders of granulomas can produce increased amounts of ACE. The level of ACE in the blood may increase when sarcoidosis-related granulomas develop.

Hepatitis A is a highly contagious liver infection caused by the hepatitis A virus (HAV). It is one of several various causes of hepatitis, a condition characterized by inflammation and enlargement of the liver. This test detects antibodies in the blood that are produced by the immune system in response to a hepatitis A infection.

Hepatitis A is one of five “hepatitis viruses” identified so far, including B, C, D, and E, that are known to cause the disease. While hepatitis A can cause a severe, acute disease that typically lasts 1 to 2 months, it does not cause a chronic infection as do some of the other hepatitis viruses.

Hepatitis A is spread, most commonly, from person-to person through stool (fecal) contamination or by ingesting food or water contaminated by the stool of an infected person (a foodborne illness). Recognized risk factors for hepatitis A include close contact with an infected person, international travel, household or personal contact with a child who attends a child care center, household or personal contact with a newly arriving international adoptee, a recognized foodborne outbreak, men who have sex with men, and use of illegal drugs.

Although there are many causes of hepatitis, the symptoms remain the same. In hepatitis, the liver is damaged and unable to function normally. It cannot process toxins or waste products such as bilirubin for their removal from the body. During the course of the disease, bilirubin and liver enzyme levels in the blood can increase. While tests such as bilirubin or a liver panel can tell a healthcare practitioner that someone has hepatitis, they do not identify the cause. Antibody tests for hepatitis viruses may help determine the cause.

There are two different classes of hepatitis A antibody that may be tested, IgM and IgG. When a person is exposed to hepatitis A, the body first produces hepatitis A IgM antibodies. These antibodies typically develop 2 to 3 weeks after first being infected (and are detectable before the onset of symptoms) and persist for about 3 to 6 months. Hepatitis A IgG antibodies are produced within 1 to 2 weeks of the IgM antibodies and usually persist for life. (See below for more details.)

A vaccine that prevents hepatitis A has been available since 1995. Historically, infection rates varied cyclically, with nationwide increases every 10-15 years. However, according to the Centers for Disease Control and Prevention (CDC), hepatitis A rates have declined by more than 95% since the vaccine first became available. In 2015, the number of acute hepatitis A cases reported nationwide was an estimated 2,800.

Hepatitis B is an infection of the liver caused by the hepatitis B virus (HBV). Hepatitis B blood tests detect viral proteins (antigens), the antibodies that are produced in response to an infection, or detect or evaluate the genetic material (DNA) of the virus. The pattern of test results can identify a person who has a current active infection, was exposed to HBV in the past, or has immunity as a result of vaccination.

For details on the various tests, see the table under Common Questions: How is it used?

Hepatitis is a condition characterized by inflammation and, sometimes, enlargement of the liver. It has various causes, one of which is infection by a virus. HBV is one of five “hepatitis viruses” identified so far that are known to mainly infect the liver. The other four are hepatitis A, hepatitis C, hepatitis D, and hepatitis E.

HBV is spread through contact with blood or other body fluids from an infected person. Exposure can occur, for example, through sharing of needles for IV drug use or through unprotected sex. People who live in or travel to areas of the world where hepatitis B is prevalent are at a greater risk. Mothers who are infected can pass the infection to their babies, usually during or after birth. The virus is not spread through casual contact such as holding hands, coughing or sneezing. However, the virus can survive outside the body for up to seven days, including in dried blood, and can be passed by sharing items such as razors or toothbrushes with an infected person.

Effective hepatitis B vaccines have been available in the U.S. since 1981, and beginning in 1991, healthcare providers in the U.S. began vaccinating infants at birth. Still, the Centers for Disease Control and Prevention (CDC) estimates that between 850,000 and 2.2 million people in the U.S. are chronically infected with the virus, most of whom are not aware that they are infected.

The course of HBV infections can vary from a mild form that lasts only a few weeks to a more serious chronic form lasting years. Sometimes chronic HBV leads to serious complications such as cirrhosis or liver cancer. Some of the various stages or forms of hepatitis B include:

• Acute infection — presence of typical signs and symptoms with a positive HBV test
• Chronic infection — persistent infection with the virus detected by laboratory tests accompanied by inflammation of the liver
• Carrier state — persistent infection (determined by HBV tests) but no liver inflammation (a carrier is someone who may appear to be in good health but harbors the virus and can potentially infect others)
• Resolved or inactive infection — no longer has any evidence of infection; viral antigen and DNA tests are negative and no signs or symptoms of liver inflammation (although, in many cases, the virus is present in an inactive state in the liver and can potentially reactivate)
• Reactivation — return of HBV infection (detected by HBV tests) with liver damage in a person who was a carrier or who had a resolved, inactive infection. This most commonly occurs in persons treated with chemotherapy for cancer or with drugs that suppress the immune system used to treat autoimmune diseases or following an organ transplant. It also can occur during treatment for hepatitis C (HCV) in people who also had been exposed to HBV in the past.

Though a potentially serious infection, acute HBV infection usually resolves on its own in most adults. Infants and children tend to develop a chronic infection more often than adults. Approximately 90% of infants infected with HBV will develop a chronic condition. For children between the ages of one and five, the risk of developing chronic hepatitis drops to between 25% and 50%. Over the age of five, less than 5% of HBV infections become chronic.

The vast majority of those with chronic infections will have no symptoms. For acute infections, the symptoms (when present) are very similar to those of other types of acute hepatitis, although no symptoms occur in over half of those with acute HBV infection. Symptoms include fever, fatigue, nausea, vomiting, and jaundice. With acute hepatitis, the liver is damaged and is not able to function normally. It may not process toxins or waste products such as bilirubin for their removal from the body. During the course of disease, bilirubin and liver enzyme levels in the blood may increase. While tests such as bilirubin or a liver panel can tell a healthcare practitioner that someone has hepatitis, they will not indicate what is causing it. Tests that detect infection with a hepatitis virus may help determine the cause.

Hepatitis B is an infection of the liver caused by the hepatitis B virus (HBV). Hepatitis B blood tests detect viral proteins (antigens), the antibodies that are produced in response to an infection, or detect or evaluate the genetic material (DNA) of the virus. The pattern of test results can identify a person who has a current active infection, was exposed to HBV in the past, or has immunity as a result of vaccination.

For details on the various tests, see the table under Common Questions: How is it used?

Hepatitis is a condition characterized by inflammation and, sometimes, enlargement of the liver. It has various causes, one of which is infection by a virus. HBV is one of five “hepatitis viruses” identified so far that are known to mainly infect the liver. The other four are hepatitis A, hepatitis C, hepatitis D, and hepatitis E.

HBV is spread through contact with blood or other body fluids from an infected person. Exposure can occur, for example, through sharing of needles for IV drug use or through unprotected sex. People who live in or travel to areas of the world where hepatitis B is prevalent are at a greater risk. Mothers who are infected can pass the infection to their babies, usually during or after birth. The virus is not spread through casual contact such as holding hands, coughing or sneezing. However, the virus can survive outside the body for up to seven days, including in dried blood, and can be passed by sharing items such as razors or toothbrushes with an infected person.

Effective hepatitis B vaccines have been available in the U.S. since 1981, and beginning in 1991, healthcare providers in the U.S. began vaccinating infants at birth. Still, the Centers for Disease Control and Prevention (CDC) estimates that between 850,000 and 2.2 million people in the U.S. are chronically infected with the virus, most of whom are not aware that they are infected.

The course of HBV infections can vary from a mild form that lasts only a few weeks to a more serious chronic form lasting years. Sometimes chronic HBV leads to serious complications such as cirrhosis or liver cancer. Some of the various stages or forms of hepatitis B include:

• Acute infection — presence of typical signs and symptoms with a positive HBV test
• Chronic infection — persistent infection with the virus detected by laboratory tests accompanied by inflammation of the liver
• Carrier state — persistent infection (determined by HBV tests) but no liver inflammation (a carrier is someone who may appear to be in good health but harbors the virus and can potentially infect others)
• Resolved or inactive infection — no longer has any evidence of infection; viral antigen and DNA tests are negative and no signs or symptoms of liver inflammation (although, in many cases, the virus is present in an inactive state in the liver and can potentially reactivate)
• Reactivation — return of HBV infection (detected by HBV tests) with liver damage in a person who was a carrier or who had a resolved, inactive infection. This most commonly occurs in persons treated with chemotherapy for cancer or with drugs that suppress the immune system used to treat autoimmune diseases or following an organ transplant. It also can occur during treatment for hepatitis C (HCV) in people who also had been exposed to HBV in the past.

Though a potentially serious infection, acute HBV infection usually resolves on its own in most adults. Infants and children tend to develop a chronic infection more often than adults. Approximately 90% of infants infected with HBV will develop a chronic condition. For children between the ages of one and five, the risk of developing chronic hepatitis drops to between 25% and 50%. Over the age of five, less than 5% of HBV infections become chronic.

The vast majority of those with chronic infections will have no symptoms. For acute infections, the symptoms (when present) are very similar to those of other types of acute hepatitis, although no symptoms occur in over half of those with acute HBV infection. Symptoms include fever, fatigue, nausea, vomiting, and jaundice. With acute hepatitis, the liver is damaged and is not able to function normally. It may not process toxins or waste products such as bilirubin for their removal from the body. During the course of disease, bilirubin and liver enzyme levels in the blood may increase. While tests such as bilirubin or a liver panel can tell a healthcare practitioner that someone has hepatitis, they will not indicate what is causing it. Tests that detect infection with a hepatitis virus may help determine the cause.

Hepatitis B is an infection of the liver caused by the hepatitis B virus (HBV). Hepatitis B blood tests detect viral proteins (antigens), the antibodies that are produced in response to an infection, or detect or evaluate the genetic material (DNA) of the virus. The pattern of test results can identify a person who has a current active infection, was exposed to HBV in the past, or has immunity as a result of vaccination.

For details on the various tests, see the table under Common Questions: How is it used?

Hepatitis is a condition characterized by inflammation and, sometimes, enlargement of the liver. It has various causes, one of which is infection by a virus. HBV is one of five “hepatitis viruses” identified so far that are known to mainly infect the liver. The other four are hepatitis A, hepatitis C, hepatitis D, and hepatitis E.

HBV is spread through contact with blood or other body fluids from an infected person. Exposure can occur, for example, through sharing of needles for IV drug use or through unprotected sex. People who live in or travel to areas of the world where hepatitis B is prevalent are at a greater risk. Mothers who are infected can pass the infection to their babies, usually during or after birth. The virus is not spread through casual contact such as holding hands, coughing or sneezing. However, the virus can survive outside the body for up to seven days, including in dried blood, and can be passed by sharing items such as razors or toothbrushes with an infected person.

Effective hepatitis B vaccines have been available in the U.S. since 1981, and beginning in 1991, healthcare providers in the U.S. began vaccinating infants at birth. Still, the Centers for Disease Control and Prevention (CDC) estimates that between 850,000 and 2.2 million people in the U.S. are chronically infected with the virus, most of whom are not aware that they are infected.

The course of HBV infections can vary from a mild form that lasts only a few weeks to a more serious chronic form lasting years. Sometimes chronic HBV leads to serious complications such as cirrhosis or liver cancer. Some of the various stages or forms of hepatitis B include:

• Acute infection — presence of typical signs and symptoms with a positive HBV test
• Chronic infection — persistent infection with the virus detected by laboratory tests accompanied by inflammation of the liver
• Carrier state — persistent infection (determined by HBV tests) but no liver inflammation (a carrier is someone who may appear to be in good health but harbors the virus and can potentially infect others)
• Resolved or inactive infection — no longer has any evidence of infection; viral antigen and DNA tests are negative and no signs or symptoms of liver inflammation (although, in many cases, the virus is present in an inactive state in the liver and can potentially reactivate)
• Reactivation — return of HBV infection (detected by HBV tests) with liver damage in a person who was a carrier or who had a resolved, inactive infection. This most commonly occurs in persons treated with chemotherapy for cancer or with drugs that suppress the immune system used to treat autoimmune diseases or following an organ transplant. It also can occur during treatment for hepatitis C (HCV) in people who also had been exposed to HBV in the past.

Though a potentially serious infection, acute HBV infection usually resolves on its own in most adults. Infants and children tend to develop a chronic infection more often than adults. Approximately 90% of infants infected with HBV will develop a chronic condition. For children between the ages of one and five, the risk of developing chronic hepatitis drops to between 25% and 50%. Over the age of five, less than 5% of HBV infections become chronic.

The vast majority of those with chronic infections will have no symptoms. For acute infections, the symptoms (when present) are very similar to those of other types of acute hepatitis, although no symptoms occur in over half of those with acute HBV infection. Symptoms include fever, fatigue, nausea, vomiting, and jaundice. With acute hepatitis, the liver is damaged and is not able to function normally. It may not process toxins or waste products such as bilirubin for their removal from the body. During the course of disease, bilirubin and liver enzyme levels in the blood may increase. While tests such as bilirubin or a liver panel can tell a healthcare practitioner that someone has hepatitis, they will not indicate what is causing it. Tests that detect infection with a hepatitis virus may help determine the cause.

Hepatitis C (HCV) is a virus that causes an infection of the liver that is marked by liver inflammation and damage. Hepatitis C tests are a group of tests that are performed to diagnose hepatitis C infection and to guide and monitor treatment of the infection.

Hepatitis C tests include:

• HCV antibody test—detects antibodies in your blood that are produced in response to an HCV infection
• HCV RNA test—detects and measures viral hepatitis C RNA in the blood
• HCV genotype test—determines the specific subtype of the virus; this information is useful in guiding treatment.

Hepatitis C is one of five viruses identified so far, including A, B, D, and E, that are known to cause hepatitis.

HCV is spread when contaminated blood enters the body, primarily though sharing needles and syringes during IV drug use. HCV is spread less commonly by sharing personal items contaminated with blood (e.g., razors, toothbrushes), through sex with an infected person, needlestick injuries to healthcare workers, unregulated tattooing, and from mother to baby during pregnancy and childbirth. Before tests for HCV became available in the 1990s, HCV was often transmitted by blood transfusions. Currently, there is no vaccine to prevent hepatitis C.

• Acute hepatitis C—for some people, infection with HCV is a short-term illness, usually with few, mild symptoms or no symptoms, and the virus is cleared from the body without specific treatment. Occasionally (about 20 to 30% of the time), this acute stage of infection can cause more severe symptoms, particularly jaundice and fatigue.
• Chronic hepatitis C—more than half of people infected develop chronic hepatitis C that, without treatment, can lead to serious, long-term health problems like cirrhosis and liver cancer, and may be fatal. Chronic hepatitis progresses slowly over time, so infected individuals may not be aware they have the condition until it causes enough liver damage to affect liver function.

The Centers for Disease Control and Prevention (CDC) estimates that there were approximately 44,700 cases of acute hepatitis C in the U.S. in 2017 and that there are 2.4 million people in the U.S. living with chronic hepatitis C. Many of these people don’t know they are infected. The only way to know is to get tested for hepatitis C.

If you are diagnosed with hepatitis C, your healthcare practitioner may recommend an antiviral treatment to cure your infection or refer you to a healthcare practitioner who specializes in treating liver diseases or infectious diseases. The HCV RNA test may be repeated prior to starting treatment to determine whether the virus is still present and your infection persists, and also to provide a baseline to compare to during treatment.

There are several antiviral treatments available to treat hepatitis C. While some treat specific types (genotypes) of the virus, there are some that treat all genotypes. Treatment typically involves taking medication by mouth (oral) for about 8 to 12 weeks, although it can be longer in some cases. These medications can cure over 90% of people with chronic hepatitis C with relatively few side effects. Your infection is considered cured if you have no detectable HCV in your blood 12 weeks after completing treatment.

Susceptibility is a term used when microbe such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed on bacteria or fungi causing an individual’s infection after they have been recovered in a culture of the specimen. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

Although viruses are microbes, testing for their resistance to antiviral drugs is performed less frequently and by different test methods. This article is limited to the discussion of bacterial and fungal susceptibility testing.

During the culture process, pathogens are isolated (separated out from any other microbes present). Each pathogen, if present, is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, it is possible to determine whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. For example, strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus), can be treated with ampicillin and does not require a test to predict susceptibility to this class of antibiotics.

Susceptibility testing is performed on each type of bacteria or fungi that may be relevant to the individual’s treatment and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria. Resistance can also be determined by detection of a gene that is known to cause resistance to specific antibiotics.

Susceptibility is a term used when microbe such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed on bacteria or fungi causing an individual’s infection after they have been recovered in a culture of the specimen. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

Although viruses are microbes, testing for their resistance to antiviral drugs is performed less frequently and by different test methods. This article is limited to the discussion of bacterial and fungal susceptibility testing.

During the culture process, pathogens are isolated (separated out from any other microbes present). Each pathogen, if present, is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, it is possible to determine whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. For example, strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus), can be treated with ampicillin and does not require a test to predict susceptibility to this class of antibiotics.

Susceptibility testing is performed on each type of bacteria or fungi that may be relevant to the individual’s treatment and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria. Resistance can also be determined by detection of a gene that is known to cause resistance to specific antibiotics.

Antistreptolysin O (ASO) is an antibody targeted against streptolysin O, a toxic enzyme produced by group A Streptococcus bacteria. ASO and anti-DNase B are the most common of several antibodies that are produced by the body’s immune system in response to a strep infection with group A Streptococcus. This test measures the amount of ASO in the blood.

Group A Streptococcus (Streptococcus pyogenes) is the bacterium responsible for causing strep throat and a variety of other infections, including skin infections (pyoderma, impetigo, cellulitis). In most cases, strep infections are identified and treated with antibiotics, and the infections resolve.

When a strep infection does not cause identifiable symptoms, goes untreated, or is treated ineffectively, however, complications, namely rheumatic fever and a type of kidney disease (glomerulonephritis), can sometimes develop, especially in young children. These secondary conditions have become much less prevalent in the U.S. because of routine strep testing, but they still do occur. These conditions can cause serious complications such as heart damage, acute kidney dysfunction, tissue swelling (edema), and high blood pressure (hypertension). The ASO test can be used to help determine if these are due to a recent group A strep infection.

Apolipoprotein A-I (apo A-I) is a protein that has specific roles in the transportation and metabolism of lipids and is the main protein component in high-density lipoprotein (HDL, the “good cholesterol”). This test measures the amount of apo A-I in the blood.

Lipids alone cannot dissolve in the blood; they are like oil that floats on water. Apolipoproteins are the proteins that combine with lipids to make lipoprotein particles that can transport lipids throughout the bloodstream. Apolipoproteins provide structural integrity to lipoproteins and shield the water-repellent (hydrophobic) lipids at their center.

Most lipoproteins are cholesterol- or triglyceride-rich (two main lipids) and carry them throughout the body for uptake by cells. HDL, however, is like an empty taxi. It goes out to the tissues and picks up excess cholesterol, then transports it back to the liver. In the liver, the cholesterol is either recycled for future use or excreted in bile. HDL’s reverse transport is the only way that cells can get rid of excess cholesterol. This reverse transport helps protect the arteries and, if there is enough HDL present, it can even reverse the build-up of fatty plaques, deposits resulting from atherosclerosis that can lead to cardiovascular disease (CVD).

Apolipoprotein A is the taxi driver. It activates the enzymes that load cholesterol from the tissues into HDL and allows HDL to be recognized and bound by receptors in the liver at the end of the transport. There are two forms of apolipoprotein A: apo A-I and apo A-II. Apo A-I is found in greater proportion than apo A-II (about 3 to 1). The concentration of apo A-I can be measured directly and tends to rise and fall with HDL levels. Deficiencies in apo A-I correlate with an increased risk of developing CVD. Apo A-I levels provide more information to help evaluate CVD risk, especially when HDL levels are low.

Apolipoprotein B-100 (also called apolipoprotein B or apo B) is a protein that is involved in the metabolism of lipids and is the main protein constituent of lipoproteins such as very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL, the “bad cholesterol”). This test measures the amount of apo B in the blood.

Apolipoproteins combine with lipids to transport them throughout the bloodstream. Apolipoproteins provide structural integrity to lipoproteins and shield the water-repellent (hydrophobic) lipids at their center. Most lipoproteins are cholesterol- or triglyceride-rich and carry lipids through the body for uptake by cells.

Chylomicrons are the lipoprotein particles that carry dietary lipids from the digestive tract, via the bloodstream, to tissue – mainly the liver. In the liver, the body repackages these dietary lipids and combines them with apo B-100 to form triglyceride-rich VLDL. This combination is like a taxi full of passengers with apo B-100 as the taxi driver. In the bloodstream, the taxi moves from place to place, releasing one passenger at a time.

An enzyme called lipoprotein lipase (LPL) removes triglycerides from VLDL to produce intermediate density lipoproteins (IDL) first and then LDL. Each VLDL particle contains one molecule of apo B-100, which is retained as VLDL loses triglycerides and shrinks to become the more cholesterol-rich LDL. Apo B-100 is recognized by receptors found on the surface of many of the body’s cells. These receptors promote the uptake of cholesterol into the cells.

The cholesterol that LDL and apo B-100 transport is vital for cell membrane integrity, sex hormone production, and steroid production. In excess, however, LDL can lead to fatty deposits (plaques) in artery walls and lead to hardening and scarring of the blood vessels. These fatty depositions narrow the vessels in a process termed atherosclerosis. The atherosclerotic process increases the risk of heart attack.

Apo B-100 levels tend to mirror LDL-C levels, a test routinely ordered as part of a lipid profile. Many experts think that apo B levels may eventually prove to be a better indicator of risk of cardiovascular disease (CVD) than LDL-C. Some recommend the measurement of apo B to help with risk prediction when a person has multiple risk factors. Other experts disagree; they feel that apo B is only a marginally better alternative and do not recommend its routine use. The clinical utility of apo B and that of other emerging cardiac risk markers such as apo A-I, Lp(a), and hs-CRP has yet to be fully established.

The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) is a screening test that helps evaluate a person’s ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets stick to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. This forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.

With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered “prolonged.”

When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A healthcare practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways.

• The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
• A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).

For more on this, see the article on the Coagulation Cascade.

The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) is a screening test that helps evaluate a person’s ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets stick to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. This forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.

With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered “prolonged.”

When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A healthcare practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways.

• The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
• A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).

For more on this, see the article on the Coagulation Cascade.

Aspartate aminotransferase (AST) is an enzyme found in cells throughout the body but mostly in the heart and liver and, to a lesser extent, in the kidneys and muscles. In healthy individuals, levels of AST in the blood are low. When liver or muscle cells are injured, they release AST into the blood. This makes AST a useful test for detecting or monitoring liver damage or related infections and some side effects of medications.

The liver is a vital organ located in the upper right-hand side of the abdominal area just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body’s nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors, and breaks down potentially toxic substances into harmless ones that the body can use or excrete. It also helps metabolize some medications.

A number of conditions can cause injury to the liver and may cause increases in AST. The test is most useful in detecting liver damage due to hepatitis, drugs toxic to the liver, cirrhosis, or alcoholism. AST, however, is not specific for the liver and may be increased in conditions affecting other parts of the body.

An AST test is often performed along with an alanine aminotransferase (ALT) test. Both are enzymes found in the liver that become elevated in the blood when the liver is damaged. A calculated AST/ALT ratio is useful for differentiating between different causes of liver injury and in recognizing when the increased levels may be coming from another source, such as heart or muscle injury.

Aspartate aminotransferase (AST) is an enzyme found in cells throughout the body but mostly in the heart and liver and, to a lesser extent, in the kidneys and muscles. In healthy individuals, levels of AST in the blood are low. When liver or muscle cells are injured, they release AST into the blood. This makes AST a useful test for detecting or monitoring liver damage or related infections and some side effects of medications.

The liver is a vital organ located in the upper right-hand side of the abdominal area just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body’s nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors, and breaks down potentially toxic substances into harmless ones that the body can use or excrete. It also helps metabolize some medications.

A number of conditions can cause injury to the liver and may cause increases in AST. The test is most useful in detecting liver damage due to hepatitis, drugs toxic to the liver, cirrhosis, or alcoholism. AST, however, is not specific for the liver and may be increased in conditions affecting other parts of the body.

An AST test is often performed along with an alanine aminotransferase (ALT) test. Both are enzymes found in the liver that become elevated in the blood when the liver is damaged. A calculated AST/ALT ratio is useful for differentiating between different causes of liver injury and in recognizing when the increased levels may be coming from another source, such as heart or muscle injury.

The stool culture is a test that detects and identifies bacteria that cause infections of the lower digestive tract. The test distinguishes between the types of bacteria that cause disease (pathogenic) and the types that are normally found in the digestive tract (normal flora). The test helps to determine if pathogenic bacteria are the cause of a person’s gastrointestinal symptoms (gastroenteritis).

The bacteria found in stool are representative of the bacteria that are present in the digestive system (gastrointestinal tract). Certain bacteria and fungi called normal flora inhabit everyone’s gastrointestinal tract. They play an important role in the digestion of food and their presence keeps a check on the growth of disease-causing bacteria.

Sometimes, the balance of the normal flora may be affected by the administration of broad-spectrum antibiotics; the drugs inhibit the growth of normal flora and allow the bacteria Clostridium difficile that is resistant to the antibiotics to survive and overgrow the digestive tract, leading to symptoms such as diarrhea and abdominal pain.

Pathogenic bacteria can enter and infect the digestive tract when someone eats food or drinks water that is contaminated. Examples of contaminated sources include raw or undercooked eggs, poultry or beef, unpasteurized milk, and untreated water from lakes, streams, and (occasionally) from community water supplies. (For more on this, see the article Food and Waterborne Illness.)

People who travel outside the U.S., especially to developing nations, may face a greater risk of being exposed to disease-causing bacteria. Some of these bacteria may be true pathogens while others are strains of gastrointestinal bacteria that are normal flora for the local inhabitants but cause gastrointestinal distress to the tourist. Visitors may become infected by eating or drinking anything that has been contaminated with the bacteria, even things as simple as tap water, ice cubes in a drink, a fresh salad, or food from a vendor’s stall.

The most common symptoms of a pathogenic bacterial infection are prolonged diarrhea, bloody diarrhea, mucus in the stool, abdominal pain and cramping, and nausea. If diarrhea lasts more than a few days, it may lead to complications such as dehydration and electrolyte imbalance, which can be dangerous conditions, especially for children and the elderly. Dehydration can cause symptoms such as dry skin, fatigue, and light-headedness.

Severely affected people may require hospitalization to replace lost fluids and electrolytes. Hemolytic uremic syndrome is a serious complication characterized by the destruction of red blood cells and kidney failure that may occasionally arise from an infection with a toxin-producing strain of the bacteria Escherichia coli. The condition is most frequently seen in children, the elderly, and those with weakened immune systems.

If a person’s illness is uncomplicated and goes away within a few days, a healthcare practitioner may not order testing. However, if symptoms are severe, if there is bloody diarrhea or mucus present in the stool, or if it is continuing unabated, then a stool culture may be ordered. This is especially true if the person has been outside the U.S. and/or has eaten or drunk anything that has also made someone close to them ill.

To aid diagnosis, a stool culture may be done in conjunction with or following a GI pathogens panel that simultaneously tests for multiple disease-causing bacteria, viruses, and parasites. Other tests that may be done include an ova and parasite exam or antigen tests to identify specific microbes.

How is the sample collected for testing?

A fresh stool sample is collected in a clean container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should be taken to the laboratory within two hours after collection or should be transferred into a vial containing a preservative and taken to the lab as soon as possible. For infants, a stool sample is usually collected with a swab of the rectum.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Sputum is the thick mucus or phlegm that is expelled from the lower respiratory tract (bronchi and lungs) through coughing; it is not saliva or spit. Care must be taken in the sample collection process to ensure that the sample is from the lower airways and not from the upper respiratory tract. Sputum samples may be expectorated or induced (See the section below on sample collection.)

Bacterial sputum cultures detect the presence of disease-causing bacteria (pathogens) in people who are suspected of having bacterial pneumonia or other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testing is performed to guide antibiotic treatment.

Sometimes a respiratory infection is caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture. Other tests, such as an AFB smear and culture, fungal culture, or viral culture, may be ordered in addition to or instead of a routine culture.

Typically, the first step in the routine analysis of a sputum sample is a Gram stain to identify the general type of bacteria that may be present. The sample is then placed on or in appropriate nutrient media and incubated. The media encourages the growth of bacteria that are present, allowing for further testing and identification.

Sputum is not sterile. That means that when a person has a bacterial respiratory infection, there will typically be harmless bacteria that are normally present in the mouth, throat, etc. (normal flora) as well as disease-causing (pathogenic) bacteria present.

A trained laboratorian differentiates normal flora from pathogenic bacteria and identifies the various types of bacteria present in the culture. Identification is a step-by-step process that may involve several biochemical, immunological, and/or molecular tests and observations of the organism’s growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment and to determine whether the bacteria present are likely to respond to specific antibiotics.

The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report that informs the health practitioner which pathogen(s) are present and which antibiotic therapies are likely to inhibit their growth.

How is the sample collected for testing?

Sputum samples may be coughed up or induced. Samples that are coughed up are expelled into a sterile cup provided by the laboratory. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by following instructions provided and inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Steam inhalation or a hot shower can also be useful in loosening the phlegm. Sometimes, induction of sputum might be assisted by a respiratory therapist technician.

All samples collected should be taken to the laboratory promptly for processing while they are fresh. Sputum samples must be evaluated and accepted by the laboratory before they are processed.

Useful sputum culture results rely heavily on good sample collection. If examination of a Gram stain of the sample reveals that it contains a significant number of normal cells that line the mouth (squamous epithelial cells), then the sample is not generally considered adequate for culture and a re-collection of the sample may be required. If the sample contains a majority of white blood cells that indicate a body’s response to an infection, then it is considered to be an adequate sample for culturing.

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to brush your teeth and rinse your mouth with water prior to sample collection. You may also be instructed to avoid food for at least 1-2 hours before the sample is collected, which is usually first thing in the morning.

Human chorionic gonadotropin (hCG) is a hormone, made up of an alpha and beta subunit, that is produced by the placenta and normally is only measurable during pregnancy. Some abnormal tissues, tumors, and cancers, however, may also produce hCG, making the hCG test useful as a tumor marker. This test measures the amount of intact hCG, and sometimes the beta subunit of hCG, in the blood.

An increased level of hCG is seen with gestational trophoblastic disease and some germ cell tumors (benign and cancerous). If hCG is increased with these conditions, then the hCG test can be used as a diagnostic and monitoring tool.

Gestational trophoblastic disease (GTD) is a group of tumor types that develop in a woman’s uterus from the layer of cells surrounding an embryo that creates the placenta during a normal pregnancy (trophoblasts) and produces hCG. GTD usually occurs at the beginning of pregnancy after an egg has been fertilized, but instead of supporting the growth of a fetus, the cells form abnormal tissue masses. In most cases, the tumors are benign, but in a small percentage of people, they are cancerous. According to the American Cancer Society, GTD occurs in about 1 in 1,000 pregnancies. It can also occur after a normal pregnancy or after a miscarriage or abortion. The primary forms of GTD are:

• Hydatidiform mole – also called a “molar pregnancy,” which may be complete (only tumor tissue) or a mixture of tumor and fetal tissue but does not develop into a viable baby; these are usually benign but must be surgically removed.
• Invasive mole – a hydatidiform mole that grows into the uterus wall; it must be surgically removed; however, the condition can persist if GTD tissue remains.
• Choriocarcinoma – a rare cancer that may develop from the GTD tumor tissue in about 2 to 7 per 100,000 pregnancies; these cancers can grow quickly and spread to other parts of the body.
• Placental site trophoblastic tumor – also rare, this tumor arises at the site of placental attachment in the uterus. This tumor usually develops after a normal or aborted pregnancy but doesn’t often spread through the body.
• Epithelioid trophoblastic tumor – extremely rare, this tumor is similar in nature to the choriocarcinoma but is now considered a separate disease. It may take many years after a pregnancy for this tumor to be detected and may have already spread to other parts of the body.

Note: With appropriate treatment, the cure rate for most GTD is very high. For more on this, see the links in the Related Content section below.

Germ cell tumors and cancers occur primarily in the ovaries and testicles but can also rarely develop in other locations such as the chest.

• Germ cell tumors can occur in the egg-producing cells of the ovaries and are more often seen in younger women (for more, see Ovarian Cancer).
• Germ cell tumors can affect cells within the testicles that make sperm and account for more than 90% of testicular cancers (for more, see Testicular Cancer).

Levels of hCG may also be elevated in other diseases such as liver, breast, lung, skin, and stomach cancers. Increased levels may also be seen in non-cancer conditions such as cirrhosis, duodenal ulcer, and inflammatory bowel disease.

Beta-2 microglobulin (B2M) is a protein that is found on the surface of nucleated cells (contain a nucleus) and functions as part of the human immune system. This protein is routinely shed by cells into the blood and is present in most body fluids, with highest levels in the blood, generally lower levels in spinal fluid, and trace levels in urine.

In the kidneys, B2M passes through blood-filtering units called the glomeruli and is then reabsorbed by the renal proximal tubules, structures that reclaim water, proteins, vitamins, minerals, and other vital substances. Normally, only small amounts of B2M are present in the urine, but when the renal tubules become damaged or diseased, B2M concentrations increase due to the decreased ability to reabsorb this protein. When the glomeruli in the kidneys are damaged, they are unable to filter out B2M, so the level in the blood rises.

Beta-2 microglobulin (B2M) is a protein that is found on the surface of almost all cells in the body and is shed by cells into the blood, particularly by B lymphocytes and tumor cells. It is present in most body fluids and its level rises with conditions that increase cell production and/or destruction, or that activate the immune system. This test measures B2M in the blood, urine, or rarely in the cerebrospinal fluid (CSF).

B2M is frequently elevated in the blood with cancers such as multiple myeloma and lymphoma and with inflammatory disorders and infections (e.g., HIV, CMV). Because B2M is increased with blood cell cancers, it may be useful as a tumor marker. Though it can be used to assess kidney function, this article focuses on its use as a tumor marker.

The B2M level can be increased in the CSF of individuals with blood cell cancers that have spread (metastasized) to the brain, such as lymphoma, but also with some chronic disorders such as multiple sclerosis and with viral infections such as HIV.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. A 24-hour urine sample may also be collected. Rarely, a CSF sample may be collected from the lower back using a procedure called a lumbar puncture or spinal tap.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Group B strep (GBS) is the common name for the bacterium Streptococcus agalactiae that can be present (colonizing) in the digestive tract and genital tract. It rarely causes symptoms or problems in healthy adults but can cause infections and serious illness in newborns who become infected before or during labor and delivery. GBS screening identifies the presence of the bacteria in the vaginal/rectal area of a pregnant woman.

GBS can cause early-onset GBS disease that occurs within the first week after birth. Signs and symptoms in newborns include fever, difficulty with feeding and breathing, irritability or lethargy, and a blue tint to the skin. GBS can cause serious infections such as pneumonia, sepsis, and meningitis. According to the Centers for Disease Control and Prevention, it is the most common cause of life-threatening infections in newborns.

Approximately 25% of pregnant women carry group B strep in their rectum or vagina. However, the number of infants with GBS disease has decreased significantly in recent years because of a concerted effort by healthcare practitioners to screen pregnant women for GBS late in their pregnancy and, when they are positive for GBS, to treat them with intravenous antibiotics (usually penicillin or ampicillin) during labor. This prevents or greatly decreases the risk of passing the bacteria to the newborn. Nevertheless, GBS disease remains the primary cause of early-onset sepsis, a serious and life-threatening infection in newborns.

Currently there is no vaccine available to prevent GBS, and treating all pregnant women with antibiotics is not practical. Screening for GBS and appropriate treatment continues to be the best means for preventing GBS disease in newborns.

How is the sample collected for testing?

For screening pregnant women, a swab is typically obtained from the vagina and rectum. Urine collected during pregnancy may be cultured for significant numbers of GBS.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Bicarbonate is an electrolyte, a negatively charged ion that is used by the body to help maintain the body’s acid-base (pH) balance. It also works with the other electrolytes (sodium, potassium, and chloride) to maintain electrical neutrality at the cellular level. This test measures the total amount of carbon dioxide (CO₂) in the blood, which occurs mostly in the form of bicarbonate (HCO₃^–). The CO₂ is mainly a by-product of various metabolic processes.

Measuring bicarbonate as part of an electrolyte or metabolic panel may help diagnose an electrolyte imbalance or acidosis or alkalosis. Acidosis and alkalosis describe the abnormal conditions that result from an imbalance in the pH of the blood caused by an excess of acid or alkali (base). This imbalance is typically caused by some underlying condition or disease.

The lungs and kidneys are the major organs involved in regulating blood pH through the removal of excess bicarbonate.

• The lungs flush acid out of the body by exhaling CO₂. Raising and lowering the respiratory rate alters the amount of CO₂ that is breathed out, and this can affect blood pH within minutes.
• The kidneys eliminate acids in the urine and they regulate the concentration of bicarbonate (HCO₃^–, a base) in blood. Acid-base changes due to increases or decreases in HCO₃^– concentration occur more slowly than changes in CO₂, taking hours or days.

Any disease or condition that affects the lungs, kidneys, metabolism, or breathing has the potential to cause acidosis or alkalosis.

The bicarbonate test gives a healthcare practitioner a rough estimate of your acid-base balance. This is usually sufficient, but measurements of gases dissolved in the blood (blood gases) may be done if more information is needed. Bicarbonate is typically measured along with sodium, potassium, and possibly chloride in an electrolyte panel as it is the balance of these molecules that gives the healthcare practitioner the most information.

Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of heme. Heme is a component of hemoglobin, which is found in red blood cells (RBCs). Bilirubin is ultimately processed by the liver so that it can be removed from the body. This test measures the amount of bilirubin in the blood to evaluate a person’s liver function or to help diagnose anemias caused by RBC destruction (hemolytic anemia).

RBCs normally degrade after about 120 days in circulation. Bilirubin is formed as the liver breaks down and recycles aged red blood cells.

Two forms of bilirubin can be measured or estimated by laboratory tests:

• Unconjugated bilirubin is formed when heme is released from hemoglobin. It is carried by proteins to the liver. In the liver, sugars are attached (conjugated) to bilirubin to form conjugated bilirubin.
• Conjugated bilirubin enters the bile and passes from the liver to the small intestines, where it is further broken down by bacteria and eventually eliminated in the stool. Thus, the breakdown products of bilirubin give stool its characteristic brown color. Normally, the level of conjugated bilirubin in the blood is very low.

The bilirubin test is included in the comprehensive metabolic panel (CMP) and the liver panel, which are often used as general health screenings.

• Usually, an initial test measures the total bilirubin level (unconjugated plus conjugated bilirubin).
• If the total bilirubin level is increased, the laboratory can use a second test to detect water-soluble forms of bilirubin, called “direct” bilirubin. The direct bilirubin test provides an estimate of the amount of conjugated bilirubin present.
• Subtracting the direct bilirubin level from the total bilirubin level helps estimate the “indirect” level of unconjugated bilirubin.

A small amount (approximately 250 to 350 milligrams, or about 4 milligrams per kilogram of body weight) of bilirubin is produced daily in a normal, healthy adult. Most bilirubin (70%-90%) comes from damaged or degraded RBCs, with the remaining amount coming from the bone marrow or liver. Normally, small amounts of unconjugated bilirubin are released into the blood, but almost no conjugated bilirubin is present.

If the bilirubin level increases in the blood, a person may appear jaundiced, with a yellowing of the skin and/or whites of the eyes. The pattern of bilirubin test results can give the healthcare practitioner information regarding the condition that may be present. (For details, see “What does the test result mean?” under Common Questions.)

Cholesterol is a substance (a steroid) that is essential for life. It forms the membranes for cells in all organs and tissues in the body. It is used to make hormones that are essential for development, growth, and reproduction. It forms bile acids that are needed to absorb nutrients from food. The test for cholesterol measures total cholesterol that is carried in the blood by lipoproteins.

A small amount of cholesterol circulates in the blood in complex particles called lipoproteins. Each particle contains a combination of protein, cholesterol, triglyceride, and phospholipid molecules and the particles are classified by their density into high-density lipoproteins (HDL), low-density lipoproteins (LDL), and very low-density lipoproteins (VLDL). HDL-C particles, sometimes called “good” cholesterol, carry excess cholesterol away for disposal and LDL-C particles, or “bad” cholesterol, deposit cholesterol in tissues and organs.

Monitoring and maintaining healthy levels of cholesterol is important for staying healthy. The body produces the cholesterol needed to work properly, but the source for some cholesterol is diet. If an individual has an inherited predisposition for high cholesterol levels or eats too much of the foods that are high in saturated fats and trans unsaturated fats (trans fats), then the level of cholesterol in that person’s blood may increase and have a negative impact on the person’s health. The extra cholesterol in the blood may be deposited in plaques on the walls of blood vessels. Plaques can narrow or eventually block the opening of blood vessels, leading to hardening of the arteries (atherosclerosis) and increased risk of numerous health problems, including heart disease and stroke.

Coagulation factors are proteins circulating in the blood that are essential for proper blood clot formation. Coagulation factor tests measure the function of or sometimes the amount of these proteins in the blood.

Blood clotting is a complex process that involves numerous coagulation factors, which are produced by the liver and blood vessels. Each coagulation factor is evaluated with one or more tests. When factor levels are low, it can cause blood clotting to fail, leading to unexplained bleeding episodes. Measuring coagulation factors can help a healthcare practitioner determine the cause of the bleeding and the best treatment.

Coagulation factors are usually tested by measuring the factor’s activity level in the blood. Activity assays can detect reduced levels of protein or proteins that don’t function properly. Rarely, the amount (antigen level) of a coagulation factor may also be measured. Coagulation factor antigen tests can tell how much of the protein is present, but not whether its function is normal.

When someone bleeds (e.g., with an injury), the coagulation system is activated, plugging the leaking blood vessel with a clot. The coagulation system consists of a series of coagulation factors that activate in a step-by-step process called the coagulation cascade. The end result is the formation of insoluble fibrin threads that link together at the site of injury, along with aggregated cell fragments called platelets, to form a stable blood clot. The clot prevents additional blood loss and remains in place until the injured area has healed.

Blood clotting is dynamic; once a clot is formed, other factors are activated that slow clotting or dissolve the clot in a process called fibrinolysis. The clot is eventually removed after the injury site heals. In normal healthy individuals, this balance between clot formation and removal ensures that bleeding does not become excessive and that clots are removed once they are no longer needed.

For people with bleeding disorders, clotting does not work properly because they lack platelets or coagulation factors, or their platelets or factors don’t work properly. There are a variety of bleeding disorders that may be passed through families (inherited) or acquired after birth. If a person has signs and symptoms of one of these disorders, coagulation factor testing may be ordered to help determine the diagnosis and treatment.

There are nine coagulation factor proteins that can be measured clinically (see table below). These factors are referred to by a name or Roman numeral or both in some cases. For example, coagulation factor II is also known as prothrombin. When one or more of these factors are produced in too small a quantity, or are not functioning correctly, they can cause excessive bleeding.

Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is removed from the body by the kidneys, which filter almost all of it from the blood and release it into the urine. This test measures the amount of creatinine in the blood and/or urine.

Creatine is part of the cycle that produces energy needed to contract muscles. Both creatine and creatinine are produced by the body at a relatively constant rate. Since almost all creatinine is filtered from the blood by the kidneys and released into the urine, blood levels are usually a good indicator of how well the kidneys are working. The amount of creatinine you produce depends on your body size and your muscle mass. For this reason, creatinine levels are usually slightly higher in men than in women and children.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Results from a blood creatinine test may be used in combination with results from other tests, such as a 24-hour urine creatinine test, to calculate values that are used to evaluate kidney function.

Blood cultures are procedures done to detect an infection in the blood and identify the cause. Infections of the bloodstream are most commonly caused by bacteria (bacteremia) but can also be caused by yeasts or other fungi (fungemia) or by a virus (viremia). Although blood can be used to test for viruses, this article focuses on the use of blood cultures to detect and identify bacteria and fungi in the blood.

A blood infection typically originates from some other specific site within the body, spreading from that site when a person has a severe infection and/or the immune system cannot confine it to its source. For example, a urinary tract infection may spread from the bladder and/or kidneys into the blood and then be carried throughout the body, infecting other organs and causing a serious and sometimes life-threatening systemic infection. The terms septicemia and sepsis are sometimes used interchangeably to describe this condition. Septicemia refers to an infection of the blood while sepsis is the body’s serious, overwhelming, and sometimes life-threatening response to infection. This condition often requires prompt and aggressive treatment, usually in an intensive care unit of a hospital.

Other serious complications can result from an infection of the blood. Endocarditis, an inflammation and infection of the lining of the heart and/or of the heart valves, can result from a bloodstream infection. People who have prosthetic heart valves or prosthetic joints have a higher risk of a systemic infection following their surgery, although these infections are not common.

Anyone with a weakened immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive drugs such as those given for chemotherapy is at a higher risk for blood infections as their immune system is less capable of killing the microbes that occasionally enter the blood. Bacteria and yeasts may also be introduced directly into the bloodstream through intravenous drug use or through intravenous catheters or surgical drains.

For blood cultures, multiple blood samples are usually collected for testing and from different veins to increase the likelihood of detecting the bacteria or fungi that may be present in small numbers and/or may enter the blood intermittently. This is also done to help ensure that any bacteria or fungi detected are the ones causing the infection and are not contaminants.

Blood cultures are incubated for several days before being reported as negative. Some types of bacteria and fungi grow more slowly than others and/or may take longer to detect if initially present in low numbers.

When a blood culture is positive, the specific microbe causing the infection is identified and susceptibility testing is performed to inform the healthcare practitioner which antibiotics are most likely to be effective for treatment.

In many laboratories, the blood culture testing process is automated with instruments continuously monitoring the samples for growth of bacteria or fungi. This allows for timely reporting of results and for the healthcare practitioner to direct antimicrobial therapy to the specific microbe present in the blood. Because treatment must be given as soon as possible in cases of sepsis, broad-spectrum antimicrobials that are effective against several types of bacteria are usually given intravenously while waiting for blood culture results. Antimicrobial therapy may be changed to a more targeted antibiotic therapy once the microbe causing the infection is identified.

How is the sample collected for testing?

Usually, two blood samples are collected from different veins to increase the likelihood of detecting bacteria or fungi if they are present in the blood. Multiple blood samples help to differentiate true pathogens, which will be present in more than one blood culture, from skin bacteria that may contaminate one of several blood cultures during the collection process.

Blood is obtained by inserting a needle into a vein in the arm. The phlebotomist will put the blood into two culture bottles containing broth to grow microbes. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different site, immediately after the first venipuncture. A single blood culture may be collected from children since they often have high numbers of bacteria present in their blood when they have an infection. For infants and young children, the quantity of each blood sample will be smaller and appropriate for their body size.

Blood cultures are procedures done to detect an infection in the blood and identify the cause. Infections of the bloodstream are most commonly caused by bacteria (bacteremia) but can also be caused by yeasts or other fungi (fungemia) or by a virus (viremia). Although blood can be used to test for viruses, this article focuses on the use of blood cultures to detect and identify bacteria and fungi in the blood.

A blood infection typically originates from some other specific site within the body, spreading from that site when a person has a severe infection and/or the immune system cannot confine it to its source. For example, a urinary tract infection may spread from the bladder and/or kidneys into the blood and then be carried throughout the body, infecting other organs and causing a serious and sometimes life-threatening systemic infection. The terms septicemia and sepsis are sometimes used interchangeably to describe this condition. Septicemia refers to an infection of the blood while sepsis is the body’s serious, overwhelming, and sometimes life-threatening response to infection. This condition often requires prompt and aggressive treatment, usually in an intensive care unit of a hospital.

Other serious complications can result from an infection of the blood. Endocarditis, an inflammation and infection of the lining of the heart and/or of the heart valves, can result from a bloodstream infection. People who have prosthetic heart valves or prosthetic joints have a higher risk of a systemic infection following their surgery, although these infections are not common.

Anyone with a weakened immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive drugs such as those given for chemotherapy is at a higher risk for blood infections as their immune system is less capable of killing the microbes that occasionally enter the blood. Bacteria and yeasts may also be introduced directly into the bloodstream through intravenous drug use or through intravenous catheters or surgical drains.

For blood cultures, multiple blood samples are usually collected for testing and from different veins to increase the likelihood of detecting the bacteria or fungi that may be present in small numbers and/or may enter the blood intermittently. This is also done to help ensure that any bacteria or fungi detected are the ones causing the infection and are not contaminants.

Blood cultures are incubated for several days before being reported as negative. Some types of bacteria and fungi grow more slowly than others and/or may take longer to detect if initially present in low numbers.

When a blood culture is positive, the specific microbe causing the infection is identified and susceptibility testing is performed to inform the healthcare practitioner which antibiotics are most likely to be effective for treatment.

In many laboratories, the blood culture testing process is automated with instruments continuously monitoring the samples for growth of bacteria or fungi. This allows for timely reporting of results and for the healthcare practitioner to direct antimicrobial therapy to the specific microbe present in the blood. Because treatment must be given as soon as possible in cases of sepsis, broad-spectrum antimicrobials that are effective against several types of bacteria are usually given intravenously while waiting for blood culture results. Antimicrobial therapy may be changed to a more targeted antibiotic therapy once the microbe causing the infection is identified.

How is the sample collected for testing?

Usually, two blood samples are collected from different veins to increase the likelihood of detecting bacteria or fungi if they are present in the blood. Multiple blood samples help to differentiate true pathogens, which will be present in more than one blood culture, from skin bacteria that may contaminate one of several blood cultures during the collection process.

Blood is obtained by inserting a needle into a vein in the arm. The phlebotomist will put the blood into two culture bottles containing broth to grow microbes. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different site, immediately after the first venipuncture. A single blood culture may be collected from children since they often have high numbers of bacteria present in their blood when they have an infection. For infants and young children, the quantity of each blood sample will be smaller and appropriate for their body size.

White blood cells (WBCs), also called leukocytes, are cells that circulate in the blood and the lymphatic system that help protect the body against infections. They are an important part of the body’s immune system and also have a role in inflammation, allergic responses, and protection against cancer. A WBC differential totals the number of each of the different types of WBCs in a person’s sample of blood.

There are five types of white blood cells, each with different functions. The differential reveals if the cells are present in normal proportion to one another, if the number of one cell type is increased or decreased, or if abnormal and/or immature cells are present. This information is helpful in diagnosing specific types of illnesses that affect the immune system and the bone marrow.

A differential may be performed in conjunction with a complete blood count (CBC), a test often used as a general health check, or it may be performed in follow-up to abnormal results on a CBC. Most often, a differential is performed on an automated blood analyzer but also may be performed manually by a trained laboratorian who examines a blood smear under a microscope. The values are typically reported as absolute numbers of cells but may be expressed as the relative percentages of the total number of WBCs.

White blood cells develop from precursor cells produced in the bone marrow. The five different types of WBCs include:

• Granulocytes—these white blood cells have granules in their cytoplasm. The granules contain chemicals and other substances that are released as part of an immune response. The three types of granulocytes include:
  • Neutrophils (neu) normally make up the largest number of circulating WBCs. They move into an area of damaged or infected tissue, where they engulf and destroy bacteria or sometimes fungi.
  • Eosinophils (eos) respond to infections caused by parasites, play a role in allergic reactions (hypersensitivities), and control the extent of immune responses and inflammation.
  • Basophils (baso) usually make up the fewest number of circulating WBCs and are thought to be involved in allergic reactions.
• Lymphocytes (lymphs) exist in both the blood and the lymphatic system. They are divided into three types, but the differential does not distinguish among them. All lymphocytes differentiate from common lymphoid progenitor cells in the bone marrow. The differential counts and reports all lymphocytes together. Separate specialized testing (like immunophenotyping) must be done to differentiate the three types:
  • B lymphocytes (B cells) are antibody-producing cells that are essential for acquired, antigen-specific immune responses. Plasma cells are fully differentiated B-cells that produce antibodies, immune proteins that target and destroy bacteria, viruses and other “non-self” foreign antigens.
  • T lymphocytes (T cells) finish maturing in the thymus and consist of a few different types. Some T cells help the body distinguish between “self” and “non-self” antigens. Others initiate and control the extent of an immune response, boosting it as needed and then slowing it as the condition resolves. Other types of T cells directly attack and neutralize virus-infected or cancerous cells.
  • Natural killer cells (NK cells) directly attack and kill abnormal cells such as cancer cells or those infected with a virus.
• Monocytes (mono), similar to neutrophils, move to an area of infection and engulf and destroy bacteria. They are associated more often with chronic rather than acute infections. They are also involved in tissue repair and other functions involving the immune system.

When there is an infection or an inflammatory process somewhere in the body, the bone marrow produces more WBCs, releasing them into the blood. Depending on the cause of infection or inflammation, one particular type of WBC may be increased as opposed to other types. As the condition resolves, the production of that type of WBC subsides and the number drops to normal levels again.

In addition to infections and inflammation, there are a variety of conditions that can affect the production of WBCs by the bone marrow or their survival in the blood, resulting in either increased or decreased numbers. The differential, along with the other components of the CBC, alerts the healthcare provider to possible health issues. Results are often interpreted in conjunction with additional tests such as a blood smear review, which can reveal the presence of abnormal and/or immature populations of WBCs.

In a few serious diseases, some immature forms of the cells are released from the bone marrow into the circulation and may be detected by the WBC differential. This may occur with bacterial infection, leukemia, bone marrow involvement by solid tumor, myelodysplastic syndrome, or myeloproliferative neoplasms, for example. Some immature cells that may be detected include metamyelocytes, myelocytes, promyelocytes, and/or blasts.

If results indicate a problem, a wide variety of other tests may be performed in order to help determine the cause. A healthcare provider will typically consider an individual’s signs and symptoms, medical history, and results of a physical examination to decide what other tests may be necessary. For example, as needed, a bone marrow biopsy will be performed to evaluate the bone marrow status.

How is the sample collected for testing?

A blood sample is drawn from a vein in the arm or from a fingerstick (for children and adults) or heelstick (for infants).

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Glucose (commonly called “blood sugar”) is the primary energy source for the body’s cells and the only short-term energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood. Glucose tests measure the level of glucose in your blood or detect glucose in your urine.

A few different protocols may be used to evaluate glucose levels. This article focuses on:

• Fasting blood glucose (commonly called fasting blood sugar)—this test measures the level after a fast of at least 8 hours.
• Random blood glucose—sometimes your blood glucose will be measured when you have not fasted (randomly).

Other types of glucose tests include:

• A glucose tolerance test measures glucose levels after fasting and after you drink liquid containing a specific amount of glucose (see Glucose Tolerance Test).
• A specific protocol is used to help diagnose gestational diabetes, which is diabetes that first develops during pregnancy (see Glucose Tests for Gestational Diabetes).
• Urine is routinely tested for glucose as part of a urinalysis.

During digestion, the carbohydrates that you eat are broken down into glucose (and other nutrients). They are absorbed by the digestive tract, move into the blood, and circulate throughout the body. Normally, blood glucose rises slightly after a meal and the hormone insulin is released by the pancreas into the blood in response. The amount of insulin released corresponds to the size and content of the meal. Insulin helps transport glucose into the body’s cells, where it is used for energy. As glucose moves into the cells and is broken down (metabolized), the blood glucose level drops and the pancreas responds by decreasing the release of insulin.

If this glucose/insulin blood feedback system is working properly, the amount of glucose in the blood remains fairly stable. If the feedback system is disrupted and the glucose level in the blood rises, then the body tries to restore the balance by increasing insulin production.

Diabetes is the most common disease resulting from an imbalance between glucose and insulin.

• Type 1 diabetes results when the body is not able to produce sufficient insulin to control blood glucose levels. Usually in type 1 diabetes, the cells that produce insulin (beta cells) have been destroyed by the person’s own immune system.
• Type 2 diabetes results from a combination of insulin resistance (the body does not react normally to insulin) and a relative decline in insulin production.
• Some women may develop gestational diabetes, which is high blood glucose that develops during pregnancy. (For more information, see the article on Glucose Tests for Gestational Diabetes.)

Severe, acute changes in blood glucose, either high (hyperglycemia) or low (hypoglycemia), can be life-threatening, causing organ failure, brain damage, coma, and, in some cases, death. Chronically high blood glucose levels that can occur with untreated or poorly controlled diabetes can cause progressive damage to body organs such as the kidneys, eyes, heart and blood vessels, and nerves. Chronic hypoglycemia can lead to brain and nerve damage.

Ketones or ketone bodies are byproducts of fat metabolism. This test measures the amount of ketones in the blood.

Ketones are produced when glucose is not available to the body’s cells as an energy source and/or when the body cannot use glucose as a fuel source because there is no insulin or not enough insulin. Fat is used as fuel instead. When fat is metabolized, byproducts called ketone bodies build up in the blood, causing first ketosis and then progressing to ketoacidosis, a form of metabolic acidosis. This condition is most frequently seen with uncontrolled type 1 diabetes and can be a medical emergency.

Diabetic ketoacidosis (DKA) is associated with sudden and severe high blood glucose (acute hyperglycemia), a severe insulin deficiency, and a disruption of the body’s acid-base balance. Excess ketones and glucose are dumped into the urine by the kidneys in an effort to flush them from the body. This causes increased urination, thirst, dehydration, and a loss of electrolytes. Symptoms may also include rapid breathing, shortness of breath, a fruity scent to the breath, nausea, vomiting, fatigue, confusion, and eventually coma.

Ketosis and ketoacidosis may also be seen with prolonged starvation, alcoholism, and with high-fat, low-carbohydrate diets (keto diets). It may be induced on purpose in some children with epilepsy who have frequent seizures and do not respond to available medications or other treatments.

There are three ketone bodies – acetoacetate, beta-hydroxybutyrate, and acetone.

• Acetoacetate is created first when fat is metabolized.
• Beta-hydroxybutyrate is created from acetoacetate. Beta-hydroxybutyrate is the predominant ketone body present in severe diabetic ketoacidosis (DKA).
• Acetone is a spontaneously created side product of acetoacetate.

Different ketone tests measure one or more ketone bodies, and their results are not interchangeable.

Blood testing gives a snapshot of the status of ketone accumulation at the time that the sample was collected. Urine ketone testing reflects recent rather than current blood ketones. Urine testing is much more common than blood ketones testing. It may be performed by itself, with a urine glucose test, or as part of a urinalysis. The urine methods measure either acetoacetate or acetoacetate and acetone but do not usually detect beta-hydroxybutyrate.

Blood ketones may be measured in a laboratory or with a handheld monitor. The laboratory test uses serum, the liquid portion of the blood, and typically measures acetoacetate. Beta-hydroxybutyrate can be ordered as a separate blood test.

When whole blood from a fingerstick is tested for ketones using a handheld monitor, the monitor measures beta-hydroxybutyrate. This test may be performed at the bedside in a hospital or emergency room, in a healthcare practitioner’s office, or performed at home.

Blood types are based on the markers (specific carbohydrates or proteins) or antigens on the surface of red blood cells (RBCs). Two major antigens or surface identifiers on human RBCs are the A and B antigens. Another important surface antigen is called Rh. Blood typing detects the presence or absence of these antigens to determine a person’s ABO blood group and Rh type.

People whose red blood cells have A antigens are in blood group A, those with B antigens are group B, those with both A and B antigens are in group AB, and those who do not have either of these markers are in blood group O.

If the Rh protein is present on the red blood cells, a person’s blood type is Rh+ (positive); if it is absent, the person’s blood is type Rh- (negative).

Our bodies naturally produce antibodies against the A and B antigens that we do not have on our red blood cells. For example, a person who is blood type A will have anti-B antibodies directed against the B antigens on red blood cells and someone who is type B will have anti-A antibodies directed against the A antigens. People with type AB blood have neither of these antibodies, while those with type O blood have both.

The following table indicates the type of antibodies a person is expected to have based on their blood type.

These antibodies are useful for determining a person’s blood type and help determine the types of blood that he or she can safely receive (compatibility). If a person who is group A with antibodies directed against the B antigen, for example, were to be transfused with blood that is type B, his or her own antibodies would target and destroy the transfused red blood cells, causing severe, potentially fatal complications. Thus, it is critical to match a person’s blood type with the blood that is to be transfused.

Unlike antibodies to A and B antigens, antibodies to Rh are not produced naturally. That is, Rh antibodies develop only after a person who does not have Rh factor on his or her red blood cells (Rh negative) is exposed to Rh positive red blood cells. This can happen during pregnancy or birth when an Rh-negative woman is pregnant with an Rh-positive baby, or sometimes when an Rh-negative person is transfused with Rh-positive blood. In either case, the first exposure to the Rh antigen may not result in a strong response against the Rh positive cells, but subsequent exposures may cause severe reactions.

Urea is a waste product formed in the liver when protein is metabolized into its component parts (amino acids). This process produces ammonia, which is then converted into the less toxic waste product urea. This test measures the blood urea nitrogen (BUN) level in the blood. Sometimes, a BUN to creatinine ratio is calculated to help determine the cause of elevated levels.

Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the “transport method” used by the body to rid itself of excess nitrogen. Urea is formed in and released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or if the kidneys are not working properly and have difficulty filtering wastes out of the blood, then urea levels will rise in the blood. If significant liver damage or disease inhibits the production of urea, then BUN levels may fall.

Urea is a waste product formed in the liver when protein is metabolized into its component parts (amino acids). This process produces ammonia, which is then converted into the less toxic waste product urea. This test measures the blood urea nitrogen (BUN) level in the blood. Sometimes, a BUN to creatinine ratio is calculated to help determine the cause of elevated levels.

Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the “transport method” used by the body to rid itself of excess nitrogen. Urea is formed in and released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or if the kidneys are not working properly and have difficulty filtering wastes out of the blood, then urea levels will rise in the blood. If significant liver damage or disease inhibits the production of urea, then BUN levels may fall.

Urea is a waste product formed in the liver when protein is metabolized into its component parts (amino acids). This process produces ammonia, which is then converted into the less toxic waste product urea. This test measures the blood urea nitrogen (BUN) level in the blood. Sometimes, a BUN to creatinine ratio is calculated to help determine the cause of elevated levels.

Nitrogen is a component of both ammonia and urea. Urea and urea nitrogen are referred to somewhat interchangeably because urea contains nitrogen and because urea/urea nitrogen is the “transport method” used by the body to rid itself of excess nitrogen. Urea is formed in and released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Most diseases or conditions that affect the kidneys or liver have the potential to affect the amount of urea present in the blood. If increased amounts of urea are produced by the liver or if the kidneys are not working properly and have difficulty filtering wastes out of the blood, then urea levels will rise in the blood. If significant liver damage or disease inhibits the production of urea, then BUN levels may fall.

Susceptibility is a term used when microbe such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed on bacteria or fungi causing an individual’s infection after they have been recovered in a culture of the specimen. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

Although viruses are microbes, testing for their resistance to antiviral drugs is performed less frequently and by different test methods. This article is limited to the discussion of bacterial and fungal susceptibility testing.

During the culture process, pathogens are isolated (separated out from any other microbes present). Each pathogen, if present, is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, it is possible to determine whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. For example, strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus), can be treated with ampicillin and does not require a test to predict susceptibility to this class of antibiotics.

Susceptibility testing is performed on each type of bacteria or fungi that may be relevant to the individual’s treatment and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria. Resistance can also be determined by detection of a gene that is known to cause resistance to specific antibiotics.

C-reactive protein (CRP) is a protein made by the liver. CRP levels in the blood increase when there is a condition causing inflammation somewhere in the body. A CRP test measures the amount of CRP in the blood to detect inflammation due to acute conditions or to monitor the severity of disease in chronic conditions.

CRP is a non-specific indicator of inflammation and one of the most sensitive acute phase reactants. That means that it is released into the blood within a few hours after an injury, the start of an infection, or other cause of inflammation. Markedly increased levels can occur, for example, after trauma or a heart attack, with active or untreated autoimmune disorders, and with serious bacterial infections, such as in sepsis. The level of CRP can jump as much as a thousand-fold in response to bacterial infection, and its rise in the blood can precede pain, fever, or other signs and symptoms.

The CRP test is not diagnostic, but it provides information to your healthcare practitioner as to whether inflammation is present, without identifying the source of the inflammation. This information can be used in conjunction with other factors such as signs and symptoms, physical exam, and other tests to determine if you have an acute inflammatory condition or are experiencing a flare-up of a chronic inflammatory disease. Your healthcare practitioner may then follow up with further testing and treatment.

This standard CRP test is not to be confused with an hs-CRP test. These are two different tests that measure CRP and each test measures a different range of CRP level in the blood for different purposes:

The standard CRP test measures high levels of the protein observed in diseases that cause significant inflammation. It measures CRP in the range from 8 to 1000 mg/L (or 0.8 to 100 mg/dL).
The hs-CRP test precisely detects lower levels of the protein than that measured by the standard CRP test and is used to evaluate individuals for risk of cardiovascular disease. It measures CRP in the range from 0.3 to 10 mg/L. (See the article on hs-CRP.)

The complement system consists of almost 60 proteins, approximately 30 of which are circulating blood proteins that work together to promote immune and inflammatory responses. Complement tests measure the amount or activity of complement proteins in the blood.

The complement system’s principal role is to help identify, destroy and remove foreign pathogens like bacteria and viruses, as well as damaged “self” materials (e.g., cells and proteins). The complement system is so named because it “complements” or aids the natural body defenses, such as antibodies. So, the complement system is also activated when the body makes antibodies, whether against itself or foreign invaders. The body makes antibodies against its own tissues that it thinks are foreign (autoantibodies) in various autoimmune diseases.

The complement system is part of the body’s innate immune system. Unlike the acquired immune system, which produces antibodies that target and protect against specific threats, the innate immune system is non-specific and can quickly respond to foreign substances. It does not require previous exposure to an invading microbe or offending substance and does not maintain a memory of previous encounters.

There are nine primary complement proteins that are designated C1 through C9. These components, in addition to the remaining proteins, work together in a cascade-like approach by activating, amplifying, breaking apart, and forming protein complexes that respond to infections, “non-self” tissues (e.g., transplants), dead cell debris (e.g., from apoptosis), or inflammation.

The complement cascade consists of 3 separate pathways that may be activated to converge in a final common pathway. The pathways include the “classical pathway” (including components C1qrs, C2, C4), the “alternative pathway” (including components C3, factor B, properdin), and the “lectin pathway” (a.k.a. mannan-binding lectin [MBL]). The end result of all three activation pathways is the same – the formation of the membrane attack complex (MAC). Complement activation causes several things to happen (“complement cascade”):

• The MAC binds to the surface of each microbe, abnormal cell, or substance that has been targeted for destruction. It creates a lesion (hole) in the membrane wall and causes lysis, which is destruction of the cell by letting the contents out – like piercing a water-filled balloon.
• It increases the permeability of blood vessels, allowing infection-fighting white blood cells (WBCs) to move out of the bloodstream and into tissues.
• It attracts WBCs to the site of the infection.
• It stimulates phagocytosis, a process in which microbes are engulfed by macrophages and neutrophils and killed.
• It also labels immune complexes with complement components so that WBCs will engulf them and clear them out of the blood.

Complement tests measure the amount or the function (activity) of complement proteins in the blood. Complement components may be measured individually or together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity can be measured if a healthcare practitioner suspects a deficiency that is not measured by C3 or C4. The CH50 functional test measures the function of the complete classical complement pathway, mediated by components C1 – C9. If this measurement is outside the normal range, then each of the nine different complement levels can be measured individually to look for hereditary or acquired deficiencies.

The complement system consists of almost 60 proteins, approximately 30 of which are circulating blood proteins that work together to promote immune and inflammatory responses. Complement tests measure the amount or activity of complement proteins in the blood.

The complement system’s principal role is to help identify, destroy and remove foreign pathogens like bacteria and viruses, as well as damaged “self” materials (e.g., cells and proteins). The complement system is so named because it “complements” or aids the natural body defenses, such as antibodies. So, the complement system is also activated when the body makes antibodies, whether against itself or foreign invaders. The body makes antibodies against its own tissues that it thinks are foreign (autoantibodies) in various autoimmune diseases.

The complement system is part of the body’s innate immune system. Unlike the acquired immune system, which produces antibodies that target and protect against specific threats, the innate immune system is non-specific and can quickly respond to foreign substances. It does not require previous exposure to an invading microbe or offending substance and does not maintain a memory of previous encounters.

There are nine primary complement proteins that are designated C1 through C9. These components, in addition to the remaining proteins, work together in a cascade-like approach by activating, amplifying, breaking apart, and forming protein complexes that respond to infections, “non-self” tissues (e.g., transplants), dead cell debris (e.g., from apoptosis), or inflammation.

The complement cascade consists of 3 separate pathways that may be activated to converge in a final common pathway. The pathways include the “classical pathway” (including components C1qrs, C2, C4), the “alternative pathway” (including components C3, factor B, properdin), and the “lectin pathway” (a.k.a. mannan-binding lectin [MBL]). The end result of all three activation pathways is the same – the formation of the membrane attack complex (MAC). Complement activation causes several things to happen (“complement cascade”):

• The MAC binds to the surface of each microbe, abnormal cell, or substance that has been targeted for destruction. It creates a lesion (hole) in the membrane wall and causes lysis, which is destruction of the cell by letting the contents out – like piercing a water-filled balloon.
• It increases the permeability of blood vessels, allowing infection-fighting white blood cells (WBCs) to move out of the bloodstream and into tissues.
• It attracts WBCs to the site of the infection.
• It stimulates phagocytosis, a process in which microbes are engulfed by macrophages and neutrophils and killed.
• It also labels immune complexes with complement components so that WBCs will engulf them and clear them out of the blood.

Complement tests measure the amount or the function (activity) of complement proteins in the blood. Complement components may be measured individually or together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity can be measured if a healthcare practitioner suspects a deficiency that is not measured by C3 or C4. The CH50 functional test measures the function of the complete classical complement pathway, mediated by components C1 – C9. If this measurement is outside the normal range, then each of the nine different complement levels can be measured individually to look for hereditary or acquired deficiencies.

Cancer antigen 19-9 (CA 19-9) is a protein that exists on the surface of certain cancer cells. CA 19-9 does not cause cancer; rather, it is shed by the tumor cells and can be detected by laboratory tests in blood and sometimes other body fluids. This test measures the level of CA19-9.

Since CA 19-9 can be measured in blood, it is useful as a tumor marker to follow the course of the cancer. CA 19-9 is elevated in about 70% to 95% of people with advanced pancreatic cancer. (Read more in the “How is the test used?” section under Common Questions and in the Pancreatic Cancer article.)

However, CA 19-9 may also be elevated in other cancers, conditions, and diseases such as: gallbladder and bile duct cancers (cholangiocarcinoma), colorectal cancer, gastric cancers, ovarian cancer, lung cancer, liver cancer, bile duct obstruction (e.g., gallstones), pancreatitis, cystic fibrosis, thyroid disease, and liver disease. Small amounts of CA 19-9 are present in the blood of healthy people. Since CA 19-9 is not specific for pancreatic cancer, it cannot be used by itself for screening or diagnosis.

The European Group on Tumor Markers (EGTM) and National Comprehensive Cancer Network (NCCN) recommend use of CA 19-9 as a tumor marker for pancreatic cancer in addition to other tests and examinations to diagnose and monitor the disease.

Cancer Antigen 125 (CA-125) is a protein that is present on the surface of most, but not all, ovarian cancer cells. This makes the test useful as a tumor marker in specific circumstances. The CA-125 test measures the amount of CA-125 protein in your blood.

Significantly elevated concentrations of CA-125 may be present in the blood of a woman who has ovarian cancer. Thus, the test may be used to monitor the effectiveness of treatment and/or for recurrence of the cancer. However, not all women with ovarian cancer will have elevated CA-125 so the test may not be useful in all cases.

Ovarian cancer is the fifth most common cause of cancer death in women. According to the American Cancer Society (ACS), the lifetime risk of a woman developing ovarian cancer is about 1 in 78 and the lifetime risk of death is 1 in 108. ACS estimates that in 2019 about 22,530 women will be diagnosed with ovarian cancer in the U.S. and about 13,980 women will die of it.

Currently, less than 20% of ovarian cancers are found in the early stages before they have spread outside the ovary. The primary reason they go undetected is that the symptoms of ovarian cancer are fairly non-specific.

The need for a reliable method for early detection of ovarian cancer among asymptomatic women continues to drive ongoing research. In the meantime, regular physicals, pelvic exams, and an awareness of your family history and symptoms are important.

CA-125 is not recommended as a screening test for asymptomatic women because it is non-specific. Small quantities of CA-125 are produced by normal tissues throughout your body and by some other cancers. Levels in your blood may be moderately elevated with a variety of non-cancerous conditions, including menstruation, pregnancy, and pelvic inflammatory disease.

Cancer antigen 15-3 (CA 15-3) is a protein that is produced by normal breast cells. In many people with cancerous breast tumors, there is an increased production of CA 15-3 and the related cancer antigen 27.29. CA 15-3 does not cause cancer; rather, it is shed by the tumor cells and enters the blood. This test measures CA 15-3 in the blood.

Since CA 15-3 can be measured in the blood, it is useful as a tumor marker to follow the course of the cancer. CA 15-3 is elevated in fewer than 50% of women with early localized, breast cancer or with a small tumor, but is elevated in about 80% of those with breast cancer that has spread (metastatic). Because not all women with invasive breast cancer will have elevated CA 15-3, the test is not useful in all cases.

CA 15-3 is not recommended as a screening test to detect breast cancer in women because it is non-specific. It may also be elevated in healthy people and in individuals with other cancers such as colon, lung, pancreas, ovarian, or prostate malignancies or certain conditions such as cirrhosis, hepatitis, and benign breast disease.

Cancer antigen 19-9 (CA 19-9) is a protein that exists on the surface of certain cancer cells. CA 19-9 does not cause cancer; rather, it is shed by the tumor cells and can be detected by laboratory tests in blood and sometimes other body fluids. This test measures the level of CA19-9.

Since CA 19-9 can be measured in blood, it is useful as a tumor marker to follow the course of the cancer. CA 19-9 is elevated in about 70% to 95% of people with advanced pancreatic cancer. (Read more in the “How is the test used?” section under Common Questions and in the Pancreatic Cancer article.)

However, CA 19-9 may also be elevated in other cancers, conditions, and diseases such as: gallbladder and bile duct cancers (cholangiocarcinoma), colorectal cancer, gastric cancers, ovarian cancer, lung cancer, liver cancer, bile duct obstruction (e.g., gallstones), pancreatitis, cystic fibrosis, thyroid disease, and liver disease. Small amounts of CA 19-9 are present in the blood of healthy people. Since CA 19-9 is not specific for pancreatic cancer, it cannot be used by itself for screening or diagnosis.

The European Group on Tumor Markers (EGTM) and National Comprehensive Cancer Network (NCCN) recommend use of CA 19-9 as a tumor marker for pancreatic cancer in addition to other tests and examinations to diagnose and monitor the disease.

Glomerular filtration rate (GFR) is a measure of how well your kidneys are working. The kidney’s primary function is to filter blood. Waste and excess water gets removed and turned into urine. The levels of salts and minerals in blood are adjusted to maintain a healthy balance. In addition, kidneys produce hormones that regulate blood pressure, maintain bone health, and control production of red blood cells.

Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important substances, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to build up in the blood.

Chronic kidney disease (CKD) is a common disease associated with a slow and progressive decrease in kidney function. Diabetes and high blood pressure are the two main causes of CKD. Aging is another risk factor for CKD that is often overlooked. As we age, so do our kidneys, losing about 1% of kidney filtration per year after age of 40.

Kidney diseases tend to progress silently and most cases of early stages of CKD go undiagnosed. Most people have no symptoms until 30-40% of kidney function is lost. Early detection of kidney dysfunction is crucial because it can help to minimize the damage by starting a suitable treatment. Screening for CKD is recommended for high risk groups, such as people with diabetes, high blood pressure (hypertension), heart disease, family history of kidney failure, and the elderly (age 60 and older).

Simple laboratory tests used to evaluate kidney function include the blood creatinine test and/or blood cystatin C test to estimate GFR and urine albumin and urine creatinine clearance.

Measuring glomerular filtration rate (GFR) directly is considered the most accurate way to detect changes in kidney function, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. For this reason, the estimated GFR (eGFR), which represents the best routinely available measurement of kidney function, is usually used. See “How can my actual GFR be determined?” under Common Questions for more information.

The eGFR is calculated according to the formulae recommended by the National Kidney Foundation using measured test results of blood creatinine levels and in special circumstances blood cystatin C levels.

• The most common approach is the eGFR based on blood creatinine level along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and levels increase in the blood. The variations in muscle metabolism and muscle mass can lead to significant differences in measured creatinine levels. The eGFR may not be reliable in amputees, body builders, vegetarians, patients at the extremes of weight and age, and those with rapidly changing kidney function (e.g., acute kidney injury). In addition, some medications can increase creatinine.
• The eGFR calculated based on blood cystatin C level is recommended for confirmatory testing when eGFR based on serum creatinine is less accurate (e.g., elderly, overweight or people with abnormal muscle mass). Cystatin C is a small protein produced at a constant rate by all nucleated cells in the body and is found in various body fluids, including blood. It is filtered by the kidneys and broken down at a constant rate. It does not reenter the blood circulation or get eliminated in the urine. Unlike creatinine, which is affected by muscle mass, cystatin C is affected by age, inflammation, obesity, and diabetes. Cystatin C is a relatively new biomarker and extensive research is ongoing to define its optimal use. See “Is there anything else I should know?” under Common Questions for more information.

Different equations may be used to calculate eGFR. The following two are most common and require a blood creatinine result, age, and assigned values based upon sex and race.

• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (2009)—recommended by the National Kidney Foundation for calculating eGFR in adults.
• Modification of Diet in Renal Disease Study (MDRD) equation—some laboratories continue to use this equation.

Additional sets of CKD-EPI equations recommended by the National Kidney Foundation for adults that use cystatin C level to calculate eGFR, along with age and gender, include:

• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation (2012)
• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine and cystatin C equation (2012)

The results reported using one of the above equations versus the other will not be identical but should give a healthcare practitioner similar information.

Separate equations, based on serum creatinine levels and height, are recommended for use in youth age 18 years and under. One is the updated Schwartz equation that takes the blood urea nitrogen (BUN) level into consideration.

The eGFR equations are not valid for those who are 70 years of age or older because muscle mass normally decreases with age.

See “Could I calculate my own eGFR?” under Common Questions for more information about these eGFR equations.

Cancer antigen 19-9 (CA 19-9) is a protein that exists on the surface of certain cancer cells. CA 19-9 does not cause cancer; rather, it is shed by the tumor cells and can be detected by laboratory tests in blood and sometimes other body fluids. This test measures the level of CA19-9.

Since CA 19-9 can be measured in blood, it is useful as a tumor marker to follow the course of the cancer. CA 19-9 is elevated in about 70% to 95% of people with advanced pancreatic cancer. (Read more in the “How is the test used?” section under Common Questions and in the Pancreatic Cancer article.)

However, CA 19-9 may also be elevated in other cancers, conditions, and diseases such as: gallbladder and bile duct cancers (cholangiocarcinoma), colorectal cancer, gastric cancers, ovarian cancer, lung cancer, liver cancer, bile duct obstruction (e.g., gallstones), pancreatitis, cystic fibrosis, thyroid disease, and liver disease. Small amounts of CA 19-9 are present in the blood of healthy people. Since CA 19-9 is not specific for pancreatic cancer, it cannot be used by itself for screening or diagnosis.

The European Group on Tumor Markers (EGTM) and National Comprehensive Cancer Network (NCCN) recommend use of CA 19-9 as a tumor marker for pancreatic cancer in addition to other tests and examinations to diagnose and monitor the disease.

Cancer antigen 15-3 (CA 15-3) is a protein that is produced by normal breast cells. In many people with cancerous breast tumors, there is an increased production of CA 15-3 and the related cancer antigen 27.29. CA 15-3 does not cause cancer; rather, it is shed by the tumor cells and enters the blood. This test measures CA 15-3 in the blood.

Since CA 15-3 can be measured in the blood, it is useful as a tumor marker to follow the course of the cancer. CA 15-3 is elevated in fewer than 50% of women with early localized, breast cancer or with a small tumor, but is elevated in about 80% of those with breast cancer that has spread (metastatic). Because not all women with invasive breast cancer will have elevated CA 15-3, the test is not useful in all cases.

CA 15-3 is not recommended as a screening test to detect breast cancer in women because it is non-specific. It may also be elevated in healthy people and in individuals with other cancers such as colon, lung, pancreas, ovarian, or prostate malignancies or certain conditions such as cirrhosis, hepatitis, and benign breast disease.

Cannabis, also referred to as marijuana, is a plant of the species Cannabis sativa that has psychoactive effects. Marijuana contains many chemical compounds that interact with the body, called cannabinoids. The main mind-altering cannabinoid in marijuana is THC (delta-9-tetrahydrocannabinol). The body breaks down THC into several inactive metabolites (e.g., THC-COOH, 11-nor-carboxy-delta-9-tetrahydrocannabinol). Since the metabolites of THC stay in the body for a longer period of time than THC does, most marijuana testing detects the presence of THC-COOH or other metabolites in urine. Some tests also detect the active compound, THC, for example when marijuana testing is done using blood or saliva.

Marijuana leaves can be smoked, prepared and eaten in food products, or ingested as a tincture. Smoking or ingesting marijuana causes a faster heartbeat and pulse rate, bloodshot eyes, and dry mouth and throat. It may cause short- and long-term effects, including impaired short-term memory, altered sense of time, decreased ability to concentrate, altered reaction time, decreased coordination, increased risk of psychosis, and cyclic episodes of nausea, stomach pain and vomiting. Marijuana use impairs driving ability, and accident risk greatly increases if the driver is also drinking alcohol. Marijuana today has higher THC concentrations than in the past, which can lead to greater levels of impairment.

Marijuana is the most commonly used illicit substance as defined by U.S. federal law. However, marijuana is used both recreationally and medicinally, and its use has been legalized in a number of states and Canada.

Marijuana as medicine
Some people use marijuana to treat a variety of conditions. Due to legal restrictions on marijuana, its therapeutic effects and safety have been difficult to research.

Natural cannabis may be obtained in some states with a medical marijuana card, an identification card issued by the state with a healthcare practitioner’s recommendation. It allows a patient to obtain, possess, and/or grow marijuana for medicinal use. The process for obtaining these cards varies by state.

Synthetic forms or purified forms of THC and other cannabinoids are available as prescription medications that can be taken by mouth, inhaled, or sprayed under the tongue.

• Dronabinol and nabilone are two drugs approved by the U.S. Food and Drug Administration (FDA) that contain THC. They are sometimes prescribed to treat chemotherapy-associated nausea that does not respond to standard treatments.
• Cannabidiol (CBD) is a compound related to THC that is a product of the marijuana plant. It is available in liquid form (CBD oil) as an FDA-approved medication that may be used to treat pain and inflammation and some epileptic seizures. Some “herbal supplements” also contain CBD but are not regulated by the FDA. Unlike THC, CBD is not intoxicating—it does not make people “high.” However, because CBD is derived from the marijuana plant, it may contain variable amounts of THC.
• Nabiximols is a marijuana-derived prescription drug that contains THC and CBD. It may be used to treat muscle control problems in people with multiple sclerosis (MS). Though this drug has not been approved by the FDA, it has been approved for use in Canada, the United Kingdom, and some European countries.

Anything that contains THC has the potential to be detected as THC or THC-COOH in a marijuana test. In medical cases, drug testing results are typically reported to the physicians who ordered the tests and they interpret the meaning of results in the context of the medical cases. To aid the interpretation of results, patients should notify their physicians of any over-the-counter or prescription medications (including marijuana) or supplements they are taking.

Marijuana laws and policies
Drug testing policy for THC can become confusing due to conflicting state and federal laws and policies. Marijuana is a Schedule 1 drug according to U.S. federal law. The government considers Schedule 1 drugs to have high potential for abuse, lack evidence of safety and effectiveness, with no currently accepted medical use in the U.S.

Since 1986, U.S. federal employees and employees in federally-regulated jobs that affect public safety like transportation and air traffic control have been prohibited from using illegal drugs. In U.S. federally regulated tests, no marijuana use is considered legitimate, except in the case of prescription synthetic THC, such as dronabinol. Private employers not under federal drug testing regulations are free to determine their own drug testing policies and may or may not test for marijuana use.

If someone is given a drug test for legal or employment purposes while taking a prescription form of THC, they should report their prescription to the physician who will review the test results (Medical Review Officer or MRO) and/or list it under ‘prescriptions’ in the appropriate space on the specimen collection form. Positive drug testing results that are due to prescription medications may be reported by the MRO as “negative.”

Marijuana testing
To test for marijuana, healthcare practitioners may send urine samples to a laboratory for screening or they may perform a test in their office. Testing may be performed with point-of-care tests, which are typically small strips that are dipped into the urine and interpreted by the appearance or absence of a colored line on the strip.

Positive screening tests for marijuana are presumptive. This is because all drug screening tests have the potential for false-positive results. Therefore, screening tests that are positive are often confirmed with a second test, which is referred to as a confirmatory test. Confirmatory tests are usually more sensitive and specific than screening tests. Confirmatory testing is usually performed with an instrument called a mass spectrometer. Types of mass spectrometry tests used for confirmatory testing include gas chromatography/mass spectrometry (GC/MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Additional testing may also be performed to identify whether any substance has been added to the urine sample, such as water, bleach, or other types of chemicals (adulterants) in an attempt to interfere with the test result. If adulteration is suspected, the test may be reported as invalid or adulterated.

Urine samples may also be identified as dilute. Dilute urine may be produced if an individual drinks large amounts of water or other liquids prior to testing. Dilute urine may also result from some medical conditions. If an individual adds liquid such as water to a urine sample, this may also cause dilute urine. It may not always be possible to determine how the urine was diluted. If the urine is so dilute that the results are not reliable, the test may be reported as invalid or adulterated.

Cancer antigen 19-9 (CA 19-9) is a protein that exists on the surface of certain cancer cells. CA 19-9 does not cause cancer; rather, it is shed by the tumor cells and can be detected by laboratory tests in blood and sometimes other body fluids. This test measures the level of CA19-9.

Since CA 19-9 can be measured in blood, it is useful as a tumor marker to follow the course of the cancer. CA 19-9 is elevated in about 70% to 95% of people with advanced pancreatic cancer. (Read more in the “How is the test used?” section under Common Questions and in the Pancreatic Cancer article.)

However, CA 19-9 may also be elevated in other cancers, conditions, and diseases such as: gallbladder and bile duct cancers (cholangiocarcinoma), colorectal cancer, gastric cancers, ovarian cancer, lung cancer, liver cancer, bile duct obstruction (e.g., gallstones), pancreatitis, cystic fibrosis, thyroid disease, and liver disease. Small amounts of CA 19-9 are present in the blood of healthy people. Since CA 19-9 is not specific for pancreatic cancer, it cannot be used by itself for screening or diagnosis.

The European Group on Tumor Markers (EGTM) and National Comprehensive Cancer Network (NCCN) recommend use of CA 19-9 as a tumor marker for pancreatic cancer in addition to other tests and examinations to diagnose and monitor the disease.

Carcinoembryonic antigen (CEA) is a protein that is present in certain tissues of a developing baby (fetus). By the time a baby is born, it drops to a very low level. In adults, CEA is normally present at very low levels in the blood but may be elevated with certain types of cancer. This test measures the amount of CEA in the blood to help evaluate individuals diagnosed with cancer.

CEA is a tumor marker. Originally, it was thought that CEA was a specific marker for colon cancer, but further study has shown that an increase in CEA may be seen in a wide variety of other cancers. CEA can also be increased in some non-cancer-related conditions, such as inflammation, cirrhosis, peptic ulcer, ulcerative colitis, rectal polyps, emphysema, and benign breast disease, and in smokers. For this reason, it is not useful as a general cancer screening tool, but it does have a role in evaluating response to cancer treatment. When you have been diagnosed with cancer, an initial baseline test for CEA may be performed. If this level is elevated, then subsequent serial testing of CEA may be performed to monitor the cancer during treatment.

Cardiac biomarkers are substances that are released into the blood when the heart is damaged or stressed. Measurements of these biomarkers are used to help diagnose acute coronary syndrome (ACS) and cardiac ischemia, conditions associated with insufficient blood flow to the heart. Tests for cardiac biomarkers can also be used to help determine a person’s risk of having these conditions or to help monitor and manage someone with suspected ACS and cardiac ischemia.

The root causes of both ACS and cardiac ischemia are usually the buildup of plaque in artery walls and hardening of the arteries (atherosclerosis). This can result in severe narrowing of the arteries leading to the heart or a sudden blockage of blood flow through these coronary arteries.

• Cardiac ischemia is caused when the supply of blood reaching heart tissue is not enough to meet the heart’s needs. When not enough blood gets to the heart, it can cause pain in the chest (angina), shortness of breath, sweating, and other symptoms. Typical angina occurs when the coronary arteries have been gradually narrowed over time. The pain starts when a person is active, making the heart work harder, and is quickly relieved by rest or by drugs that increase blood flow to the heart, such as nitroglycerine.
• ACS is caused by rupture of a plaque that results from atherosclerosis. Plaque rupture causes blood clot (thrombus) formation in coronary arteries, which results in a sudden decrease in the amount of blood and oxygen reaching the heart. A sudden decrease in the supply of blood to the heart can cause prolonged chest pain called unstable angina, often occurring at rest or not relieved by rest or nitroglycerine.When blood flow to the heart is blocked or significantly reduced for a longer period of time (usually for more than 30-60 minutes), it can cause heart cells to die and is called an acute myocardial infarction (AMI or heart attack). This leads to death of the affected portion of heart muscle with permanent damage and scarring of the heart and sometimes can cause sudden death by causing irregular heart contractions (arrhythmia). Unstable angina and AMI are together called acute coronary syndrome since they are both due to a very acute decrease in blood flow to the heart.

The symptoms of ACS and cardiac ischemia can vary greatly but frequently include chest pain, pressure, nausea, and/or shortness of breath. Though these symptoms are most often associated with heart attacks and angina, they may also be seen with non-heart-related conditions.

It is important to distinguish heart attacks from angina, heart failure, or other conditions that may have similar signs and symptoms because the treatments and monitoring requirements are different. Cardiac biomarker tests are ordered to help detect the presence of ACS and cardiac ischemia and to evaluate their severity. Increases in one or more cardiac biomarkers in the blood can identify people with ACS or cardiac ischemia, allowing rapid and accurate diagnosis and appropriate treatment of their condition.

For ACS, prompt medical intervention is crucial to prevent death and to minimize heart damage and future complications. Cardiac biomarker tests must be available to a health practitioner 24 hours a day, 7 days a week with a rapid turn-around-time. Some of the tests may be performed at the point of care (POC) – in the emergency department or at a person’s bedside. Usually, multiple cardiac biomarker tests are done over several hours to ensure that a rise in blood levels is not missed and to estimate the severity of a heart attack.

C-reactive protein (CRP) is a protein that increases in the blood with inflammation and infection as well as following a heart attack, surgery, or trauma. Studies have suggested that a persistent low level of inflammation plays a major role in atherosclerosis, the narrowing of blood vessels due to build-up of cholesterol and other lipids, which is often associated with cardiovascular disease (CVD). The hs-CRP test accurately measures low levels of CRP to identify low but persistent levels of inflammation and thus helps predict a person’s risk of developing CVD.

There are two different tests that measure CRP and each test measures a different range of CRP level in the blood for different purposes:

• The standard CRP test measures markedly high levels of the protein to detect diseases that cause significant inflammation. It measures CRP in the range from 10 to 1000 mg/L. This test may be used to detect inflammation (see the article C-Reactive Protein).
• The hs-CRP test accurately detects lower levels of the protein than the standard CRP test. It measures CRP in the range from 0.5 to 10 mg/L. This test is used to evaluate individuals for risk of CVD.

CVD causes more deaths in the U.S. each year than any other cause, according to the American Heart Association. A number of risk factors, such as family history, high cholesterol, high blood pressure, being overweight or diabetic, have been linked to the development of CVD, but a significant number of people who have few or no identified risk factors will also develop CVD. This fact has lead researchers to look for additional risk factors that might be either causing CVD or that could be used to determine lifestyle changes and/or treatments that could reduce a person’s risk.

High-sensitivity CRP is one of a growing number of cardiac risk markers that are used to help determine a person’s risk. Some studies have shown that measuring CRP with a highly sensitive assay can help identify the risk level for CVD in apparently healthy people. This more sensitive test can measure CRP levels that are within the higher end of the reference range. These normal but slightly high levels of CRP in otherwise healthy individuals can predict the future risk of a heart attack, stroke, sudden cardiac death, and peripheral arterial disease, even when cholesterol levels are within an acceptable range.

hs-CRP could be a marker not only in apparently healthy people, recent studies have shown. Adults with congenital heart disease (ACHD) with elevated CRP have worse functional status and exercise capacity, greater risk for death, or non-elective cardiovascular hospitalization.

The complete blood count (CBC) is a group of tests that evaluate the cells that circulate in blood, including red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs). The CBC can evaluate your overall health and detect a variety of diseases and conditions, such as infections, anemia and leukemia.

Blood cells are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed. The three types of cells evaluated by the CBC include:

Red Blood Cells

Red blood cells, also called erythrocytes, are produced in the bone marrow and released into the bloodstream when they mature. They contain hemoglobin, a protein that transports oxygen throughout the body. The typical lifespan of an RBC is 120 days. Thus, the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect the production of new RBCs and/or their lifespan, in addition to those conditions that may result in significant bleeding.

RBCs normally are uniform in size and shape, but their appearance can be affected by a variety of conditions, such as vitamin B12 and folate deficiencies and iron deficiency. An example of a common condition affecting RBCs is anemia, which results from low red blood cell counts and low hemoglobin. Various diseases can lead to anemia, so additional tests are often needed to determine the cause. For more details, see the articles on Red Blood Cell Count, Hemoglobin, and Hematocrit.

White Blood Cells

White blood cells, also called leukocytes, are cells that exist in the blood, the lymphatic system, and tissues and are an important part of the body’s natural defense (immune) system. They help protect against infections and also have a role in inflammation, and allergic reactions. There are five different types of WBCs and each has a different function. They include neutrophils, lymphocytes, basophils, eosinophils, and monocytes.

WBCs are present in the blood at relatively stable numbers. However, these numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate your bone marrow to produce a higher number of neutrophils to fight off a bacterial infection. With allergies, there may be an increased number of eosinophils. An increased number of lymphocytes may be produced with a viral infection. In certain diseases, such as leukemia, abnormal (immature or mature) white cells may rapidly multiply. For additional details, see the articles White Blood Cell Count and WBC Differential.

Platelets

Platelets, also called thrombocytes, are actually tiny cell fragments that circulate in blood and are essential for normal blood clotting. When there is an injury and bleeding begins, platelets help stop bleeding by adhering to the injury site and clumping together to form a temporary plug. They also release chemical signals that attract and promote clumping of additional platelets and eventually become part of a stable blood clot at the site of the injury that remains in place until the injury heals.

If you have a disease or condition that causes low platelets (thrombocytopenia) or dysfunction of platelets, you may be at an increased risk of excessive bleeding and bruising. An excess of platelets (thrombocytosis) can cause excessive clotting. For more information, see the article Platelet Count.

The complete blood count (CBC) is a group of tests that evaluate the cells that circulate in blood, including red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs). The CBC can evaluate your overall health and detect a variety of diseases and conditions, such as infections, anemia and leukemia.

Blood cells are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed. The three types of cells evaluated by the CBC include:

Red Blood Cells

Red blood cells, also called erythrocytes, are produced in the bone marrow and released into the bloodstream when they mature. They contain hemoglobin, a protein that transports oxygen throughout the body. The typical lifespan of an RBC is 120 days. Thus, the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect the production of new RBCs and/or their lifespan, in addition to those conditions that may result in significant bleeding.

RBCs normally are uniform in size and shape, but their appearance can be affected by a variety of conditions, such as vitamin B12 and folate deficiencies and iron deficiency. An example of a common condition affecting RBCs is anemia, which results from low red blood cell counts and low hemoglobin. Various diseases can lead to anemia, so additional tests are often needed to determine the cause. For more details, see the articles on Red Blood Cell Count, Hemoglobin, and Hematocrit.

White Blood Cells

White blood cells, also called leukocytes, are cells that exist in the blood, the lymphatic system, and tissues and are an important part of the body’s natural defense (immune) system. They help protect against infections and also have a role in inflammation, and allergic reactions. There are five different types of WBCs and each has a different function. They include neutrophils, lymphocytes, basophils, eosinophils, and monocytes.

WBCs are present in the blood at relatively stable numbers. However, these numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate your bone marrow to produce a higher number of neutrophils to fight off a bacterial infection. With allergies, there may be an increased number of eosinophils. An increased number of lymphocytes may be produced with a viral infection. In certain diseases, such as leukemia, abnormal (immature or mature) white cells may rapidly multiply. For additional details, see the articles White Blood Cell Count and WBC Differential.

Platelets

Platelets, also called thrombocytes, are actually tiny cell fragments that circulate in blood and are essential for normal blood clotting. When there is an injury and bleeding begins, platelets help stop bleeding by adhering to the injury site and clumping together to form a temporary plug. They also release chemical signals that attract and promote clumping of additional platelets and eventually become part of a stable blood clot at the site of the injury that remains in place until the injury heals.

If you have a disease or condition that causes low platelets (thrombocytopenia) or dysfunction of platelets, you may be at an increased risk of excessive bleeding and bruising. An excess of platelets (thrombocytosis) can cause excessive clotting. For more information, see the article Platelet Count.

Carcinoembryonic antigen (CEA) is a protein that is present in certain tissues of a developing baby (fetus). By the time a baby is born, it drops to a very low level. In adults, CEA is normally present at very low levels in the blood but may be elevated with certain types of cancer. This test measures the amount of CEA in the blood to help evaluate individuals diagnosed with cancer.

CEA is a tumor marker. Originally, it was thought that CEA was a specific marker for colon cancer, but further study has shown that an increase in CEA may be seen in a wide variety of other cancers. CEA can also be increased in some non-cancer-related conditions, such as inflammation, cirrhosis, peptic ulcer, ulcerative colitis, rectal polyps, emphysema, and benign breast disease, and in smokers. For this reason, it is not useful as a general cancer screening tool, but it does have a role in evaluating response to cancer treatment. When you have been diagnosed with cancer, an initial baseline test for CEA may be performed. If this level is elevated, then subsequent serial testing of CEA may be performed to monitor the cancer during treatment.

Cerebrospinal fluid (CSF) is a clear, watery liquid that flows around the brain and spinal cord, surrounding and protecting them. CSF testing is performed to evaluate the level or concentration of different substances and cells in CSF in order to diagnose conditions affecting the brain and spinal cord (central nervous system).

CSF is produced and secreted by the choroid plexus, a special tissue that has many blood vessels and that lines the small cavities or chambers (ventricles) in the brain. The total CSF volume is 3-5 ounces (90-150 mL) in adults and 0.3-2 ounces (10-60 mL) in newborns. CSF is continually produced, circulated, and then absorbed into the blood. About 17 ounces (500 mL) of CSF are produced each day. This rate of production means that all the CSF is replaced every few hours.

A protective, semi-permeable barrier separates the brain from the bloodstream. This blood-brain barrier allows some substances to cross and prevents other substances from crossing. Importantly, it helps keep large molecules, toxins, and most blood cells away from the central nervous system. Any condition that disrupts this protective barrier may result in a change in the normal level or makeup of CSF. Because CSF surrounds the brain and spinal cord, testing a sample of CSF can be very valuable in diagnosing a variety of conditions affecting the central nervous system.

Although a sample of CSF may be more difficult to obtain than, for example, urine or blood, the results of CSF testing may reveal more directly the cause of central nervous system conditions. The following are some examples:

• Meningitis, an infection of the layers that cover the brain and spinal cord (meninges), and encephalitis, an infection in the brain
• Autoimmune diseases that affect the central nervous system, such as multiple sclerosis
• Cancers of the central nervous system or cancers that have spread to the central nervous system, such as leukemia
• Alzheimer disease, an irreversible form of dementia

Ceruloplasmin is a copper-containing enzyme that plays a role in the body’s iron metabolism. This test measures the amount of ceruloplasmin in the blood.

Copper is an essential mineral that plays a role in the regulation of iron metabolism, formation of connective tissue, energy production within cells, and the function of the nervous system. It is absorbed from food and liquids by the intestines and then transported to the liver, where it is stored or used to produce a variety of enzymes.

The liver binds copper to a protein to produce ceruloplasmin and then releases it into the bloodstream. About 95% of the copper in the blood is bound to ceruloplasmin. Because of this, the ceruloplasmin test can be used along with one or more copper tests to help diagnose Wilson disease, an inherited disorder that can lead to excess storage of copper in the eyes, liver, brain, and other organs.

A Pap smear (Pap test) is primarily a screening test for cervical cancer. It is used to detect abnormal or potentially abnormal cells from the vagina and the cervix, the narrow bottom portion of a woman’s uterus.

Various bacterial, fungal, and viral infections of the uterus may also be detected using this test.

Cervical cancer is caused by the uncontrolled growth of cells in the cervix. Almost all cervical cancers are caused by persistent infections with high-risk types of human papillomavirus, also called HPV (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 66 and 68). Two HPV types,16 and 18, cause 80% of all cervical cancers.

HPV is a very common infection and may be spread by sexual contact (sexually transmitted disease). Many HPV infections resolve without treatment—the body is able to clear the infection—but infections with high-risk HPV types that do not go away can lead to cervical cancer. It can take many years for an HPV infection to develop into cancer.

An HPV infection can cause changes in the cells lining the inside or outside of the cervix. However, these changes can also occur temporarily in response to infections or irritation of the cervix lining and may not be related to cancer. On a Pap smear, the cells with these non-specific changes are reported as “atypical.” Further testing may be necessary to determine the significance of the atypical cells. If they are precancerous, they can become more abnormal over time and may progress to cancer if left untreated.

Pap smears, when performed routinely, improve the detection and treatment of precancerous cells, which helps to prevent cervical cancer from developing. In addition, the test can help detect cervical cancer in the early stages, when it is most treatable.

A Pap smear may be used either alone or along with an HPV molecular test for cervical cancer screening every 5 years for women 30 to 65 years of age. HPV tests are not recommended before age 30 because HPV infections are common in younger, sexually active women and usually resolve without treatment. However, if a woman between the ages of 21 through 29 has an abnormal Pap smear, then HPV testing may be done.

A Pap smear (Pap test) is primarily a screening test for cervical cancer. It is used to detect abnormal or potentially abnormal cells from the vagina and the cervix, the narrow bottom portion of a woman’s uterus.

Various bacterial, fungal, and viral infections of the uterus may also be detected using this test.

Cervical cancer is caused by the uncontrolled growth of cells in the cervix. Almost all cervical cancers are caused by persistent infections with high-risk types of human papillomavirus, also called HPV (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 66 and 68). Two HPV types,16 and 18, cause 80% of all cervical cancers.

HPV is a very common infection and may be spread by sexual contact (sexually transmitted disease). Many HPV infections resolve without treatment—the body is able to clear the infection—but infections with high-risk HPV types that do not go away can lead to cervical cancer. It can take many years for an HPV infection to develop into cancer.

An HPV infection can cause changes in the cells lining the inside or outside of the cervix. However, these changes can also occur temporarily in response to infections or irritation of the cervix lining and may not be related to cancer. On a Pap smear, the cells with these non-specific changes are reported as “atypical.” Further testing may be necessary to determine the significance of the atypical cells. If they are precancerous, they can become more abnormal over time and may progress to cancer if left untreated.

Pap smears, when performed routinely, improve the detection and treatment of precancerous cells, which helps to prevent cervical cancer from developing. In addition, the test can help detect cervical cancer in the early stages, when it is most treatable.

A Pap smear may be used either alone or along with an HPV molecular test for cervical cancer screening every 5 years for women 30 to 65 years of age. HPV tests are not recommended before age 30 because HPV infections are common in younger, sexually active women and usually resolve without treatment. However, if a woman between the ages of 21 through 29 has an abnormal Pap smear, then HPV testing may be done.

Chlamydia is one of the most common bacterial sexually transmitted diseases (STD) in the United States and can cause serious complications if not treated. Chlamydia testing identifies the bacteria Chlamydia trachomatis as the cause of your infection.

The preferred method for chlamydia testing is the nucleic acid amplification test (NAAT) that detects the genetic material (DNA) of Chlamydia trachomatis. It is generally more sensitive and specific than other chlamydia tests and can be performed on a vaginal swab on women or urine from both men and women, which eliminates the need for a pelvic exam in women.

Screening for, diagnosing, and treating chlamydia is very important in preventing long-term complications and spread of the infection to others. Chlamydia infections are especially common among people 15 to 24 years of age. The Centers for Disease Control and Prevention (CDC) estimates that 2.86 million Americans are infected with chlamydia each year and notes that women are frequently re-infected if their partners don’t get treatment. The actual number of cases may be higher since many people do not experience any symptoms and do not get tested and diagnosed. Still, over one million new cases are reported each year.

Chlamydia is generally spread through sexual contact (oral, vaginal, or anal) with an infected partner. Risk factors include having multiple sex partners, infection with another STD at the same time or previous STD infection, and not using a condom correctly and consistently.

Many people with chlamydia infections have no symptoms and some may experience only mild symptoms. Signs and symptoms of chlamydia are similar to and can be confused with those cause by another STD, gonorrhea, so tests for these infections are often done at the same time.

Chlamydia is easily treated with a course of antibiotics. If not diagnosed and treated, it can cause severe reproductive and other health problems.

In women, untreated chlamydia infections can lead to pelvic inflammatory disease (PID) from infections that start on the cervix but spread to the fallopian tubes and ovaries. This can cause:

• Long-term (chronic) pelvic pain
• Infertility
• An increased risk of tubal (ectopic) pregnancy, which can be fatal

If you are infected while pregnant, you may experience premature labor, premature rupture of the membranes and your baby may be born with a low birth weight. You can pass the infection to your baby during childbirth. Infected babies are at risk of developing complications such as pneumonia or conjunctivitis, an inflammation that, left untreated, can threaten eyesight.

Rarely, men who are not treated may experience infertility.

Chlamydia is one of the most common bacterial sexually transmitted diseases (STD) in the United States and can cause serious complications if not treated. Chlamydia testing identifies the bacteria Chlamydia trachomatis as the cause of your infection.

The preferred method for chlamydia testing is the nucleic acid amplification test (NAAT) that detects the genetic material (DNA) of Chlamydia trachomatis. It is generally more sensitive and specific than other chlamydia tests and can be performed on a vaginal swab on women or urine from both men and women, which eliminates the need for a pelvic exam in women.

Screening for, diagnosing, and treating chlamydia is very important in preventing long-term complications and spread of the infection to others. Chlamydia infections are especially common among people 15 to 24 years of age. The Centers for Disease Control and Prevention (CDC) estimates that 2.86 million Americans are infected with chlamydia each year and notes that women are frequently re-infected if their partners don’t get treatment. The actual number of cases may be higher since many people do not experience any symptoms and do not get tested and diagnosed. Still, over one million new cases are reported each year.

Chlamydia is generally spread through sexual contact (oral, vaginal, or anal) with an infected partner. Risk factors include having multiple sex partners, infection with another STD at the same time or previous STD infection, and not using a condom correctly and consistently.

Many people with chlamydia infections have no symptoms and some may experience only mild symptoms. Signs and symptoms of chlamydia are similar to and can be confused with those cause by another STD, gonorrhea, so tests for these infections are often done at the same time.

Chlamydia is easily treated with a course of antibiotics. If not diagnosed and treated, it can cause severe reproductive and other health problems.

In women, untreated chlamydia infections can lead to pelvic inflammatory disease (PID) from infections that start on the cervix but spread to the fallopian tubes and ovaries. This can cause:

• Long-term (chronic) pelvic pain
• Infertility
• An increased risk of tubal (ectopic) pregnancy, which can be fatal

If you are infected while pregnant, you may experience premature labor, premature rupture of the membranes and your baby may be born with a low birth weight. You can pass the infection to your baby during childbirth. Infected babies are at risk of developing complications such as pneumonia or conjunctivitis, an inflammation that, left untreated, can threaten eyesight.

Rarely, men who are not treated may experience infertility.

Chlamydia is one of the most common bacterial sexually transmitted diseases (STD) in the United States and can cause serious complications if not treated. Chlamydia testing identifies the bacteria Chlamydia trachomatis as the cause of your infection.

The preferred method for chlamydia testing is the nucleic acid amplification test (NAAT) that detects the genetic material (DNA) of Chlamydia trachomatis. It is generally more sensitive and specific than other chlamydia tests and can be performed on a vaginal swab on women or urine from both men and women, which eliminates the need for a pelvic exam in women.

Screening for, diagnosing, and treating chlamydia is very important in preventing long-term complications and spread of the infection to others. Chlamydia infections are especially common among people 15 to 24 years of age. The Centers for Disease Control and Prevention (CDC) estimates that 2.86 million Americans are infected with chlamydia each year and notes that women are frequently re-infected if their partners don’t get treatment. The actual number of cases may be higher since many people do not experience any symptoms and do not get tested and diagnosed. Still, over one million new cases are reported each year.

Chlamydia is generally spread through sexual contact (oral, vaginal, or anal) with an infected partner. Risk factors include having multiple sex partners, infection with another STD at the same time or previous STD infection, and not using a condom correctly and consistently.

Many people with chlamydia infections have no symptoms and some may experience only mild symptoms. Signs and symptoms of chlamydia are similar to and can be confused with those cause by another STD, gonorrhea, so tests for these infections are often done at the same time.

Chlamydia is easily treated with a course of antibiotics. If not diagnosed and treated, it can cause severe reproductive and other health problems.

In women, untreated chlamydia infections can lead to pelvic inflammatory disease (PID) from infections that start on the cervix but spread to the fallopian tubes and ovaries. This can cause:

• Long-term (chronic) pelvic pain
• Infertility
• An increased risk of tubal (ectopic) pregnancy, which can be fatal

If you are infected while pregnant, you may experience premature labor, premature rupture of the membranes and your baby may be born with a low birth weight. You can pass the infection to your baby during childbirth. Infected babies are at risk of developing complications such as pneumonia or conjunctivitis, an inflammation that, left untreated, can threaten eyesight.

Rarely, men who are not treated may experience infertility.

Chloride is an electrolyte. It is a negatively charged ion that works with other electrolytes, such as potassium, sodium, and bicarbonate, to help regulate the amount of fluid in the body and maintain the acid-base balance. This test measures the level of chloride in the blood and/or urine.

Chloride is present in all body fluids but is found in the highest concentration in the blood and in the fluid outside of the body’s cells. Most of the time, chloride concentrations mirror those of sodium, increasing and decreasing for the same reasons and in direct relationship to sodium. When there is an acid-base imbalance, however, blood chloride levels can change independently of sodium levels as chloride acts as a buffer. It helps to maintain electrical neutrality at the cellular level by moving into or out of the cells as needed.

We get chloride in our diet through food and table salt, which is made up of sodium and chloride ions. Most of the chloride is absorbed by the digestive tract, and the excess is eliminated in urine. The normal blood level remains steady, with a slight drop after meals (because the stomach produces acid after eating, using chloride from blood).

Cholesterol is a substance (a steroid) that is essential for life. It forms the membranes for cells in all organs and tissues in the body. It is used to make hormones that are essential for development, growth, and reproduction. It forms bile acids that are needed to absorb nutrients from food. The test for cholesterol measures total cholesterol that is carried in the blood by lipoproteins.

A small amount of cholesterol circulates in the blood in complex particles called lipoproteins. Each particle contains a combination of protein, cholesterol, triglyceride, and phospholipid molecules and the particles are classified by their density into high-density lipoproteins (HDL), low-density lipoproteins (LDL), and very low-density lipoproteins (VLDL). HDL-C particles, sometimes called “good” cholesterol, carry excess cholesterol away for disposal and LDL-C particles, or “bad” cholesterol, deposit cholesterol in tissues and organs.

Monitoring and maintaining healthy levels of cholesterol is important for staying healthy. The body produces the cholesterol needed to work properly, but the source for some cholesterol is diet. If an individual has an inherited predisposition for high cholesterol levels or eats too much of the foods that are high in saturated fats and trans unsaturated fats (trans fats), then the level of cholesterol in that person’s blood may increase and have a negative impact on the person’s health. The extra cholesterol in the blood may be deposited in plaques on the walls of blood vessels. Plaques can narrow or eventually block the opening of blood vessels, leading to hardening of the arteries (atherosclerosis) and increased risk of numerous health problems, including heart disease and stroke.

Creatine kinase (CK) is an enzyme found in the heart, brain, skeletal muscle, and other tissues. Increased amounts of CK are released into the blood when there is muscle damage. This test measures the amount of creatine kinase in the blood.

The small amount of CK that is normally in the blood comes primarily from skeletal muscles. Any condition that causes muscle damage and/or interferes with muscle energy production or use can cause an increase in CK. For example, strenuous exercise and inflammation of muscles, called myositis, can increase CK as can muscle diseases (myopathies) such as muscular dystrophy. Rhabdomyolysis, an extreme breakdown of skeletal muscle tissue, is associated with significantly elevated levels of CK.

Creatine kinase-MB (CK-MB) is a form of an enzyme found primarily in heart muscle cells. This test measures CK-MB in the blood.

CK-MB is one of three forms (isoenzymes) of the enzyme creatine kinase (CK). These isoenzymes include:

• CK-MM (found in skeletal muscles and the heart)
• CK-MB (found mostly in the heart, but small amounts found in skeletal muscles)
• CK-BB (found mostly in the brain and smooth muscle, such as the intestines and uterus)

CK is released from muscle cells and is detectable in the blood whenever there is muscle damage. The small amount of CK that is normally in the blood is primarily CK-MM. CK-BB almost never gets into the blood, and CK-MB will typically only be present in significant amounts when the heart is damaged. A CK test measures the total level but does not distinguish between the three isoenzymes. When there is an increased amount of CK present in the blood, the CK-MB test can be used to determine whether it is due to heart damage or is more likely to be related to skeletal muscle injury.

Creatine kinase-MB (CK-MB) is a form of an enzyme found primarily in heart muscle cells. This test measures CK-MB in the blood.

CK-MB is one of three forms (isoenzymes) of the enzyme creatine kinase (CK). These isoenzymes include:

• CK-MM (found in skeletal muscles and the heart)
• CK-MB (found mostly in the heart, but small amounts found in skeletal muscles)
• CK-BB (found mostly in the brain and smooth muscle, such as the intestines and uterus)

CK is released from muscle cells and is detectable in the blood whenever there is muscle damage. The small amount of CK that is normally in the blood is primarily CK-MM. CK-BB almost never gets into the blood, and CK-MB will typically only be present in significant amounts when the heart is damaged. A CK test measures the total level but does not distinguish between the three isoenzymes. When there is an increased amount of CK present in the blood, the CK-MB test can be used to determine whether it is due to heart damage or is more likely to be related to skeletal muscle injury.

Chloride is an electrolyte. It is a negatively charged ion that works with other electrolytes, such as potassium, sodium, and bicarbonate, to help regulate the amount of fluid in the body and maintain the acid-base balance. This test measures the level of chloride in the blood and/or urine.

Chloride is present in all body fluids but is found in the highest concentration in the blood and in the fluid outside of the body’s cells. Most of the time, chloride concentrations mirror those of sodium, increasing and decreasing for the same reasons and in direct relationship to sodium. When there is an acid-base imbalance, however, blood chloride levels can change independently of sodium levels as chloride acts as a buffer. It helps to maintain electrical neutrality at the cellular level by moving into or out of the cells as needed.

We get chloride in our diet through food and table salt, which is made up of sodium and chloride ions. Most of the chloride is absorbed by the digestive tract, and the excess is eliminated in urine. The normal blood level remains steady, with a slight drop after meals (because the stomach produces acid after eating, using chloride from blood).

Clostridium difficile (commonly called C. difficile or C. diff) is a type of bacteria that is associated with diarrhea resulting from antibiotic use. C. difficile testing and C. difficile toxin tests identify the presence of these bacteria, genes associated with toxin production, and/or detect the toxins produced by them.

Clostridium difficile has been recently reclassified and renamed as Clostridioides difficile, but since many people still use the former name, it will be used for the purposes of this article.

C. difficile may be present as part of the normal bacterial flora in the digestive tract of up to 65% of healthy infants and 3% of healthy adults. Sometimes, when broad-spectrum antibiotics are used to treat other infections, typically for an extended period, the balance of the normal flora in the digestive tract is disrupted. Normal bacterial flora that are susceptible to the antibiotic are eliminated from the digestive tract, while C. difficile that are resistant to the antibiotic remain and begin to overgrow, or new types (strains) of C. difficile are acquired.

C. difficile usually produced two toxins: toxin A and toxin B. The resulting combination of decreased normal flora, overgrowth of C. difficile, and toxin production can damage the lining of the lower portion of the digestive tract (colon, bowel) and lead to severe inflammation of the colon and prolonged diarrhea. Dead tissue, fibrin, and numerous white blood cells can form a lining over the surface of the inflamed bowel (a pseudomembrane), a condition that is referred to as pseudomembranous colitis.

C. difficile infection is the most common cause of diarrhea in people who develop diarrheal symptoms while hospitalized. C. difficile toxin is detected in the stools of up to 20-30% of those with antibiotic-associated diarrhea and greater than 95% of those with pseudomembranous colitis. While the organism is frequently carried by infants, it does not usually cause diarrhea in this population.

The risk of having symptoms increases with age and increases in those who have weakened immune systems, have acute or chronic colon conditions, have been previously affected by C. difficile, or who have had recent gastrointestinal surgery or chemotherapy. C. difficile-associated diarrhea usually occurs in people who have been taking antibiotics for several days, but it can also occur several weeks after treatment is completed.

C. difficile-associated disease is a spectrum of illness ranging from mild diarrhea to a more severe colitis, or to toxic megacolon or perforated bowel, which can result in sepsis and death. Signs and symptoms may include frequent loose stools, abdominal pain and cramps, nausea, fever, dehydration, fatigue, and high white blood cell count (leukocytosis). Treatment typically involves discontinuing use of the original antibiotic and administering specific oral antibiotic therapy targeting C. difficile. Most people improve as the normal bacterial flora re-colonize the gastrointestinal tract, but about 12-24% of those affected may have a second episode within two months.

For a description of tests used to identify C. difficile and toxins, see the “How is it used?” section.

Coagulation factors are proteins circulating in the blood that are essential for proper blood clot formation. Coagulation factor tests measure the function of or sometimes the amount of these proteins in the blood.

Blood clotting is a complex process that involves numerous coagulation factors, which are produced by the liver and blood vessels. Each coagulation factor is evaluated with one or more tests. When factor levels are low, it can cause blood clotting to fail, leading to unexplained bleeding episodes. Measuring coagulation factors can help a healthcare practitioner determine the cause of the bleeding and the best treatment.

Coagulation factors are usually tested by measuring the factor’s activity level in the blood. Activity assays can detect reduced levels of protein or proteins that don’t function properly. Rarely, the amount (antigen level) of a coagulation factor may also be measured. Coagulation factor antigen tests can tell how much of the protein is present, but not whether its function is normal.

When someone bleeds (e.g., with an injury), the coagulation system is activated, plugging the leaking blood vessel with a clot. The coagulation system consists of a series of coagulation factors that activate in a step-by-step process called the coagulation cascade. The end result is the formation of insoluble fibrin threads that link together at the site of injury, along with aggregated cell fragments called platelets, to form a stable blood clot. The clot prevents additional blood loss and remains in place until the injured area has healed.

Blood clotting is dynamic; once a clot is formed, other factors are activated that slow clotting or dissolve the clot in a process called fibrinolysis. The clot is eventually removed after the injury site heals. In normal healthy individuals, this balance between clot formation and removal ensures that bleeding does not become excessive and that clots are removed once they are no longer needed.

For people with bleeding disorders, clotting does not work properly because they lack platelets or coagulation factors, or their platelets or factors don’t work properly. There are a variety of bleeding disorders that may be passed through families (inherited) or acquired after birth. If a person has signs and symptoms of one of these disorders, coagulation factor testing may be ordered to help determine the diagnosis and treatment.

There are nine coagulation factor proteins that can be measured clinically (see table below). These factors are referred to by a name or Roman numeral or both in some cases. For example, coagulation factor II is also known as prothrombin. When one or more of these factors are produced in too small a quantity, or are not functioning correctly, they can cause excessive bleeding.

Cytomegalovirus (CMV) is a common virus that usually causes no symptoms or only mild illness. CMV testing detects antibodies in the blood that the body produces in response to the infection or detects CMV directly.

In the United States, as many as 60% of people have been exposed to CMV at some point in their life. Almost 1 out of every 3 children have been exposed to CMV by age 5 and more than half of adults are exposed to CMV by age 40.

Most people are infected as children or as young adults, but many don’t know it because CMV usually does not cause noticeable symptoms or only mild illness in otherwise healthy people. Individuals with mild illness may have non-specific signs and symptoms, such as sore throat, fever, tiredness, and swollen glands. In otherwise healthy adults, CMV infection may sometimes cause a flu-like illness or signs and symptoms similar to mononucleosis (mono), such as extreme fatigue, fever, chills, body aches, and/or headaches that usually resolve within a few weeks.

CMV is found in many body fluids during an active infection, including saliva, urine, blood, breast milk, semen, vaginal fluid, and cerebrospinal fluid. CMV easily spreads from person to person through close contact with someone who is infected or by contact with contaminated body fluids or objects, such as diapers or toys.

Once you are infected, CMV remains in your body for the rest of your life without causing symptoms. After your initial “primary” infection resolves, CMV becomes dormant or latent, like other members of the herpes family. However, if your immune system is significantly weakened, the virus can become active again (reactivate) and cause illness.

CMV can cause notable health problems in these situations:

• A pregnant woman who is infected for the first time (primary infection) during pregnancy can pass the infection to her developing baby across the placenta. This can cause serious physical and developmental problems in the baby. Most newborns (about 90%) who are infected appear healthy at birth but may develop hearing or vision problems, pneumonia, seizures, and/or delayed mental development a few months later. A few babies may be stillborn, while others may have symptoms at birth such as jaundice, anemia, an enlarged spleen or liver, and a small head.
• CMV can cause serious illness and death in people with weakened immune systems, such as those with HIV/AIDS, solid organ or bone marrow transplant recipients, and cancer patients receiving chemotherapy. These individuals might experience the most severe symptoms and their CMV infection may remain active. CMV may reactivate in those who were previously exposed. The infection could affect the:
  • Eyes, causing inflammation of the retina, which can lead to blindness
  • Digestive tract, causing bloody diarrhea and abdominal pain
  • Lungs, causing pneumonia with a non-productive cough and shortness of breath
  • Brain, causing encephalitis
  • Spleen and liver
  • Organ or bone marrow transplants, causing some degree of rejection

Active CMV also further depresses the immune system, allowing other secondary infections such as fungal infections, to occur.

Cytomegalovirus (CMV) is a common virus that usually causes no symptoms or only mild illness. CMV testing detects antibodies in the blood that the body produces in response to the infection or detects CMV directly.

In the United States, as many as 60% of people have been exposed to CMV at some point in their life. Almost 1 out of every 3 children have been exposed to CMV by age 5 and more than half of adults are exposed to CMV by age 40.

Most people are infected as children or as young adults, but many don’t know it because CMV usually does not cause noticeable symptoms or only mild illness in otherwise healthy people. Individuals with mild illness may have non-specific signs and symptoms, such as sore throat, fever, tiredness, and swollen glands. In otherwise healthy adults, CMV infection may sometimes cause a flu-like illness or signs and symptoms similar to mononucleosis (mono), such as extreme fatigue, fever, chills, body aches, and/or headaches that usually resolve within a few weeks.

CMV is found in many body fluids during an active infection, including saliva, urine, blood, breast milk, semen, vaginal fluid, and cerebrospinal fluid. CMV easily spreads from person to person through close contact with someone who is infected or by contact with contaminated body fluids or objects, such as diapers or toys.

Once you are infected, CMV remains in your body for the rest of your life without causing symptoms. After your initial “primary” infection resolves, CMV becomes dormant or latent, like other members of the herpes family. However, if your immune system is significantly weakened, the virus can become active again (reactivate) and cause illness.

CMV can cause notable health problems in these situations:

• A pregnant woman who is infected for the first time (primary infection) during pregnancy can pass the infection to her developing baby across the placenta. This can cause serious physical and developmental problems in the baby. Most newborns (about 90%) who are infected appear healthy at birth but may develop hearing or vision problems, pneumonia, seizures, and/or delayed mental development a few months later. A few babies may be stillborn, while others may have symptoms at birth such as jaundice, anemia, an enlarged spleen or liver, and a small head.
• CMV can cause serious illness and death in people with weakened immune systems, such as those with HIV/AIDS, solid organ or bone marrow transplant recipients, and cancer patients receiving chemotherapy. These individuals might experience the most severe symptoms and their CMV infection may remain active. CMV may reactivate in those who were previously exposed. The infection could affect the:
  • Eyes, causing inflammation of the retina, which can lead to blindness
  • Digestive tract, causing bloody diarrhea and abdominal pain
  • Lungs, causing pneumonia with a non-productive cough and shortness of breath
  • Brain, causing encephalitis
  • Spleen and liver
  • Organ or bone marrow transplants, causing some degree of rejection

Active CMV also further depresses the immune system, allowing other secondary infections such as fungal infections, to occur.

Coagulation factors are proteins circulating in the blood that are essential for proper blood clot formation. Coagulation factor tests measure the function of or sometimes the amount of these proteins in the blood.

Blood clotting is a complex process that involves numerous coagulation factors, which are produced by the liver and blood vessels. Each coagulation factor is evaluated with one or more tests. When factor levels are low, it can cause blood clotting to fail, leading to unexplained bleeding episodes. Measuring coagulation factors can help a healthcare practitioner determine the cause of the bleeding and the best treatment.

Coagulation factors are usually tested by measuring the factor’s activity level in the blood. Activity assays can detect reduced levels of protein or proteins that don’t function properly. Rarely, the amount (antigen level) of a coagulation factor may also be measured. Coagulation factor antigen tests can tell how much of the protein is present, but not whether its function is normal.

When someone bleeds (e.g., with an injury), the coagulation system is activated, plugging the leaking blood vessel with a clot. The coagulation system consists of a series of coagulation factors that activate in a step-by-step process called the coagulation cascade. The end result is the formation of insoluble fibrin threads that link together at the site of injury, along with aggregated cell fragments called platelets, to form a stable blood clot. The clot prevents additional blood loss and remains in place until the injured area has healed.

Blood clotting is dynamic; once a clot is formed, other factors are activated that slow clotting or dissolve the clot in a process called fibrinolysis. The clot is eventually removed after the injury site heals. In normal healthy individuals, this balance between clot formation and removal ensures that bleeding does not become excessive and that clots are removed once they are no longer needed.

For people with bleeding disorders, clotting does not work properly because they lack platelets or coagulation factors, or their platelets or factors don’t work properly. There are a variety of bleeding disorders that may be passed through families (inherited) or acquired after birth. If a person has signs and symptoms of one of these disorders, coagulation factor testing may be ordered to help determine the diagnosis and treatment.

There are nine coagulation factor proteins that can be measured clinically (see table below). These factors are referred to by a name or Roman numeral or both in some cases. For example, coagulation factor II is also known as prothrombin. When one or more of these factors are produced in too small a quantity, or are not functioning correctly, they can cause excessive bleeding.

Cold agglutinins are autoantibodies produced by a person’s immune system that mistakenly target red blood cells (RBCs). They cause RBCs to clump together when a person is exposed to cold temperatures and increase the likelihood that the affected RBCs will be destroyed by the body. This test detects and measures the amount of cold agglutinins in the blood.

When the presence of cold agglutinins in a person’s blood leads to significant RBC destruction, it can cause hemolytic anemia and lead to a low RBC count and hemoglobin. This rare form of autoimmune hemolytic anemia is known as cold agglutinin disease. Cold agglutinin disease may be primary or secondary, induced by some other disease or condition.

Primary cold agglutinin disease typically affects those who are middle age to elderly, and it tends to continue over time (chronic). Secondary cold agglutinin disease may affect anyone and may be acute or chronic, temporary or persistent. It may cause hemolytic anemia to a greater or lesser degree and is associated with a variety of conditions. For details see the “What does the test result mean?” section in Common Questions below.

The cold agglutinin test is not routinely ordered. It is a test that has been available for a long time, but it has become less commonly used as more specific tests for secondary causes, such as Mycoplasma pneumoniae infection, have become available.

The complement system consists of almost 60 proteins, approximately 30 of which are circulating blood proteins that work together to promote immune and inflammatory responses. Complement tests measure the amount or activity of complement proteins in the blood.

The complement system’s principal role is to help identify, destroy and remove foreign pathogens like bacteria and viruses, as well as damaged “self” materials (e.g., cells and proteins). The complement system is so named because it “complements” or aids the natural body defenses, such as antibodies. So, the complement system is also activated when the body makes antibodies, whether against itself or foreign invaders. The body makes antibodies against its own tissues that it thinks are foreign (autoantibodies) in various autoimmune diseases.

The complement system is part of the body’s innate immune system. Unlike the acquired immune system, which produces antibodies that target and protect against specific threats, the innate immune system is non-specific and can quickly respond to foreign substances. It does not require previous exposure to an invading microbe or offending substance and does not maintain a memory of previous encounters.

There are nine primary complement proteins that are designated C1 through C9. These components, in addition to the remaining proteins, work together in a cascade-like approach by activating, amplifying, breaking apart, and forming protein complexes that respond to infections, “non-self” tissues (e.g., transplants), dead cell debris (e.g., from apoptosis), or inflammation.

The complement cascade consists of 3 separate pathways that may be activated to converge in a final common pathway. The pathways include the “classical pathway” (including components C1qrs, C2, C4), the “alternative pathway” (including components C3, factor B, properdin), and the “lectin pathway” (a.k.a. mannan-binding lectin [MBL]). The end result of all three activation pathways is the same – the formation of the membrane attack complex (MAC). Complement activation causes several things to happen (“complement cascade”):

The MAC binds to the surface of each microbe, abnormal cell, or substance that has been targeted for destruction. It creates a lesion (hole) in the membrane wall and causes lysis, which is destruction of the cell by letting the contents out – like piercing a water-filled balloon.
It increases the permeability of blood vessels, allowing infection-fighting white blood cells (WBCs) to move out of the bloodstream and into tissues.
It attracts WBCs to the site of the infection.
It stimulates phagocytosis, a process in which microbes are engulfed by macrophages and neutrophils and killed.
It also labels immune complexes with complement components so that WBCs will engulf them and clear them out of the blood.
Complement tests measure the amount or the function (activity) of complement proteins in the blood. Complement components may be measured individually or together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity can be measured if a healthcare practitioner suspects a deficiency that is not measured by C3 or C4. The CH50 functional test measures the function of the complete classical complement pathway, mediated by components C1 – C9. If this measurement is outside the normal range, then each of the nine different complement levels can be measured individually to look for hereditary or acquired deficiencies.

The complete blood count (CBC) is a group of tests that evaluate the cells that circulate in blood, including red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs). The CBC can evaluate your overall health and detect a variety of diseases and conditions, such as infections, anemia and leukemia.

Blood cells are produced and mature primarily in the bone marrow and, under normal circumstances, are released into the bloodstream as needed. The three types of cells evaluated by the CBC include:

Red Blood Cells

Red blood cells, also called erythrocytes, are produced in the bone marrow and released into the bloodstream when they mature. They contain hemoglobin, a protein that transports oxygen throughout the body. The typical lifespan of an RBC is 120 days. Thus, the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect the production of new RBCs and/or their lifespan, in addition to those conditions that may result in significant bleeding.

RBCs normally are uniform in size and shape, but their appearance can be affected by a variety of conditions, such as vitamin B12 and folate deficiencies and iron deficiency. An example of a common condition affecting RBCs is anemia, which results from low red blood cell counts and low hemoglobin. Various diseases can lead to anemia, so additional tests are often needed to determine the cause. For more details, see the articles on Red Blood Cell Count, Hemoglobin, and Hematocrit.

White Blood Cells

White blood cells, also called leukocytes, are cells that exist in the blood, the lymphatic system, and tissues and are an important part of the body’s natural defense (immune) system. They help protect against infections and also have a role in inflammation, and allergic reactions. There are five different types of WBCs and each has a different function. They include neutrophils, lymphocytes, basophils, eosinophils, and monocytes.

WBCs are present in the blood at relatively stable numbers. However, these numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate your bone marrow to produce a higher number of neutrophils to fight off a bacterial infection. With allergies, there may be an increased number of eosinophils. An increased number of lymphocytes may be produced with a viral infection. In certain diseases, such as leukemia, abnormal (immature or mature) white cells may rapidly multiply. For additional details, see the articles White Blood Cell Count and WBC Differential.

Platelets

Platelets, also called thrombocytes, are actually tiny cell fragments that circulate in blood and are essential for normal blood clotting. When there is an injury and bleeding begins, platelets help stop bleeding by adhering to the injury site and clumping together to form a temporary plug. They also release chemical signals that attract and promote clumping of additional platelets and eventually become part of a stable blood clot at the site of the injury that remains in place until the injury heals.

If you have a disease or condition that causes low platelets (thrombocytopenia) or dysfunction of platelets, you may be at an increased risk of excessive bleeding and bruising. An excess of platelets (thrombocytosis) can cause excessive clotting. For more information, see the article Platelet Count.

What is being tested?
Comparing COVID-19 tests
Click to view or download
Coronavirus disease 2019 (COVID-19) is the name of the illness caused by the new strain of coronavirus called SARS-CoV-2. Diagnostic tests detect either the genetic material (RNA) of the virus or viral proteins (antigens) in a sample from the respiratory tract. COVID-19 serologic blood tests detect antibodies produced in response to the SARS-CoV-2infection.

SARS-CoV-2 is a new (novel) virus that first appeared in December 2019 and spread throughout the globe at an alarming rate, prompting the World Health Organization to declare the outbreak a pandemic and the U.S. Department of Health and Human Services to declare a public health emergency. As the pandemic continues, scientists continue to study the virus and learn more about COVID-19.

There are seven coronaviruses that are known to infect humans, and most cause mild to moderate respiratory symptoms. However, the disease resulting from SARS-CoV-2 infection is like MERS (Middle East respiratory syndrome) and SARS (severe acute respiratory syndrome) because it can cause more severe illness and, in some cases, lead to pneumonia and death.

Because SARS-CoV-2 is a novel virus, everyone is potentially susceptible to infection and, at this time, it is not known who may develop serious complications. Unlike seasonal influenza, which can also cause serious illness and death, there is no vaccine or specific treatment for SARS-CoV-2 infection yet.

About 1 to 2 weeks after infection, the body begins to produce antibodies to the virus, with the level gradually increasing over time. However, it is not yet known how long people continue to produce antibodies and whether the antibodies protect against re-infection, providing immunity. (For more general information on antibodies, including IgG, IgM and IgA, read the article on Immunoglobulins)

Some infected people may be asymptomatic or have no noticeable symptoms and yet be contagious, potentially spreading the virus to others (silent carriers). Many of those infected have no symptoms or mild to moderate illness and will recover within one to two weeks. If symptoms of COVID-19 develop, they typically appear within 2 to 14 days of exposure to the virus. Research suggests that the average time from first exposure to getting sick (incubation period) is about 5 days, and about 97% of people who develop symptoms will do so within 11 days.

The key symptoms of COVID-19 are coughing, shortness of breath or difficulty breathing. Additional symptoms may include fever, chills, repeated shaking with chills, muscle aches, headache, sore throat and loss of smell or taste. Symptoms may come and go, and there may be periods of time where someone with COVID-19 feels better. Children and babies generally have a milder illness but will often have the same symptoms as an adult.

The risk of serious disease increases with age and with having underlying health conditions, such as heart disease, lung disease, high blood pressure, diabetes or a weakened immune system. Some people with COVID-19 may develop pneumonia, a lung infection, and in severe cases, a ventilator may be needed to ensure enough oxygen. Although COVID-19 is primarily a respiratory infection, researchers are learning that the illness may affect other organs besides the lungs, such as the heart, brain, and kidneys. In the most severe cases, COVID-19 may lead to organ failure or death.

Tests for COVID-19

The initial signs and symptoms of COVID-19 are frequently difficult to distinguish from those of a common cold or of other respiratory illnesses, so testing is necessary to help diagnose a current or past infection.

Reverse Transcription Polymerase Chain Reaction (RT-PCR): Most tests to check for current SARS-CoV-2 infection rely on RT-PCR testing to detect the virus’s RNA in a respiratory tract sample from a patient. PCR is a laboratory method used for making a very large number of copies of short sections of DNA from a very small sample of DNA so that it can be detected. This process is called “amplifying” the DNA. (See the article on PCR for more details.) The reverse transcription step allows the viral RNA to be converted into DNA so that the PCR technique can be used.
Rapid Antigen Tests: These tests detect the viral proteins of SARS-CoV-2 in respiratory samples. The main advantages of antigen tests are that they can provide results in minutes, are simpler than RT-PCR tests to perform, and are sometimes used at the point of care, such as at a health clinic. However, they are not as sensitive as RT-PCR tests, so negative results do not rule out infection.
Blood Test for Antibodies (Total, IgG, IgM) to SARS-CoV-2 (Serology): These tests detect antibodies produced by the body’s immune system in response to SARS-CoV-2. COVID-19 serology tests can tell whether or not you have had the viral infection in the past. However, antibody tests are not the preferred tests to diagnose current infections. Antibodies don’t show up for about 1 to 2 weeks after you first become sick so antibody tests could miss some early infections. (For more general information on antibodies, also called immunoglobulins, see the article on Immunoglobulins.)
How is the sample collected for testing?

Proper collection of the appropriate samples is essential for accurate COVID-19 test results.

For RT-PCR or Antigen Testing: The preferred sample is a swab from the back of your nose. This is called a nasopharyngeal swab, or NP swab. It is collected by having you tip your head back and then a swab (like a long Q-tip with a small head) is gently inserted through one of the nostrils until resistance is met (about 2 inches). It is left in place for several seconds, then rotated several times and withdrawn. This is not painful, but it may be uncomfortable, cause your eyes to tear and provoke a coughing spell. CDC guidance says that other samples from the respiratory tract may be collected when it is not possible to collect an NP swab. These include a swab from the back of the throat (oropharyngeal swab) or a swab from the front of your nose (nostril). Sometimes a saliva sample may be collected by having the patient spit into a container.
For Antibody Testing: A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a fingertip and collecting a few drops of blood.

Coronavirus disease 2019 (COVID-19) is the name of the illness caused by the new strain of coronavirus called SARS-CoV-2. Diagnostic tests detect either the genetic material (RNA) of the virus or viral proteins (antigens) in a sample from the respiratory tract. COVID-19 serologic blood tests detect antibodies produced in response to the SARS-CoV-2infection.

SARS-CoV-2 is a new (novel) virus that first appeared in December 2019 and spread throughout the globe at an alarming rate, prompting the World Health Organization to declare the outbreak a pandemic and the U.S. Department of Health and Human Services to declare a public health emergency. As the pandemic continues, scientists continue to study the virus and learn more about COVID-19.

There are seven coronaviruses that are known to infect humans, and most cause mild to moderate respiratory symptoms. However, the disease resulting from SARS-CoV-2 infection is like MERS (Middle East respiratory syndrome) and SARS (severe acute respiratory syndrome) because it can cause more severe illness and, in some cases, lead to pneumonia and death.

Because SARS-CoV-2 is a novel virus, everyone is potentially susceptible to infection and, at this time, it is not known who may develop serious complications. Unlike seasonal influenza, which can also cause serious illness and death, there is no vaccine or specific treatment for SARS-CoV-2 infection yet.

About 1 to 2 weeks after infection, the body begins to produce antibodies to the virus, with the level gradually increasing over time. However, it is not yet known how long people continue to produce antibodies and whether the antibodies protect against re-infection, providing immunity. (For more general information on antibodies, including IgG, IgM and IgA, read the article on Immunoglobulins)

Some infected people may be asymptomatic or have no noticeable symptoms and yet be contagious, potentially spreading the virus to others (silent carriers). Many of those infected have no symptoms or mild to moderate illness and will recover within one to two weeks. If symptoms of COVID-19 develop, they typically appear within 2 to 14 days of exposure to the virus. Research suggests that the average time from first exposure to getting sick (incubation period) is about 5 days, and about 97% of people who develop symptoms will do so within 11 days.

The key symptoms of COVID-19 are coughing, shortness of breath or difficulty breathing. Additional symptoms may include fever, chills, repeated shaking with chills, muscle aches, headache, sore throat and loss of smell or taste. Symptoms may come and go, and there may be periods of time where someone with COVID-19 feels better. Children and babies generally have a milder illness but will often have the same symptoms as an adult.

The risk of serious disease increases with age and with having underlying health conditions, such as heart disease, lung disease, high blood pressure, diabetes or a weakened immune system. Some people with COVID-19 may develop pneumonia, a lung infection, and in severe cases, a ventilator may be needed to ensure enough oxygen. Although COVID-19 is primarily a respiratory infection, researchers are learning that the illness may affect other organs besides the lungs, such as the heart, brain, and kidneys. In the most severe cases, COVID-19 may lead to organ failure or death.

Tests for COVID-19

The initial signs and symptoms of COVID-19 are frequently difficult to distinguish from those of a common cold or of other respiratory illnesses, so testing is necessary to help diagnose a current or past infection.

• Reverse Transcription Polymerase Chain Reaction (RT-PCR): Most tests to check for current SARS-CoV-2 infection rely on RT-PCR testing to detect the virus’s RNA in a respiratory tract sample from a patient. PCR is a laboratory method used for making a very large number of copies of short sections of DNA from a very small sample of DNA so that it can be detected. This process is called “amplifying” the DNA. (See the article on PCR for more details.) The reverse transcription step allows the viral RNA to be converted into DNA so that the PCR technique can be used.
• Rapid Antigen Tests: These tests detect the viral proteins of SARS-CoV-2 in respiratory samples. The main advantages of antigen tests are that they can provide results in minutes, are simpler than RT-PCR tests to perform, and are sometimes used at the point of care, such as at a health clinic. However, they are not as sensitive as RT-PCR tests, so negative results do not rule out infection.
• Blood Test for Antibodies (Total, IgG, IgM) to SARS-CoV-2 (Serology): These tests detect antibodies produced by the body’s immune system in response to SARS-CoV-2. COVID-19 serology tests can tell whether or not you have had the viral infection in the past. However, antibody tests are not the preferred tests to diagnose current infections. Antibodies don’t show up for about 1 to 2 weeks after you first become sick so antibody tests could miss some early infections. (For more general information on antibodies, also called immunoglobulins, see the article on Immunoglobulins.)

How is the sample collected for testing?

Proper collection of the appropriate samples is essential for accurate COVID-19 test results.

• For RT-PCR or Antigen Testing: The preferred sample is a swab from the back of your nose. This is called a nasopharyngeal swab, or NP swab. It is collected by having you tip your head back and then a swab (like a long Q-tip with a small head) is gently inserted through one of the nostrils until resistance is met (about 2 inches). It is left in place for several seconds, then rotated several times and withdrawn. This is not painful, but it may be uncomfortable, cause your eyes to tear and provoke a coughing spell. CDC guidance says that other samples from the respiratory tract may be collected when it is not possible to collect an NP swab. These include a swab from the back of the throat (oropharyngeal swab) or a swab from the front of your nose (nostril). Sometimes a saliva sample may be collected by having the patient spit into a container.
• For Antibody Testing: A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a fingertip and collecting a few drops of blood.

Reticulocytes are newly produced, relatively immature red blood cells (RBCs). A reticulocyte count helps to determine the number and/or percentage of reticulocytes in the blood and is a reflection of recent bone marrow function or activity.

Red blood cells are produced in the bone marrow, where blood-forming (hematopoietic) stem cells differentiate and develop, eventually forming reticulocytes and finally becoming mature RBCs. Reticulocytes are visually, slightly larger than mature RBCs. Unlike most other cells in the body, mature RBCs have no nucleus, but reticulocytes still have some remnant genetic material (RNA). As reticulocytes mature, they lose the last residual RNA and most are fully developed within one day of being released from the bone marrow into the blood. The reticulocyte count or percentage is a good indicator of the ability of a person’s bone marrow to adequately produce red blood cells (erythropoiesis).

RBCs typically survive for about 120 days in circulation, and the bone marrow is continually producing new RBCs to replace those that age and degrade or are lost through bleeding. Normally, a stable number of RBCs is maintained in the blood through continual replacement of degraded or lost RBCs.

A variety of diseases and conditions can affect the production of new RBCs and/or their survival, in addition to those conditions that may result in significant bleeding. These conditions may lead to a rise or drop in the number of RBCs and may affect the reticulocyte count.

Higher than normal percentage of reticulocytes: Acute or chronic bleeding (hemorrhage) or increased RBC destruction (hemolysis) can lead to fewer RBCs in the blood, resulting in anemia. The body compensates for this loss or to treatment of deficiency anemias (such as iron deficiency anemia or pernicious anemia) by increasing the rate of RBC production and by releasing RBCs sooner into the blood, before they become more mature. When this happens, the number and percentage of reticulocytes in the blood increases until a sufficient number of RBCs replaces those that were lost or until the production capacity of the bone marrow is reached.

Lower than normal percentage of reticulocytes: Decreased RBC production may occur when the bone marrow is not functioning normally. This can result from a bone marrow disorder such as aplastic anemia. Diminished production can also be due to other factors, for example, cirrhosis of the liver, kidney disease, radiation or chemotherapy treatments for cancer, a low level of the hormone erythropoietin, or deficiencies in certain nutrients such as iron, vitamin B12 or folate. Decreased production leads to fewer RBCs in circulation, decreased hemoglobin and oxygen-carrying capacity, a lower hematocrit, and a reduced number of reticulocytes as old RBCs are removed from the blood but not fully replaced.

Occasionally, both the reticulocyte count and the RBC count will be increased because of excess RBC production by the bone marrow. This may be due to an increased production of erythropoietin, disorders that cause chronic overproduction of RBCs (polycythemia vera), and cigarette smoking.

Some drugs may increase or decrease reticulocyte counts.

Cortisol is a hormone that plays a role in the metabolism of proteins, lipids, and carbohydrates. It affects blood glucose levels, helps maintain blood pressure, and helps regulate the immune system. Most cortisol in the blood is bound to a protein; only a small percentage is “free” and biologically active. Free cortisol is secreted into the urine and is present in the saliva. This test measures the amount of cortisol in the blood, urine, or saliva.

The level of cortisol in the blood (as well as the urine and saliva) normally rises and falls in a “diurnal variation” pattern. It peaks early in the morning, then declines throughout the day, reaching its lowest level about midnight. This pattern can change when a person works irregular shifts (such as the night shift) and sleeps at different times of the day, and it can become disrupted when a disease or condition either limits or stimulates cortisol production.

Cortisol is produced and secreted by the adrenal glands, two triangular organs that sit on top of the kidneys. Production of the hormone is regulated by the hypothalamus in the brain and by the pituitary gland, a tiny organ located below the brain. When the blood cortisol level falls, the hypothalamus releases corticotropin-releasing hormone (CRH), which directs the pituitary gland to produce ACTH (adrenocorticotropic hormone). ACTH stimulates the adrenal glands to produce and release cortisol. In order for appropriate amounts of cortisol to be made, the hypothalamus, the pituitary, and the adrenal glands must be functioning properly.

The group of signs and symptoms that are seen with an abnormally high level of cortisol is called Cushing syndrome. Increased cortisol production may be seen with:

Administration of large amounts of glucocorticosteroid hormones (such as prednisone, prednisolone, or dexamethasone) to treat a variety of conditions, such as autoimmune disease and some tumors
ACTH-producing tumors, in the pituitary gland and/or in other parts of the body
Increased cortisol production by the adrenal glands, due to a tumor or due to excessive growth of adrenal tissues (hyperplasia)
Rarely, with tumors in various parts of the body that produce CRH
Decreased cortisol production may be seen with:

An underactive pituitary gland or a pituitary gland tumor that inhibits ACTH production; this is known as secondary adrenal insufficiency.
Underactive or damaged adrenal glands (adrenal insufficiency) that limit cortisol production; this is referred to as primary adrenal insufficiency and is also known as Addison disease.
After stopping treatment with glucocorticosteroid hormones, especially if stopped very quickly after a long period of use
How is the sample collected for testing?
Typically, blood will be drawn from a vein in the arm, but sometimes urine or saliva may be tested. Cortisol blood tests may be drawn at about 8 am, when cortisol should be at its peak, and again at about 4 pm, when the level should have dropped significantly.

Sometimes a resting sample will be obtained to measure cortisol when it should be at its lowest level (just before sleep); this is often done by measuring cortisol in saliva rather than blood to make it easier to obtain the sample. Saliva for cortisol testing is usually collected by inserting a swab into the mouth and waiting a few minutes while the swab becomes saturated with saliva. Obtaining more than one sample allows the health practitioner to evaluate the daily pattern of cortisol secretion (the diurnal variation).

Sometimes urine is tested for cortisol; this usually requires collecting all of the urine produced during a day and night (a 24-hour urine) but sometimes may be done on a single sample of urine collected in the morning.

Is any test preparation needed to ensure the quality of the sample?
Some test preparation may be needed. Follow any instructions that are given as far as timing of sample collection, resting, and/or any other specific pre-test preparation.

A saliva test requires special care in obtaining the sample. You may be instructed to refrain from eating, drinking, or brushing your teeth for a period of time (may be some time between 15 to 30 minutes) prior to the test. Follow any specific instructions that are provided.

A stimulation or suppression test requires that you have a baseline blood sample drawn and then a specified amount of drug is given. Subsequent blood samples are drawn at specified times.

Coronavirus disease 2019 (COVID-19) is the name of the illness caused by the new strain of coronavirus called SARS-CoV-2. Diagnostic tests detect either the genetic material (RNA) of the virus or viral proteins (antigens) in a sample from the respiratory tract. COVID-19 serologic blood tests detect antibodies produced in response to the SARS-CoV-2infection.

SARS-CoV-2 is a new (novel) virus that first appeared in December 2019 and spread throughout the globe at an alarming rate, prompting the World Health Organization to declare the outbreak a pandemic and the U.S. Department of Health and Human Services to declare a public health emergency. As the pandemic continues, scientists continue to study the virus and learn more about COVID-19.

There are seven coronaviruses that are known to infect humans, and most cause mild to moderate respiratory symptoms. However, the disease resulting from SARS-CoV-2 infection is like MERS (Middle East respiratory syndrome) and SARS (severe acute respiratory syndrome) because it can cause more severe illness and, in some cases, lead to pneumonia and death.

Because SARS-CoV-2 is a novel virus, everyone is potentially susceptible to infection and, at this time, it is not known who may develop serious complications. Unlike seasonal influenza, which can also cause serious illness and death, there is no vaccine or specific treatment for SARS-CoV-2 infection yet.

About 1 to 2 weeks after infection, the body begins to produce antibodies to the virus, with the level gradually increasing over time. However, it is not yet known how long people continue to produce antibodies and whether the antibodies protect against re-infection, providing immunity. (For more general information on antibodies, including IgG, IgM and IgA, read the article on Immunoglobulins)

Some infected people may be asymptomatic or have no noticeable symptoms and yet be contagious, potentially spreading the virus to others (silent carriers). Many of those infected have no symptoms or mild to moderate illness and will recover within one to two weeks. If symptoms of COVID-19 develop, they typically appear within 2 to 14 days of exposure to the virus. Research suggests that the average time from first exposure to getting sick (incubation period) is about 5 days, and about 97% of people who develop symptoms will do so within 11 days.

The key symptoms of COVID-19 are coughing, shortness of breath or difficulty breathing. Additional symptoms may include fever, chills, repeated shaking with chills, muscle aches, headache, sore throat and loss of smell or taste. Symptoms may come and go, and there may be periods of time where someone with COVID-19 feels better. Children and babies generally have a milder illness but will often have the same symptoms as an adult.

The risk of serious disease increases with age and with having underlying health conditions, such as heart disease, lung disease, high blood pressure, diabetes or a weakened immune system. Some people with COVID-19 may develop pneumonia, a lung infection, and in severe cases, a ventilator may be needed to ensure enough oxygen. Although COVID-19 is primarily a respiratory infection, researchers are learning that the illness may affect other organs besides the lungs, such as the heart, brain, and kidneys. In the most severe cases, COVID-19 may lead to organ failure or death.

Tests for COVID-19

The initial signs and symptoms of COVID-19 are frequently difficult to distinguish from those of a common cold or of other respiratory illnesses, so testing is necessary to help diagnose a current or past infection.

• Reverse Transcription Polymerase Chain Reaction (RT-PCR): Most tests to check for current SARS-CoV-2 infection rely on RT-PCR testing to detect the virus’s RNA in a respiratory tract sample from a patient. PCR is a laboratory method used for making a very large number of copies of short sections of DNA from a very small sample of DNA so that it can be detected. This process is called “amplifying” the DNA. (See the article on PCR for more details.) The reverse transcription step allows the viral RNA to be converted into DNA so that the PCR technique can be used.
• Rapid Antigen Tests: These tests detect the viral proteins of SARS-CoV-2 in respiratory samples. The main advantages of antigen tests are that they can provide results in minutes, are simpler than RT-PCR tests to perform, and are sometimes used at the point of care, such as at a health clinic. However, they are not as sensitive as RT-PCR tests, so negative results do not rule out infection.
• Blood Test for Antibodies (Total, IgG, IgM) to SARS-CoV-2 (Serology): These tests detect antibodies produced by the body’s immune system in response to SARS-CoV-2. COVID-19 serology tests can tell whether or not you have had the viral infection in the past. However, antibody tests are not the preferred tests to diagnose current infections. Antibodies don’t show up for about 1 to 2 weeks after you first become sick so antibody tests could miss some early infections. (For more general information on antibodies, also called immunoglobulins, see the article on Immunoglobulins.)

How is the sample collected for testing?

Proper collection of the appropriate samples is essential for accurate COVID-19 test results.

• For RT-PCR or Antigen Testing: The preferred sample is a swab from the back of your nose. This is called a nasopharyngeal swab, or NP swab. It is collected by having you tip your head back and then a swab (like a long Q-tip with a small head) is gently inserted through one of the nostrils until resistance is met (about 2 inches). It is left in place for several seconds, then rotated several times and withdrawn. This is not painful, but it may be uncomfortable, cause your eyes to tear and provoke a coughing spell. CDC guidance says that other samples from the respiratory tract may be collected when it is not possible to collect an NP swab. These include a swab from the back of the throat (oropharyngeal swab) or a swab from the front of your nose (nostril). Sometimes a saliva sample may be collected by having the patient spit into a container.
• For Antibody Testing: A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a fingertip and collecting a few drops of blood.

Coronavirus disease 2019 (COVID-19) is the name of the illness caused by the new strain of coronavirus called SARS-CoV-2. Diagnostic tests detect either the genetic material (RNA) of the virus or viral proteins (antigens) in a sample from the respiratory tract. COVID-19 serologic blood tests detect antibodies produced in response to the SARS-CoV-2infection.

SARS-CoV-2 is a new (novel) virus that first appeared in December 2019 and spread throughout the globe at an alarming rate, prompting the World Health Organization to declare the outbreak a pandemic and the U.S. Department of Health and Human Services to declare a public health emergency. As the pandemic continues, scientists continue to study the virus and learn more about COVID-19.

There are seven coronaviruses that are known to infect humans, and most cause mild to moderate respiratory symptoms. However, the disease resulting from SARS-CoV-2 infection is like MERS (Middle East respiratory syndrome) and SARS (severe acute respiratory syndrome) because it can cause more severe illness and, in some cases, lead to pneumonia and death.

Because SARS-CoV-2 is a novel virus, everyone is potentially susceptible to infection and, at this time, it is not known who may develop serious complications. Unlike seasonal influenza, which can also cause serious illness and death, there is no vaccine or specific treatment for SARS-CoV-2 infection yet.

About 1 to 2 weeks after infection, the body begins to produce antibodies to the virus, with the level gradually increasing over time. However, it is not yet known how long people continue to produce antibodies and whether the antibodies protect against re-infection, providing immunity. (For more general information on antibodies, including IgG, IgM and IgA, read the article on Immunoglobulins)

Some infected people may be asymptomatic or have no noticeable symptoms and yet be contagious, potentially spreading the virus to others (silent carriers). Many of those infected have no symptoms or mild to moderate illness and will recover within one to two weeks. If symptoms of COVID-19 develop, they typically appear within 2 to 14 days of exposure to the virus. Research suggests that the average time from first exposure to getting sick (incubation period) is about 5 days, and about 97% of people who develop symptoms will do so within 11 days.

The key symptoms of COVID-19 are coughing, shortness of breath or difficulty breathing. Additional symptoms may include fever, chills, repeated shaking with chills, muscle aches, headache, sore throat and loss of smell or taste. Symptoms may come and go, and there may be periods of time where someone with COVID-19 feels better. Children and babies generally have a milder illness but will often have the same symptoms as an adult.

The risk of serious disease increases with age and with having underlying health conditions, such as heart disease, lung disease, high blood pressure, diabetes or a weakened immune system. Some people with COVID-19 may develop pneumonia, a lung infection, and in severe cases, a ventilator may be needed to ensure enough oxygen. Although COVID-19 is primarily a respiratory infection, researchers are learning that the illness may affect other organs besides the lungs, such as the heart, brain, and kidneys. In the most severe cases, COVID-19 may lead to organ failure or death.

Tests for COVID-19

The initial signs and symptoms of COVID-19 are frequently difficult to distinguish from those of a common cold or of other respiratory illnesses, so testing is necessary to help diagnose a current or past infection.

• Reverse Transcription Polymerase Chain Reaction (RT-PCR): Most tests to check for current SARS-CoV-2 infection rely on RT-PCR testing to detect the virus’s RNA in a respiratory tract sample from a patient. PCR is a laboratory method used for making a very large number of copies of short sections of DNA from a very small sample of DNA so that it can be detected. This process is called “amplifying” the DNA. (See the article on PCR for more details.) The reverse transcription step allows the viral RNA to be converted into DNA so that the PCR technique can be used.
• Rapid Antigen Tests: These tests detect the viral proteins of SARS-CoV-2 in respiratory samples. The main advantages of antigen tests are that they can provide results in minutes, are simpler than RT-PCR tests to perform, and are sometimes used at the point of care, such as at a health clinic. However, they are not as sensitive as RT-PCR tests, so negative results do not rule out infection.
• Blood Test for Antibodies (Total, IgG, IgM) to SARS-CoV-2 (Serology): These tests detect antibodies produced by the body’s immune system in response to SARS-CoV-2. COVID-19 serology tests can tell whether or not you have had the viral infection in the past. However, antibody tests are not the preferred tests to diagnose current infections. Antibodies don’t show up for about 1 to 2 weeks after you first become sick so antibody tests could miss some early infections. (For more general information on antibodies, also called immunoglobulins, see the article on Immunoglobulins.)

How is the sample collected for testing?

Proper collection of the appropriate samples is essential for accurate COVID-19 test results.

• For RT-PCR or Antigen Testing: The preferred sample is a swab from the back of your nose. This is called a nasopharyngeal swab, or NP swab. It is collected by having you tip your head back and then a swab (like a long Q-tip with a small head) is gently inserted through one of the nostrils until resistance is met (about 2 inches). It is left in place for several seconds, then rotated several times and withdrawn. This is not painful, but it may be uncomfortable, cause your eyes to tear and provoke a coughing spell. CDC guidance says that other samples from the respiratory tract may be collected when it is not possible to collect an NP swab. These include a swab from the back of the throat (oropharyngeal swab) or a swab from the front of your nose (nostril). Sometimes a saliva sample may be collected by having the patient spit into a container.
• For Antibody Testing: A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a fingertip and collecting a few drops of blood.

Creatine kinase (CK) is an enzyme found in the heart, brain, skeletal muscle, and other tissues. Increased amounts of CK are released into the blood when there is muscle damage. This test measures the amount of creatine kinase in the blood.

The small amount of CK that is normally in the blood comes primarily from skeletal muscles. Any condition that causes muscle damage and/or interferes with muscle energy production or use can cause an increase in CK. For example, strenuous exercise and inflammation of muscles, called myositis, can increase CK as can muscle diseases (myopathies) such as muscular dystrophy. Rhabdomyolysis, an extreme breakdown of skeletal muscle tissue, is associated with significantly elevated levels of CK.

Creatine kinase-MB (CK-MB) is a form of an enzyme found primarily in heart muscle cells. This test measures CK-MB in the blood.

CK-MB is one of three forms (isoenzymes) of the enzyme creatine kinase (CK). These isoenzymes include:

• CK-MM (found in skeletal muscles and the heart)
• CK-MB (found mostly in the heart, but small amounts found in skeletal muscles)
• CK-BB (found mostly in the brain and smooth muscle, such as the intestines and uterus)

CK is released from muscle cells and is detectable in the blood whenever there is muscle damage. The small amount of CK that is normally in the blood is primarily CK-MM. CK-BB almost never gets into the blood, and CK-MB will typically only be present in significant amounts when the heart is damaged. A CK test measures the total level but does not distinguish between the three isoenzymes. When there is an increased amount of CK present in the blood, the CK-MB test can be used to determine whether it is due to heart damage or is more likely to be related to skeletal muscle injury.

Creatine kinase (CK) is an enzyme found in the heart, brain, skeletal muscle, and other tissues. Increased amounts of CK are released into the blood when there is muscle damage. This test measures the amount of creatine kinase in the blood.

The small amount of CK that is normally in the blood comes primarily from skeletal muscles. Any condition that causes muscle damage and/or interferes with muscle energy production or use can cause an increase in CK. For example, strenuous exercise and inflammation of muscles, called myositis, can increase CK as can muscle diseases (myopathies) such as muscular dystrophy. Rhabdomyolysis, an extreme breakdown of skeletal muscle tissue, is associated with significantly elevated levels of CK.

Creatine kinase (CK) is an enzyme found in the heart, brain, skeletal muscle, and other tissues. Increased amounts of CK are released into the blood when there is muscle damage. This test measures the amount of creatine kinase in the blood.

The small amount of CK that is normally in the blood comes primarily from skeletal muscles. Any condition that causes muscle damage and/or interferes with muscle energy production or use can cause an increase in CK. For example, strenuous exercise and inflammation of muscles, called myositis, can increase CK as can muscle diseases (myopathies) such as muscular dystrophy. Rhabdomyolysis, an extreme breakdown of skeletal muscle tissue, is associated with significantly elevated levels of CK.

Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is removed from the body by the kidneys, which filter almost all of it from the blood and release it into the urine. This test measures the amount of creatinine in the blood and/or urine.

Creatine is part of the cycle that produces energy needed to contract muscles. Both creatine and creatinine are produced by the body at a relatively constant rate. Since almost all creatinine is filtered from the blood by the kidneys and released into the urine, blood levels are usually a good indicator of how well the kidneys are working. The amount of creatinine you produce depends on your body size and your muscle mass. For this reason, creatinine levels are usually slightly higher in men than in women and children.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what still can be used by the body. The remaining waste forms urine.

Results from a blood creatinine test may be used in combination with results from other tests, such as a 24-hour urine creatinine test, to calculate values that are used to evaluate kidney function.

Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is filtered from the blood by the kidneys and released into the urine. A creatinine clearance test measures creatinine levels in both a sample of blood and a sample of urine from a 24-hour urine collection. The results are used to calculate the amount of creatinine that has been cleared from the blood and passed into the urine. This calculation allows for a general evaluation of the amount of blood that is being filtered by the kidneys in a 24-hour time period.

The amount of creatinine produced in the body is dependent on muscle mass and is relatively constant for an individual. The amount of creatinine removed from the blood depends on both the filtering ability of the kidneys and the rate at which blood is carried to the kidneys.

The amount of blood filtered per minute by the kidneys is known as the glomerular filtration rate (GFR). If the kidneys are damaged or diseased, or if blood circulation is slowed, then less creatinine will be removed from the blood and released into the urine and the GFR will be decreased.

GFR is difficult to measure directly. Therefore, it is recommended to estimate GFR by measuring the creatinine level in the blood and using the results in an equation to calculate estimated GFR. The calculation that takes into account several factors, such as age, gender and race of the person tested (see the article on Estimated Glomerular Filtration Rate).

Another, less common way to estimate GFR is to calculate creatinine clearance. There are several versions of the creatinine clearance calculation. All of them include the measurement of the amount of creatinine in a blood sample collected just before or after the urine collection, the amount of creatinine in a 24-hour urine sample, and the 24-hour urine volume. Since the amount of creatinine produced depends on muscle mass, some calculations also use a correction factor that takes into account a person’s body surface area (using their height and weight).

How is the sample collected for testing?
The test requires a 24-hour urine collection and a blood sample drawn either at the beginning or end of the urine collection. The blood sample is drawn by needle from a vein in the arm. The person being tested will also usually be asked to provide their current height and weight.

Is any test preparation needed to ensure the quality of the sample?
You may be instructed to fast overnight or refrain from eating cooked meat. Some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.

Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is filtered from the blood by the kidneys and released into the urine. A creatinine clearance test measures creatinine levels in both a sample of blood and a sample of urine from a 24-hour urine collection. The results are used to calculate the amount of creatinine that has been cleared from the blood and passed into the urine. This calculation allows for a general evaluation of the amount of blood that is being filtered by the kidneys in a 24-hour time period.

The amount of creatinine produced in the body is dependent on muscle mass and is relatively constant for an individual. The amount of creatinine removed from the blood depends on both the filtering ability of the kidneys and the rate at which blood is carried to the kidneys.

The amount of blood filtered per minute by the kidneys is known as the glomerular filtration rate (GFR). If the kidneys are damaged or diseased, or if blood circulation is slowed, then less creatinine will be removed from the blood and released into the urine and the GFR will be decreased.

GFR is difficult to measure directly. Therefore, it is recommended to estimate GFR by measuring the creatinine level in the blood and using the results in an equation to calculate estimated GFR. The calculation that takes into account several factors, such as age, gender and race of the person tested (see the article on Estimated Glomerular Filtration Rate).

Another, less common way to estimate GFR is to calculate creatinine clearance. There are several versions of the creatinine clearance calculation. All of them include the measurement of the amount of creatinine in a blood sample collected just before or after the urine collection, the amount of creatinine in a 24-hour urine sample, and the 24-hour urine volume. Since the amount of creatinine produced depends on muscle mass, some calculations also use a correction factor that takes into account a person’s body surface area (using their height and weight).

How is the sample collected for testing?

The test requires a 24-hour urine collection and a blood sample drawn either at the beginning or end of the urine collection. The blood sample is drawn by needle from a vein in the arm. The person being tested will also usually be asked to provide their current height and weight.

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to fast overnight or refrain from eating cooked meat. Some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.

C-reactive protein (CRP) is a protein made by the liver. CRP levels in the blood increase when there is a condition causing inflammation somewhere in the body. A CRP test measures the amount of CRP in the blood to detect inflammation due to acute conditions or to monitor the severity of disease in chronic conditions.

CRP is a non-specific indicator of inflammation and one of the most sensitive acute phase reactants. That means that it is released into the blood within a few hours after an injury, the start of an infection, or other cause of inflammation. Markedly increased levels can occur, for example, after trauma or a heart attack, with active or untreated autoimmune disorders, and with serious bacterial infections, such as in sepsis. The level of CRP can jump as much as a thousand-fold in response to bacterial infection, and its rise in the blood can precede pain, fever, or other signs and symptoms.

The CRP test is not diagnostic, but it provides information to your healthcare practitioner as to whether inflammation is present, without identifying the source of the inflammation. This information can be used in conjunction with other factors such as signs and symptoms, physical exam, and other tests to determine if you have an acute inflammatory condition or are experiencing a flare-up of a chronic inflammatory disease. Your healthcare practitioner may then follow up with further testing and treatment.

This standard CRP test is not to be confused with an hs-CRP test. These are two different tests that measure CRP and each test measures a different range of CRP level in the blood for different purposes:

The standard CRP test measures high levels of the protein observed in diseases that cause significant inflammation. It measures CRP in the range from 8 to 1000 mg/L (or 0.8 to 100 mg/dL).
The hs-CRP test precisely detects lower levels of the protein than that measured by the standard CRP test and is used to evaluate individuals for risk of cardiovascular disease. It measures CRP in the range from 0.3 to 10 mg/L. (See the article on hs-CRP.)

C-reactive protein (CRP) is a protein that increases in the blood with inflammation and infection as well as following a heart attack, surgery, or trauma. Studies have suggested that a persistent low level of inflammation plays a major role in atherosclerosis, the narrowing of blood vessels due to build-up of cholesterol and other lipids, which is often associated with cardiovascular disease (CVD). The hs-CRP test accurately measures low levels of CRP to identify low but persistent levels of inflammation and thus helps predict a person’s risk of developing CVD.

There are two different tests that measure CRP and each test measures a different range of CRP level in the blood for different purposes:

• The standard CRP test measures markedly high levels of the protein to detect diseases that cause significant inflammation. It measures CRP in the range from 10 to 1000 mg/L. This test may be used to detect inflammation (see the article C-Reactive Protein).
• The hs-CRP test accurately detects lower levels of the protein than the standard CRP test. It measures CRP in the range from 0.5 to 10 mg/L. This test is used to evaluate individuals for risk of CVD.

CVD causes more deaths in the U.S. each year than any other cause, according to the American Heart Association. A number of risk factors, such as family history, high cholesterol, high blood pressure, being overweight or diabetic, have been linked to the development of CVD, but a significant number of people who have few or no identified risk factors will also develop CVD. This fact has lead researchers to look for additional risk factors that might be either causing CVD or that could be used to determine lifestyle changes and/or treatments that could reduce a person’s risk.

High-sensitivity CRP is one of a growing number of cardiac risk markers that are used to help determine a person’s risk. Some studies have shown that measuring CRP with a highly sensitive assay can help identify the risk level for CVD in apparently healthy people. This more sensitive test can measure CRP levels that are within the higher end of the reference range. These normal but slightly high levels of CRP in otherwise healthy individuals can predict the future risk of a heart attack, stroke, sudden cardiac death, and peripheral arterial disease, even when cholesterol levels are within an acceptable range.

hs-CRP could be a marker not only in apparently healthy people, recent studies have shown. Adults with congenital heart disease (ACHD) with elevated CRP have worse functional status and exercise capacity, greater risk for death, or non-elective cardiovascular hospitalization.

Cerebrospinal fluid (CSF) is a clear, watery liquid that flows around the brain and spinal cord, surrounding and protecting them. CSF testing is performed to evaluate the level or concentration of different substances and cells in CSF in order to diagnose conditions affecting the brain and spinal cord (central nervous system).

CSF is produced and secreted by the choroid plexus, a special tissue that has many blood vessels and that lines the small cavities or chambers (ventricles) in the brain. The total CSF volume is 3-5 ounces (90-150 mL) in adults and 0.3-2 ounces (10-60 mL) in newborns. CSF is continually produced, circulated, and then absorbed into the blood. About 17 ounces (500 mL) of CSF are produced each day. This rate of production means that all the CSF is replaced every few hours.

A protective, semi-permeable barrier separates the brain from the bloodstream. This blood-brain barrier allows some substances to cross and prevents other substances from crossing. Importantly, it helps keep large molecules, toxins, and most blood cells away from the central nervous system. Any condition that disrupts this protective barrier may result in a change in the normal level or makeup of CSF. Because CSF surrounds the brain and spinal cord, testing a sample of CSF can be very valuable in diagnosing a variety of conditions affecting the central nervous system.

Although a sample of CSF may be more difficult to obtain than, for example, urine or blood, the results of CSF testing may reveal more directly the cause of central nervous system conditions. The following are some examples:

• Meningitis, an infection of the layers that cover the brain and spinal cord (meninges), and encephalitis, an infection in the brain
• Autoimmune diseases that affect the central nervous system, such as multiple sclerosis
• Cancers of the central nervous system or cancers that have spread to the central nervous system, such as leukemia
• Alzheimer disease, an irreversible form of dementia

Lactate is one of the substances produced by cells as the body turns food into energy (cell metabolism), with the highest level of production occurring in the muscles. Depending on pH, it is sometimes present in the form of lactic acid. However, with the neutral pH maintained by the body, most of it will be present in the blood in the form of lactate. This test measures the amount of lactate in the blood or, less commonly, in the cerebrospinal fluid (CSF).

Normally, the level of lactate in blood and CSF is low. Lactate is produced in excess by muscle cells, red blood cells, brain, and other tissues when there is insufficient oxygen at the cellular level or when the primary way of producing energy in the body’s cells is disrupted. Excess lactate can lead to lactic acidosis.

The principal means of producing energy within cells occurs in the mitochondria, tiny power stations inside most cells of the body. The mitochondria use glucose and oxygen to produce ATP (adenosine triphosphate), the body’s primary source of energy. This is called aerobic energy production.

Whenever cellular oxygen levels decrease and/or the mitochondria are not functioning properly, the body must turn to less efficient energy production to metabolize glucose and produce ATP. This is called anaerobic energy production and the primary byproduct is lactic acid, which is processed (metabolized) by the liver.

Lactic acid can accumulate in the body and blood when it is produced faster than the liver can break it down.

Excess lactate may indicate one or a combination of the following:

• Lack of oxygen (hypoxia)
• The presence of a condition that causes increased lactate production
• The presence of a condition that causes decreased clearance of lactate from the body

When lactic acid production increases significantly, the affected person is said to have hyperlactatemia, which can then progress to lactic acidosis as more lactic acid accumulates. The body can often compensate for the effects of hyperlactatemia, but lactic acidosis can be severe enough to disrupt a person’s acid/base (pH) balance and cause symptoms such as muscular weakness, rapid breathing, nausea, vomiting, sweating, and even coma.

There are a number of conditions that can cause high levels of lactate. See “What does the test result mean?” in Common Questions below for more information.

Susceptibility is a term used when microbe such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed on bacteria or fungi causing an individual’s infection after they have been recovered in a culture of the specimen. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

Although viruses are microbes, testing for their resistance to antiviral drugs is performed less frequently and by different test methods. This article is limited to the discussion of bacterial and fungal susceptibility testing.

During the culture process, pathogens are isolated (separated out from any other microbes present). Each pathogen, if present, is identified using biochemical, enzymatic, or molecular tests. Once the pathogens have been identified, it is possible to determine whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. For example, strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus), can be treated with ampicillin and does not require a test to predict susceptibility to this class of antibiotics.

Susceptibility testing is performed on each type of bacteria or fungi that may be relevant to the individual’s treatment and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and the antibiotic together in a growing environment, such as nutrient media in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria. Resistance can also be determined by detection of a gene that is known to cause resistance to specific antibiotics.

The bacteria Streptococcus pyogenes, also known as group A beta-hemolytic streptococcus or group A streptococcus (GAS), causes “strep throat,” the most common bacterial cause of inflammation and soreness of the back of the throat (pharyngitis). Strep throat tests identify the presence of these bacteria as the cause of a sore throat.

While most sore throats are caused by a virus and will resolve without treatment within a few days, some people with sore throats have strep throat. Strep throat is most common in children ages 5 to 15 years old. It is important that these bacterial strep infections be promptly identified and treated with antibiotics.

Strep throat is contagious and can spread when an infected person coughs or sneezes and other people come into contact with the droplets or mucus. Touching the face, eyes or mouth after touching something that has these droplets on it can spread the infection. The best way to avoid getting strep throat is to wash hands thoroughly and often and avoid sharing items like utensils or cups. A person who has a sore throat should wash their hands often and cover their mouth with a tissue or sleeve when coughing and sneezing.

If strep throat is not diagnosed and treated, secondary complications may develop, especially in children. These complications may include rheumatic fever, which can damage the heart, and glomerulonephritis, which affects the kidneys. Because streptococcal infections are routinely diagnosed and treated, these complications are rare in the United States now, but they do still occur.

A rapid strep test and/or a throat culture is used to diagnose group A streptococci as the cause of symptoms and allows the health practitioner to prescribe the proper antibiotics for treatment.

How is the sample collected for testing?

A health practitioner uses a tongue depressor to hold down a person’s tongue and then inserts a special swab into the mouth and rubs it against the back of the throat and tonsils. The swab may be used to do a rapid strep test in a doctor’s office or clinic, or it may be sent to a laboratory. A second swab may be collected along with the first one. This extra sample is used to perform a throat culture as a follow-up test, when necessary.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. The test should be performed before antibiotics are prescribed.

Cytomegalovirus (CMV) is a common virus that usually causes no symptoms or only mild illness. CMV testing detects antibodies in the blood that the body produces in response to the infection or detects CMV directly.

In the United States, as many as 60% of people have been exposed to CMV at some point in their life. Almost 1 out of every 3 children have been exposed to CMV by age 5 and more than half of adults are exposed to CMV by age 40.

Most people are infected as children or as young adults, but many don’t know it because CMV usually does not cause noticeable symptoms or only mild illness in otherwise healthy people. Individuals with mild illness may have non-specific signs and symptoms, such as sore throat, fever, tiredness, and swollen glands. In otherwise healthy adults, CMV infection may sometimes cause a flu-like illness or signs and symptoms similar to mononucleosis (mono), such as extreme fatigue, fever, chills, body aches, and/or headaches that usually resolve within a few weeks.

CMV is found in many body fluids during an active infection, including saliva, urine, blood, breast milk, semen, vaginal fluid, and cerebrospinal fluid. CMV easily spreads from person to person through close contact with someone who is infected or by contact with contaminated body fluids or objects, such as diapers or toys.

Once you are infected, CMV remains in your body for the rest of your life without causing symptoms. After your initial “primary” infection resolves, CMV becomes dormant or latent, like other members of the herpes family. However, if your immune system is significantly weakened, the virus can become active again (reactivate) and cause illness.

CMV can cause notable health problems in these situations:

A pregnant woman who is infected for the first time (primary infection) during pregnancy can pass the infection to her developing baby across the placenta. This can cause serious physical and developmental problems in the baby. Most newborns (about 90%) who are infected appear healthy at birth but may develop hearing or vision problems, pneumonia, seizures, and/or delayed mental development a few months later. A few babies may be stillborn, while others may have symptoms at birth such as jaundice, anemia, an enlarged spleen or liver, and a small head.
CMV can cause serious illness and death in people with weakened immune systems, such as those with HIV/AIDS, solid organ or bone marrow transplant recipients, and cancer patients receiving chemotherapy. These individuals might experience the most severe symptoms and their CMV infection may remain active. CMV may reactivate in those who were previously exposed. The infection could affect the:
Eyes, causing inflammation of the retina, which can lead to blindness
Digestive tract, causing bloody diarrhea and abdominal pain
Lungs, causing pneumonia with a non-productive cough and shortness of breath
Brain, causing encephalitis
Spleen and liver
Organ or bone marrow transplants, causing some degree of rejection
Active CMV also further depresses the immune system, allowing other secondary infections such as fungal infections, to occur.

D-dimer is one of the protein fragments produced when a blood clot gets dissolved in the body. It is normally undetectable or detectable at a very low level unless the body is forming and breaking down blood clots. Then, its level in the blood can significantly rise. This test detects D-dimer in the blood.

When a blood vessel or tissue is injured and begins to bleed, a process called hemostasis is initiated by the body to create a blood clot to limit and eventually stop the bleeding. This process produces threads of a protein called fibrin, which crosslink together to form a fibrin net. That net, together with platelets, helps hold the forming blood clot in place at the site of the injury until it heals.

Once the area has had time to heal and the clot is no longer needed, the body uses an enzyme called plasmin to break the clot (thrombus) into small pieces so that it can be removed. The fragments of the disintegrating fibrin in the clot are called fibrin degradation products (FDP), which consist of variously sized pieces of crosslinked fibrin. One of the final fibrin degradation products produced is D-dimer, which can be measured in a blood sample when present. The level of D-dimer in the blood can significantly rise when there is significant formation and breakdown of fibrin clots in the body.

For a person who is at low or intermediate risk for blood clotting (thrombosis) and/or thrombotic embolism, the strength of the D-dimer test is that it can be used in a hospital emergency room setting to determine the likelihood of a clot’s presence. A negative D-dimer test (D-dimer level is below a predetermined cut-off threshold) indicates that it is highly unlikely that a thrombus is present. However, a positive D-dimer test cannot predict whether or not a clot is present. It indicates that further diagnostic procedures are required (e.g., ultrasound, CT angiography).

There are several factors and conditions associated with inappropriate blood clot formation. One of the most common is deep vein thrombosis (DVT), which involves clot formation in veins deep within the body, most frequently in the lower legs. These clots may grow very large and block blood flow in the legs, causing swelling, pain, and tissue damage. It is possible for a piece of the clot to break off and travel to other parts of the body. This “embolus” can lodge in the lungs, causing a pulmonary embolus or embolism (PE). Pulmonary embolisms from DVT affect more than 300,000 people in the U.S. each year.

While clots most commonly form in the veins of the legs, they may also form in other areas as well. Measurements of D-dimer can be used to help detect clots in any of these sites. For example, clots in coronary arteries are the cause of myocardial infarction (heart attacks). Clots may form on the lining of the heart or its valves, particularly when the heart is beating irregularly (atrial fibrillation) or when the valves are damaged. Clots can also form in large arteries as a result of narrowing and damage from atherosclerosis. Pieces of such clots may break off and cause an embolus that blocks an artery in another organ, such as the brain (causing a stroke) or the kidneys.

Measurements of D-dimer may also be ordered, along with other tests, to help diagnose disseminated intravascular coagulation (DIC). DIC is a condition in which clotting factors are activated and then used up throughout the body. This creates numerous tiny blood clots and at the same time leaves the affected person vulnerable to excessive bleeding. It is a complex, sometimes life-threatening condition that can arise from a variety of situations, including some surgical procedures, sepsis, poisonous snake bites, liver disease, and after childbirth. Steps are taken to support the affected person while the underlying condition resolves. The D-dimer level will typically be very elevated in DIC.

Dengue fever is a viral infection transmitted to humans by mosquitoes that live in tropical and subtropical climates and carry the virus. Blood testing detects the dengue virus or antibodies produced in response to dengue infection.

According to the Centers for Disease Control and Prevention (CDC), dengue infections have been reported in more than 100 countries from parts of Africa, the Americas, the Caribbean, the Eastern Mediterranean, Southeast Asia, and the Western Pacific. It is a fast emerging infectious disease, according to the World Health Organization (WHO), with an increasing number of cases and countries affected throughout the world. The actual number is not known because about 75% of cases are asymptomatic, but a recent estimate put the number of annual dengue infections as high as 390 million. Approximately 50 to 100 million symptomatic cases occur annually worldwide.

In the U.S., the majority of dengue cases occur in travelers returning from areas where dengue is endemic. Most dengue cases in U.S. citizens occur in people who live in Puerto Rico, the U.S. Virgin Islands, Samoa and Guam. Outbreaks where a large number of cases occur in a defined area are rare in the U.S. In recent years, there have been small outbreaks in Texas and Hawaii and a few cases diagnosed in southern Florida.

Many individuals will develop no symptoms at all, or have only a mild illness when exposed to one of the four serotypes (1-4) of the dengue virus. For those who do develop symptoms, prognosis is still very good for full recovery within a few weeks. The most common initial symptoms are a sudden high fever (104°F or 40°C) and flu-like symptoms that appear roughly 4 to 7 days after being bitten by an infected mosquito (this is called the incubation period and can range from 3 to 14 days). Additional signs and symptoms may include severe headache, especially behind the eyes, muscle and joint pain, skin rash, nausea, vomiting, and swollen glands.

Some people who develop a fever will recover on their own with no lasting ill effects while others may progress to severe dengue fever (sometimes called Dengue Hemorrhagic Fever). If the disease progresses to this form, a new wave of symptoms will appear 3 to 7 days after initial symptoms and as the fever recedes. These may include nose bleeds, vomiting blood, passing blood in the stool, difficulty breathing and cold clammy skin, especially in the extremities. During the second phase, the virus may attack blood vessels (the vascular system), causing capillaries to leak fluid into the space around the lungs (pleural effusion) or into the abdominal cavity (ascites).

The loss of blood and fluid during the second phase, if untreated, can worsen and can be fatal. In order to avoid that complication (sometimes called Dengue Shock Syndrome), a healthcare practitioner may hospitalize a patient with severe dengue fever so that falling blood pressure and dehydration caused by the loss of blood and fluids can be managed while the disease runs its course – generally a period of one to two weeks. During the following week of recovery, a person may develop a second rash that lasts a week or more.

Dengue fever is usually diagnosed via some combination of blood tests because the body’s immune response to the virus is dynamic and complex. Laboratory tests may include:

• Molecular tests for dengue virus (PCR)—detect the presence of the virus itself; these tests can diagnose dengue fever up to 7 days after the onset of symptoms and can be used to determine which of the 4 different serotypes of dengue virus is causing the infection.
• Antibody tests, IgM and IgG—detect antibodies produced by the immune system when a person has been exposed to the virus; these tests are most effective when performed at least 4 days after exposure.
• Complete blood count (CBC)—to look for low platelet count typical of the later stages of the illness and to detect the decrease in hemoglobin, hematocrit, and red blood cell (RBC) count (evidence of anemia) that would occur with blood loss associated with severe dengue fever
• Basic metabolic panel (BMP) – to monitor kidney function and look for evidence of dehydration that can occur with severe illness

How is the sample collected for testing?

A blood sample is collected by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Dengue fever is a viral infection transmitted to humans by mosquitoes that live in tropical and subtropical climates and carry the virus. Blood testing detects the dengue virus or antibodies produced in response to dengue infection.

According to the Centers for Disease Control and Prevention (CDC), dengue infections have been reported in more than 100 countries from parts of Africa, the Americas, the Caribbean, the Eastern Mediterranean, Southeast Asia, and the Western Pacific. It is a fast emerging infectious disease, according to the World Health Organization (WHO), with an increasing number of cases and countries affected throughout the world. The actual number is not known because about 75% of cases are asymptomatic, but a recent estimate put the number of annual dengue infections as high as 390 million. Approximately 50 to 100 million symptomatic cases occur annually worldwide.

In the U.S., the majority of dengue cases occur in travelers returning from areas where dengue is endemic. Most dengue cases in U.S. citizens occur in people who live in Puerto Rico, the U.S. Virgin Islands, Samoa and Guam. Outbreaks where a large number of cases occur in a defined area are rare in the U.S. In recent years, there have been small outbreaks in Texas and Hawaii and a few cases diagnosed in southern Florida.

Many individuals will develop no symptoms at all, or have only a mild illness when exposed to one of the four serotypes (1-4) of the dengue virus. For those who do develop symptoms, prognosis is still very good for full recovery within a few weeks. The most common initial symptoms are a sudden high fever (104°F or 40°C) and flu-like symptoms that appear roughly 4 to 7 days after being bitten by an infected mosquito (this is called the incubation period and can range from 3 to 14 days). Additional signs and symptoms may include severe headache, especially behind the eyes, muscle and joint pain, skin rash, nausea, vomiting, and swollen glands.

Some people who develop a fever will recover on their own with no lasting ill effects while others may progress to severe dengue fever (sometimes called Dengue Hemorrhagic Fever). If the disease progresses to this form, a new wave of symptoms will appear 3 to 7 days after initial symptoms and as the fever recedes. These may include nose bleeds, vomiting blood, passing blood in the stool, difficulty breathing and cold clammy skin, especially in the extremities. During the second phase, the virus may attack blood vessels (the vascular system), causing capillaries to leak fluid into the space around the lungs (pleural effusion) or into the abdominal cavity (ascites).

The loss of blood and fluid during the second phase, if untreated, can worsen and can be fatal. In order to avoid that complication (sometimes called Dengue Shock Syndrome), a healthcare practitioner may hospitalize a patient with severe dengue fever so that falling blood pressure and dehydration caused by the loss of blood and fluids can be managed while the disease runs its course – generally a period of one to two weeks. During the following week of recovery, a person may develop a second rash that lasts a week or more.

Dengue fever is usually diagnosed via some combination of blood tests because the body’s immune response to the virus is dynamic and complex. Laboratory tests may include:

Molecular tests for dengue virus (PCR)—detect the presence of the virus itself; these tests can diagnose dengue fever up to 7 days after the onset of symptoms and can be used to determine which of the 4 different serotypes of dengue virus is causing the infection.
Antibody tests, IgM and IgG—detect antibodies produced by the immune system when a person has been exposed to the virus; these tests are most effective when performed at least 4 days after exposure.
Complete blood count (CBC)—to look for low platelet count typical of the later stages of the illness and to detect the decrease in hemoglobin, hematocrit, and red blood cell (RBC) count (evidence of anemia) that would occur with blood loss associated with severe dengue fever
Basic metabolic panel (BMP) – to monitor kidney function and look for evidence of dehydration that can occur with severe illness
How is the sample collected for testing?
A blood sample is collected by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.

Dengue fever is a viral infection transmitted to humans by mosquitoes that live in tropical and subtropical climates and carry the virus. Blood testing detects the dengue virus or antibodies produced in response to dengue infection.

According to the Centers for Disease Control and Prevention (CDC), dengue infections have been reported in more than 100 countries from parts of Africa, the Americas, the Caribbean, the Eastern Mediterranean, Southeast Asia, and the Western Pacific. It is a fast emerging infectious disease, according to the World Health Organization (WHO), with an increasing number of cases and countries affected throughout the world. The actual number is not known because about 75% of cases are asymptomatic, but a recent estimate put the number of annual dengue infections as high as 390 million. Approximately 50 to 100 million symptomatic cases occur annually worldwide.

In the U.S., the majority of dengue cases occur in travelers returning from areas where dengue is endemic. Most dengue cases in U.S. citizens occur in people who live in Puerto Rico, the U.S. Virgin Islands, Samoa and Guam. Outbreaks where a large number of cases occur in a defined area are rare in the U.S. In recent years, there have been small outbreaks in Texas and Hawaii and a few cases diagnosed in southern Florida.

Many individuals will develop no symptoms at all, or have only a mild illness when exposed to one of the four serotypes (1-4) of the dengue virus. For those who do develop symptoms, prognosis is still very good for full recovery within a few weeks. The most common initial symptoms are a sudden high fever (104°F or 40°C) and flu-like symptoms that appear roughly 4 to 7 days after being bitten by an infected mosquito (this is called the incubation period and can range from 3 to 14 days). Additional signs and symptoms may include severe headache, especially behind the eyes, muscle and joint pain, skin rash, nausea, vomiting, and swollen glands.

Some people who develop a fever will recover on their own with no lasting ill effects while others may progress to severe dengue fever (sometimes called Dengue Hemorrhagic Fever). If the disease progresses to this form, a new wave of symptoms will appear 3 to 7 days after initial symptoms and as the fever recedes. These may include nose bleeds, vomiting blood, passing blood in the stool, difficulty breathing and cold clammy skin, especially in the extremities. During the second phase, the virus may attack blood vessels (the vascular system), causing capillaries to leak fluid into the space around the lungs (pleural effusion) or into the abdominal cavity (ascites).

The loss of blood and fluid during the second phase, if untreated, can worsen and can be fatal. In order to avoid that complication (sometimes called Dengue Shock Syndrome), a healthcare practitioner may hospitalize a patient with severe dengue fever so that falling blood pressure and dehydration caused by the loss of blood and fluids can be managed while the disease runs its course – generally a period of one to two weeks. During the following week of recovery, a person may develop a second rash that lasts a week or more.

Dengue fever is usually diagnosed via some combination of blood tests because the body’s immune response to the virus is dynamic and complex. Laboratory tests may include:

• Molecular tests for dengue virus (PCR)—detect the presence of the virus itself; these tests can diagnose dengue fever up to 7 days after the onset of symptoms and can be used to determine which of the 4 different serotypes of dengue virus is causing the infection.
• Antibody tests, IgM and IgG—detect antibodies produced by the immune system when a person has been exposed to the virus; these tests are most effective when performed at least 4 days after exposure.
• Complete blood count (CBC)—to look for low platelet count typical of the later stages of the illness and to detect the decrease in hemoglobin, hematocrit, and red blood cell (RBC) count (evidence of anemia) that would occur with blood loss associated with severe dengue fever
• Basic metabolic panel (BMP) – to monitor kidney function and look for evidence of dehydration that can occur with severe illness

How is the sample collected for testing?

A blood sample is collected by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Dehydroepiandrosterone sulfate (DHEAS) is a male sex hormone (androgen) that is present in both men and women. This test measures the level of DHEAS in the blood.

DHEAS:

Plays a role in developing male secondary sexual characteristics at puberty
Can be converted by the body into more potent androgens, such as testosterone and androstenedione
Can be converted into the female hormone estrogen
DHEAS is produced almost exclusively by the adrenal glands, with smaller amounts being produced by a woman’s ovaries and a man’s testicles.

A DHEAS test is useful in determining whether the adrenal glands are working properly. Adrenal tumors (cancerous and non-cancerous) and enlargement of an adrenal gland (hyperplasia) can lead to an increased level of DHEAS. Rarely, an ovarian tumor may produce DHEAS.

Excess DHEAS:

May not be noticed in adult men
Can cause early (precocious) puberty in young boys
Can lead to absence of menstrual periods (amenorrhea) and the development of masculine physical characteristics (virilization) in girls and women, such as excess body and facial hair (hirsutism)
Can cause a female baby to be born with genitals that are not distinctly male or female in appearance (ambiguous external genitalia)

A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and the blood cells on the slide are examined and evaluated. Traditionally, trained laboratorians have examined blood smears manually using a microscope. More recently, automated digital systems have become available to help analyze blood smears more efficiently.

A blood smear is a snapshot of the cells that are present in the blood at the time the sample is obtained. The blood smear allows for the evaluation of these cells:

• White blood cells (WBCs, leukocytes) — help fight infections or participate in immune responses
• Red blood cells (RBCs, erythrocytes) — carry oxygen to tissues
• Platelets (thrombocytes) — small cell fragments that are vital to proper blood clotting

These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood is dynamic but is generally maintained by the body within specific ranges.

The drop of blood on the slide used for a blood smear contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. A blood smear examination:

• Compares the WBCs’ size, shape, and general appearance to the established appearance of “normal” cells. It also determines the five different types of WBCs and their relative percentages (manual WBC differential).
• Evaluates the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology)
• Estimates the number of platelets present

A variety of diseases and conditions can affect the number and appearance of blood cells. Examination of the blood smear can be used to support findings from other tests and examinations. For example, RBCs that appear smaller and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs that are not fully mature may add to information from other tests to help make a diagnosis of infection, malignancy, or other conditions.

Digoxin is a drug used to treat heart failure and abnormal heart rhythms. Heart failure, including congestive heart failure (CHF), causes the heart to become less effective at circulating blood. As a result, blood backs up into the legs, hands, feet, lungs and liver, causing swelling, shortness of breath, and fatigue. This test measures the amount of digoxin in the blood.

Digoxin is prescribed to relieve some symptoms of heart failure. It strengthens the contractions of the heart and helps it to pump blood more efficiently. Digoxin also helps control the heart rate and abnormal heart rhythms known as arrhythmias. It will not cure heart failure or arrhythmias, which are long-term (chronic) conditions, but can help to manage the symptoms along with diet, exercise, and other medications.

Digoxin levels must be monitored because the drug has a narrow safety range. If the level in the blood is too low, symptoms may recur. If the level is too high, toxicity may occur. Digoxin dosage may be adjusted based on levels measured.

The direct antiglobulin test (DAT) determines whether your red blood cells (RBCs) circulating in the bloodstream are covered with antibodies. The antibodies that are attached to the surface of the RBCs are responsible for their destruction.

There are a few reasons why RBCs may be attacked by antibodies:

• Following a blood transfusion: If someone receives a donor’s blood that does not fully match their own type, their body will make antibodies that recognize the donor’s RBC as foreign. (For more on blood types, see the article on Blood Typing.) These antibodies will attack donor’s RBCs and destroy them. People who have many blood transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC. If someone shows symptoms of a reaction after a transfusion, a DAT will be performed to determine if those antibodies have attached to RBCs.
• Mother/baby blood type incompatibility: A mother and baby may have different blood types if the baby inherits a blood type from the father. During pregnancy or labor, the mother may be exposed to the baby’s RBCs. These RBCs may be recognized by the mother’s immune system as foreign and she will produce antibodies directed against the baby’s RBC. A baby’s RBCs might be covered with antibodies that cross the placenta from the mother’s blood into the baby’s circulation. These antibodies will destroy the baby’s circulating RBCs and cause hemolytic disease of the newborn (HDN). A DAT is performed on the blood of a baby to determine if the antibodies have attached to the baby’s RBCs.This may occur when an Rh-positive baby is born to an Rh-negative mother. This type of incompatibility usually does not affect the first baby but affects subsequent children. (For more on blood types and pregnancy, see the article on Blood Typing and RBC Antibody Screen). Formerly, antibodies to the Rh protein were the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventive treatments given to the Rh-negative mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn nowadays is an ABO incompatibility between a Group O mother and her baby. This type of fetal-maternal incompatibility is generally mild.
• Autoimmune diseases and other conditions: Some people make antibodies that target their own RBCs (autoantibodies). These antibodies are produced because the immune system mistakenly recognizes their own RBCs as foreign. Some examples of conditions that cause this include:
  • Autoimmune disorders such as lupus
  • Malignant diseases such as lymphoma and chronic lymphocytic leukemia
  • Infections such as mycoplasma pneumonia and mononucleosis
• Drug-induced anemia: Certain drugs can induce antibodies against RBCs and therefore cause their destruction (hemolysis). This is seen with some antibiotics, such as penicillin, cephalosporins and piperacillin. Be sure to tell your healthcare provider about any drugs you have been taking recently. If the healthcare provider suspects drug-induced autoimmune anemia, the suspect medication will be discontinued. Symptoms typically resolve promptly after the drug is discontinued.

The direct antiglobulin test (DAT) determines whether your red blood cells (RBCs) circulating in the bloodstream are covered with antibodies. The antibodies that are attached to the surface of the RBCs are responsible for their destruction.

There are a few reasons why RBCs may be attacked by antibodies:

Following a blood transfusion: If someone receives a donor’s blood that does not fully match their own type, their body will make antibodies that recognize the donor’s RBC as foreign. (For more on blood types, see the article on Blood Typing.) These antibodies will attack donor’s RBCs and destroy them. People who have many blood transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC. If someone shows symptoms of a reaction after a transfusion, a DAT will be performed to determine if those antibodies have attached to RBCs.
Mother/baby blood type incompatibility: A mother and baby may have different blood types if the baby inherits a blood type from the father. During pregnancy or labor, the mother may be exposed to the baby’s RBCs. These RBCs may be recognized by the mother’s immune system as foreign and she will produce antibodies directed against the baby’s RBC. A baby’s RBCs might be covered with antibodies that cross the placenta from the mother’s blood into the baby’s circulation. These antibodies will destroy the baby’s circulating RBCs and cause hemolytic disease of the newborn (HDN). A DAT is performed on the blood of a baby to determine if the antibodies have attached to the baby’s RBCs.
This may occur when an Rh-positive baby is born to an Rh-negative mother. This type of incompatibility usually does not affect the first baby but affects subsequent children. (For more on blood types and pregnancy, see the article on Blood Typing and RBC Antibody Screen). Formerly, antibodies to the Rh protein were the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventive treatments given to the Rh-negative mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn nowadays is an ABO incompatibility between a Group O mother and her baby. This type of fetal-maternal incompatibility is generally mild.

Autoimmune diseases and other conditions: Some people make antibodies that target their own RBCs (autoantibodies). These antibodies are produced because the immune system mistakenly recognizes their own RBCs as foreign. Some examples of conditions that cause this include:
Autoimmune disorders such as lupus
Malignant diseases such as lymphoma and chronic lymphocytic leukemia
Infections such as mycoplasma pneumonia and mononucleosis
Drug-induced anemia: Certain drugs can induce antibodies against RBCs and therefore cause their destruction (hemolysis). This is seen with some antibiotics, such as penicillin, cephalosporins and piperacillin. Be sure to tell your healthcare provider about any drugs you have been taking recently. If the healthcare provider suspects drug-induced autoimmune anemia, the suspect medication will be discontinued. Symptoms typically resolve promptly after the drug is discontinued.

Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of heme. Heme is a component of hemoglobin, which is found in red blood cells (RBCs). Bilirubin is ultimately processed by the liver so that it can be removed from the body. This test measures the amount of bilirubin in the blood to evaluate a person’s liver function or to help diagnose anemias caused by RBC destruction (hemolytic anemia).

RBCs normally degrade after about 120 days in circulation. Bilirubin is formed as the liver breaks down and recycles aged red blood cells.

Two forms of bilirubin can be measured or estimated by laboratory tests:

• Unconjugated bilirubin is formed when heme is released from hemoglobin. It is carried by proteins to the liver. In the liver, sugars are attached (conjugated) to bilirubin to form conjugated bilirubin.
• Conjugated bilirubin enters the bile and passes from the liver to the small intestines, where it is further broken down by bacteria and eventually eliminated in the stool. Thus, the breakdown products of bilirubin give stool its characteristic brown color. Normally, the level of conjugated bilirubin in the blood is very low.

The bilirubin test is included in the comprehensive metabolic panel (CMP) and the liver panel, which are often used as general health screenings.

• Usually, an initial test measures the total bilirubin level (unconjugated plus conjugated bilirubin).
• If the total bilirubin level is increased, the laboratory can use a second test to detect water-soluble forms of bilirubin, called “direct” bilirubin. The direct bilirubin test provides an estimate of the amount of conjugated bilirubin present.
• Subtracting the direct bilirubin level from the total bilirubin level helps estimate the “indirect” level of unconjugated bilirubin.

A small amount (approximately 250 to 350 milligrams, or about 4 milligrams per kilogram of body weight) of bilirubin is produced daily in a normal, healthy adult. Most bilirubin (70%-90%) comes from damaged or degraded RBCs, with the remaining amount coming from the bone marrow or liver. Normally, small amounts of unconjugated bilirubin are released into the blood, but almost no conjugated bilirubin is present.

If the bilirubin level increases in the blood, a person may appear jaundiced, with a yellowing of the skin and/or whites of the eyes. The pattern of bilirubin test results can give the healthcare practitioner information regarding the condition that may be present. (For details, see “What does the test result mean?” under Common Questions.)

The direct antiglobulin test (DAT) determines whether your red blood cells (RBCs) circulating in the bloodstream are covered with antibodies. The antibodies that are attached to the surface of the RBCs are responsible for their destruction.

There are a few reasons why RBCs may be attacked by antibodies:

• Following a blood transfusion: If someone receives a donor’s blood that does not fully match their own type, their body will make antibodies that recognize the donor’s RBC as foreign. (For more on blood types, see the article on Blood Typing.) These antibodies will attack donor’s RBCs and destroy them. People who have many blood transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC. If someone shows symptoms of a reaction after a transfusion, a DAT will be performed to determine if those antibodies have attached to RBCs.
• Mother/baby blood type incompatibility: A mother and baby may have different blood types if the baby inherits a blood type from the father. During pregnancy or labor, the mother may be exposed to the baby’s RBCs. These RBCs may be recognized by the mother’s immune system as foreign and she will produce antibodies directed against the baby’s RBC. A baby’s RBCs might be covered with antibodies that cross the placenta from the mother’s blood into the baby’s circulation. These antibodies will destroy the baby’s circulating RBCs and cause hemolytic disease of the newborn (HDN). A DAT is performed on the blood of a baby to determine if the antibodies have attached to the baby’s RBCs.This may occur when an Rh-positive baby is born to an Rh-negative mother. This type of incompatibility usually does not affect the first baby but affects subsequent children. (For more on blood types and pregnancy, see the article on Blood Typing and RBC Antibody Screen). Formerly, antibodies to the Rh protein were the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventive treatments given to the Rh-negative mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn nowadays is an ABO incompatibility between a Group O mother and her baby. This type of fetal-maternal incompatibility is generally mild.
• Autoimmune diseases and other conditions: Some people make antibodies that target their own RBCs (autoantibodies). These antibodies are produced because the immune system mistakenly recognizes their own RBCs as foreign. Some examples of conditions that cause this include:
  • Autoimmune disorders such as lupus
  • Malignant diseases such as lymphoma and chronic lymphocytic leukemia
  • Infections such as mycoplasma pneumonia and mononucleosis
• Drug-induced anemia: Certain drugs can induce antibodies against RBCs and therefore cause their destruction (hemolysis). This is seen with some antibiotics, such as penicillin, cephalosporins and piperacillin. Be sure to tell your healthcare provider about any drugs you have been taking recently. If the healthcare provider suspects drug-induced autoimmune anemia, the suspect medication will be discontinued. Symptoms typically resolve promptly after the drug is discontinued.

The direct low-density lipoprotein cholesterol test (direct LDL-C) measures the amount of LDL cholesterol, sometimes called “bad” cholesterol, in the blood. Elevated levels of LDL-C are associated with an increased risk of hardening of the arteries (atherosclerosis) and heart disease.

Usually, your LDL-C level is calculated using the measured values of the components of a standard lipid panel: total cholesterol, high-density lipoprotein cholesterol (HDL-C), and triglycerides. Using a mathematical equation, the amount of LDL-C can be determined from the three measured values. Calculated LDL-C is about as accurate as direct LDL-C when triglyceride levels are normal and can be done at no additional cost as part of a lipid profile.

In most cases, calculated LDL-C is a good estimate of the LDL-C, but it becomes less accurate with increasing triglyceride levels. When triglycerides are significantly elevated (above 400 mg/dL), the equation is no longer valid. Other conditions such as severe cirrhosis can also affect the accuracy of calculated LDL-C. In these situations, the only way to accurately determine LDL-C is to measure it directly or with special testing techniques (e.g., a beta quantification test).

High triglycerides may be due to a metabolic disorder affecting lipids. However, anyone may have high triglycerides after eating. In either situation, the direct LDL-C test can determine the amount of LDL in your blood.

A direct LDL-C may be ordered by your healthcare practitioner when prior test results have indicated high triglycerides. In some laboratories, the direct LDL test will automatically be performed when the triglyceride levels are too high to calculate LDL-C. This saves the healthcare practitioner time by not needing to order another test, saves you time by not needing to have a second blood sample drawn, and speeds up the time to provide the test result.

Whether calculated or measured directly, LDL-C values are used to assess your risk for heart disease and help guide decisions about what treatment may be best if you are at borderline, intermediate, or high risk. The results are considered along with other known risk factors of heart disease to develop a plan of treatment and follow up. Treatment options may involve lifestyle changes such as diet and exercise or lipid-lowering medications such as statins. LDL-C tests can also be used to monitor your response to therapy to lower cholesterol.

Read the article on LDL Cholesterol to learn more about LDL-C and what results might mean.

The direct low-density lipoprotein cholesterol test (direct LDL-C) measures the amount of LDL cholesterol, sometimes called “bad” cholesterol, in the blood. Elevated levels of LDL-C are associated with an increased risk of hardening of the arteries (atherosclerosis) and heart disease.

Usually, your LDL-C level is calculated using the measured values of the components of a standard lipid panel: total cholesterol, high-density lipoprotein cholesterol (HDL-C), and triglycerides. Using a mathematical equation, the amount of LDL-C can be determined from the three measured values. Calculated LDL-C is about as accurate as direct LDL-C when triglyceride levels are normal and can be done at no additional cost as part of a lipid profile.

In most cases, calculated LDL-C is a good estimate of the LDL-C, but it becomes less accurate with increasing triglyceride levels. When triglycerides are significantly elevated (above 400 mg/dL), the equation is no longer valid. Other conditions such as severe cirrhosis can also affect the accuracy of calculated LDL-C. In these situations, the only way to accurately determine LDL-C is to measure it directly or with special testing techniques (e.g., a beta quantification test).

High triglycerides may be due to a metabolic disorder affecting lipids. However, anyone may have high triglycerides after eating. In either situation, the direct LDL-C test can determine the amount of LDL in your blood.

A direct LDL-C may be ordered by your healthcare practitioner when prior test results have indicated high triglycerides. In some laboratories, the direct LDL test will automatically be performed when the triglyceride levels are too high to calculate LDL-C. This saves the healthcare practitioner time by not needing to order another test, saves you time by not needing to have a second blood sample drawn, and speeds up the time to provide the test result.

Whether calculated or measured directly, LDL-C values are used to assess your risk for heart disease and help guide decisions about what treatment may be best if you are at borderline, intermediate, or high risk. The results are considered along with other known risk factors of heart disease to develop a plan of treatment and follow up. Treatment options may involve lifestyle changes such as diet and exercise or lipid-lowering medications such as statins. LDL-C tests can also be used to monitor your response to therapy to lower cholesterol.

Read the article on LDL Cholesterol to learn more about LDL-C and what results might mean.

The direct low-density lipoprotein cholesterol test (direct LDL-C) measures the amount of LDL cholesterol, sometimes called “bad” cholesterol, in the blood. Elevated levels of LDL-C are associated with an increased risk of hardening of the arteries (atherosclerosis) and heart disease.

Usually, your LDL-C level is calculated using the measured values of the components of a standard lipid panel: total cholesterol, high-density lipoprotein cholesterol (HDL-C), and triglycerides. Using a mathematical equation, the amount of LDL-C can be determined from the three measured values. Calculated LDL-C is about as accurate as direct LDL-C when triglyceride levels are normal and can be done at no additional cost as part of a lipid profile.

In most cases, calculated LDL-C is a good estimate of the LDL-C, but it becomes less accurate with increasing triglyceride levels. When triglycerides are significantly elevated (above 400 mg/dL), the equation is no longer valid. Other conditions such as severe cirrhosis can also affect the accuracy of calculated LDL-C. In these situations, the only way to accurately determine LDL-C is to measure it directly or with special testing techniques (e.g., a beta quantification test).

High triglycerides may be due to a metabolic disorder affecting lipids. However, anyone may have high triglycerides after eating. In either situation, the direct LDL-C test can determine the amount of LDL in your blood.

A direct LDL-C may be ordered by your healthcare practitioner when prior test results have indicated high triglycerides. In some laboratories, the direct LDL test will automatically be performed when the triglyceride levels are too high to calculate LDL-C. This saves the healthcare practitioner time by not needing to order another test, saves you time by not needing to have a second blood sample drawn, and speeds up the time to provide the test result.

Whether calculated or measured directly, LDL-C values are used to assess your risk for heart disease and help guide decisions about what treatment may be best if you are at borderline, intermediate, or high risk. The results are considered along with other known risk factors of heart disease to develop a plan of treatment and follow up. Treatment options may involve lifestyle changes such as diet and exercise or lipid-lowering medications such as statins. LDL-C tests can also be used to monitor your response to therapy to lower cholesterol.

Read the article on LDL Cholesterol to learn more about LDL-C and what results might mean.

Drugs of abuse testing is the detection of one or more illegal and/or prescribed substances in the urine, blood, saliva, hair, or sweat. Testing detects substances not normally found in the body, with the exception of some hormones and steroids measured as part of sports testing.

Drug abuse testing usually involves an initial screening test followed by a second test that identifies and/or confirms the presence of a drug or drugs. Most laboratories use commercially available tests that have been developed and optimized to screen urine for the “major drugs of abuse.”

For most drugs of abuse testing, laboratories compare results of initial screening with a predetermined cut-off. Anything below that cut-off is considered negative; anything above is considered a positive screening result. In addition, labs might perform testing for masking agents (adulterants). These may either interfere with testing or dilute a urine sample.

Among drugs of abuse, each class of drug may contain a variety of chemically similar substances. Legal substances that are chemically similar to illegal ones can produce a positive screening result. Positive screening tests are considered presumptive. Therefore, screening tests that are positive for one or more classes of drugs are frequently confirmed with a secondary test that identifies the exact substance present using a very sensitive and specific method, such as gas chromatography/mass spectrometry (GC/MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Some of the most commonly screened drug classes are listed in the table below.

Drug class screened Examples of specific drugs identified during confirmation
Amphetamines Methamphetamine, amphetamine
Barbiturates Phenobarbital, secobarbital, pentobarbital, butalbital, amobarbital
Benzodiazepines Diazepam, lorazepam, oxazepam, temazepam, alprazolam
Cannibinoids Marijuana
Cocaine Cocaine and/or its metabolite (benzoylecognine)
Methadone Methadone, methadone metabolite (EDDP)
Opiates Codeine, morphine, metabolite of heroin, oxycodone, oxymorphone, hydrocodone, hydromorphone
Phencyclidine PCP
See a more comprehensive listing of drug classes and drugs of abuse in the table below.

Substances that are not similar to the defined classes can produce negative results even though they are present. Some drugs may be difficult to detect with the standardized assays, either because the test is not set up to detect the drug, such as methylenedioxy-methamphetamine (MDMA, also known as Ecstasy or Molly), fentanyl, methadone, oxycodone (Oxycontin), meperedine, or buprenorphine, or because the drug does not remain in the body long enough to be detected, such as gamma-hydroxybutyrate (GHB).

For sports testing of hormones and steroids, each test performed is usually specific for a single substance and may be quantitative. Athletes, especially those at the national and international levels, are tested for illegal drugs and are additionally prohibited from using a long list of substances called “performance enhancers.”

Groups of drug tests are typically ordered for medical or legal reasons, as part of a “drug-free workplace,” as part of a sports testing program, or to determine compliance with prescribed (pain) medications. People who use these substances ingest, inhale, smoke, or inject them into their bodies. How much of these drugs the body absorbs and their effects depend on the substances, how they interact, their purity and strength, their quantity, timing and method of intake, and an individual’s ability to metabolize and eliminate them from the body.

Some drugs can interfere with the action or metabolism of other medications, or have additive effects, as in the case of taking two drugs that both depress the central nervous system (CNS). Drugs may also have competing effects, as can happen when one drug that depresses the CNS and another that stimulates it are taken.

How is the sample collected for testing?
Urine is the most frequently tested sample in drug abuse screening. Other body samples, such as hair, saliva, sweat, and blood, also may be used but not interchangeably with urine.

Urine and saliva are collected in clean containers. A blood sample is obtained by inserting a needle into a vein in the arm. Hair is cut close to the scalp to collect a sample. A sweat sample is typically collected by applying a patch to the skin for a specified period of time.

Is any test preparation needed to ensure the quality of the sample?
Certain prescription and over-the-counter drugs may give a positive screening result. Examples of false-positive screening results: Vicks nasal spray can test positive for amphetamines; poppy seeds can produce a false-positive for opiates. Prior to testing, you should declare any medications that you have taken and/or for which you have prescriptions so that your results can be interpreted correctly.

Drugs of abuse testing is the detection of one or more illegal and/or prescribed substances in the urine, blood, saliva, hair, or sweat. Testing detects substances not normally found in the body, with the exception of some hormones and steroids measured as part of sports testing.

Drug abuse testing usually involves an initial screening test followed by a second test that identifies and/or confirms the presence of a drug or drugs. Most laboratories use commercially available tests that have been developed and optimized to screen urine for the “major drugs of abuse.”

For most drugs of abuse testing, laboratories compare results of initial screening with a predetermined cut-off. Anything below that cut-off is considered negative; anything above is considered a positive screening result. In addition, labs might perform testing for masking agents (adulterants). These may either interfere with testing or dilute a urine sample.

Among drugs of abuse, each class of drug may contain a variety of chemically similar substances. Legal substances that are chemically similar to illegal ones can produce a positive screening result. Positive screening tests are considered presumptive. Therefore, screening tests that are positive for one or more classes of drugs are frequently confirmed with a secondary test that identifies the exact substance present using a very sensitive and specific method, such as gas chromatography/mass spectrometry (GC/MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Some of the most commonly screened drug classes are listed in the table below.

See a more comprehensive listing of drug classes and drugs of abuse in the table below.

Substances that are not similar to the defined classes can produce negative results even though they are present. Some drugs may be difficult to detect with the standardized assays, either because the test is not set up to detect the drug, such as methylenedioxy-methamphetamine (MDMA, also known as Ecstasy or Molly), fentanyl, methadone, oxycodone (Oxycontin), meperedine, or buprenorphine, or because the drug does not remain in the body long enough to be detected, such as gamma-hydroxybutyrate (GHB).

For sports testing of hormones and steroids, each test performed is usually specific for a single substance and may be quantitative. Athletes, especially those at the national and international levels, are tested for illegal drugs and are additionally prohibited from using a long list of substances called “performance enhancers.”

Groups of drug tests are typically ordered for medical or legal reasons, as part of a “drug-free workplace,” as part of a sports testing program, or to determine compliance with prescribed (pain) medications. People who use these substances ingest, inhale, smoke, or inject them into their bodies. How much of these drugs the body absorbs and their effects depend on the substances, how they interact, their purity and strength, their quantity, timing and method of intake, and an individual’s ability to metabolize and eliminate them from the body.

Some drugs can interfere with the action or metabolism of other medications, or have additive effects, as in the case of taking two drugs that both depress the central nervous system (CNS). Drugs may also have competing effects, as can happen when one drug that depresses the CNS and another that stimulates it are taken.

How is the sample collected for testing?

Urine is the most frequently tested sample in drug abuse screening. Other body samples, such as hair, saliva, sweat, and blood, also may be used but not interchangeably with urine.

Urine and saliva are collected in clean containers. A blood sample is obtained by inserting a needle into a vein in the arm. Hair is cut close to the scalp to collect a sample. A sweat sample is typically collected by applying a patch to the skin for a specified period of time.

Is any test preparation needed to ensure the quality of the sample?

Certain prescription and over-the-counter drugs may give a positive screening result. Examples of false-positive screening results: Vicks nasal spray can test positive for amphetamines; poppy seeds can produce a false-positive for opiates. Prior to testing, you should declare any medications that you have taken and/or for which you have prescriptions so that your results can be interpreted correctly.

Drugs of abuse testing is the detection of one or more illegal and/or prescribed substances in the urine, blood, saliva, hair, or sweat. Testing detects substances not normally found in the body, with the exception of some hormones and steroids measured as part of sports testing.

Drug abuse testing usually involves an initial screening test followed by a second test that identifies and/or confirms the presence of a drug or drugs. Most laboratories use commercially available tests that have been developed and optimized to screen urine for the “major drugs of abuse.”

For most drugs of abuse testing, laboratories compare results of initial screening with a predetermined cut-off. Anything below that cut-off is considered negative; anything above is considered a positive screening result. In addition, labs might perform testing for masking agents (adulterants). These may either interfere with testing or dilute a urine sample.

Among drugs of abuse, each class of drug may contain a variety of chemically similar substances. Legal substances that are chemically similar to illegal ones can produce a positive screening result. Positive screening tests are considered presumptive. Therefore, screening tests that are positive for one or more classes of drugs are frequently confirmed with a secondary test that identifies the exact substance present using a very sensitive and specific method, such as gas chromatography/mass spectrometry (GC/MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Some of the most commonly screened drug classes are listed in the table below.

See a more comprehensive listing of drug classes and drugs of abuse in the table below.

Substances that are not similar to the defined classes can produce negative results even though they are present. Some drugs may be difficult to detect with the standardized assays, either because the test is not set up to detect the drug, such as methylenedioxy-methamphetamine (MDMA, also known as Ecstasy or Molly), fentanyl, methadone, oxycodone (Oxycontin), meperedine, or buprenorphine, or because the drug does not remain in the body long enough to be detected, such as gamma-hydroxybutyrate (GHB).

For sports testing of hormones and steroids, each test performed is usually specific for a single substance and may be quantitative. Athletes, especially those at the national and international levels, are tested for illegal drugs and are additionally prohibited from using a long list of substances called “performance enhancers.”

Groups of drug tests are typically ordered for medical or legal reasons, as part of a “drug-free workplace,” as part of a sports testing program, or to determine compliance with prescribed (pain) medications. People who use these substances ingest, inhale, smoke, or inject them into their bodies. How much of these drugs the body absorbs and their effects depend on the substances, how they interact, their purity and strength, their quantity, timing and method of intake, and an individual’s ability to metabolize and eliminate them from the body.

Some drugs can interfere with the action or metabolism of other medications, or have additive effects, as in the case of taking two drugs that both depress the central nervous system (CNS). Drugs may also have competing effects, as can happen when one drug that depresses the CNS and another that stimulates it are taken.

How is the sample collected for testing?

Urine is the most frequently tested sample in drug abuse screening. Other body samples, such as hair, saliva, sweat, and blood, also may be used but not interchangeably with urine.

Urine and saliva are collected in clean containers. A blood sample is obtained by inserting a needle into a vein in the arm. Hair is cut close to the scalp to collect a sample. A sweat sample is typically collected by applying a patch to the skin for a specified period of time.

Is any test preparation needed to ensure the quality of the sample?

Certain prescription and over-the-counter drugs may give a positive screening result. Examples of false-positive screening results: Vicks nasal spray can test positive for amphetamines; poppy seeds can produce a false-positive for opiates. Prior to testing, you should declare any medications that you have taken and/or for which you have prescriptions so that your results can be interpreted correctly.

A variety of prescription and over-the-counter (OTC) medications, illegal drugs, and household substances can cause drug overdoses. Emergency and overdose drug testing is ordered for single drugs or groups of drugs by an emergency room (ER) health practitioner to detect, evaluate, and monitor a drug overdose.

Drugs and related substances that are ingested or absorbed are typically broken down (metabolized) by the liver over a period of time and then eliminated from the body, primarily in the urine. The rate at which this happens depends on a number of factors ranging from age, weight, and sex to food intake and presence of underlying diseases. Moreover, the development of toxicity depends on the type of substance absorbed or ingested.

Some substances cause symptoms only if they are present in high concentrations or above therapeutic levels. Some common examples of these include:

• Acetaminophen—an ingredient in many over-the-counter preparations; it can cause significant liver damage if recommended doses are exceeded.
• Aspirin (also known as salicylates)—at higher levels, it can cause an acid-base imbalance.
• Therapeutic drugs—used to treat conditions such as heart failure and epilepsy

Some substances can cause symptoms at both low and high concentrations depending on the user. Long-term users of drugs such as alcohol and illegal drugs (drugs of abuse) may be able to tolerate more drug than someone who is taking it for the first time.

Other substances are toxic at any concentration and some have toxic break-down products (metabolites). Examples include:

• Toxic alcohols, including methanol and isopropanol
• Ethylene glycol (antifreeze)

Ingestion of a variety of other drugs and chemicals may cause acute toxicity. This article is limited to the drugs and a few other substances more commonly tested for in the clinical laboratory. Some of the various general categories of substances that may be tested are listed below:

Prescription and over-the-counter (OTC) medications
Overdoses caused by prescription and OTC drugs may be due to:

• Ingestion of too much of a medication
• Interaction of multiple drugs
• A decrease in the body’s ability to eliminate a drug and/or its metabolite; many drugs are processed by the liver; that is, the liver changes the drug into a different form, which is then eliminated from the body. If the liver or kidneys are not working properly, then the drug and/or its metabolite may build up in the body.

A classic example of an OTC drug with a toxic metabolite is acetaminophen, a common pain reliever that is also a component of a variety of other OTC and prescription medicines. One of the metabolites of acetaminophen is toxic to the liver, but the liver is able to detoxify “normal” amounts of it. However, if someone takes more acetaminophen than the liver can process, then the toxic metabolite builds up, damaging the liver and, in some cases, causing liver failure.

Illegal drugs
Overdoses of illegal drugs can also occur. The illicit drugs encountered in the ER depend on their prevalence in the community and whether these substances cause acute symptoms alone or in combination with other substances. Some drugs of abuse are “diverted” prescription medications, such as oxycodone or amphetamine. Some drugs, such as marijuana and other cannabinoids, can linger in the body for days to weeks but rarely cause overdose symptoms. Other substances, such as gamma-hydroxybutyrate (GHB), can cause acute symptoms such as a loss of consciousness but are metabolized so rapidly that testing for them is rarely useful. For more on these, see the article on Drugs of Abuse Testing.

Household substances
There are a wide variety of household substances that may be abused or accidentally ingested. Those commonly involved in emergency drug testing include methanol, isopropyl alcohol, and ethylene glycol (antifreeze), which some people ingest as substitutes for ethanol, also known as grain alcohol. Other poisons, such as rodenticides, aerosol and cleaning products, insecticides, and heavy metals, can also have toxic effects. (For more information, see the web site of the American Association of Poison Control Centers.)

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. Urine and saliva are collected in clean containers. A breath sample is collected by blowing through a tube, into an instrument.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Escherichia coli (E. coli) bacteria commonly occur in nature and are a necessary component of the digestive process. Most strains of E. coli are harmless, but disease-causing (pathogenic) E. coli can cause inflammation of the stomach and intestines (gastroenteritis). Laboratory tests can detect the presence of pathogenic E. coli that produce Shiga toxins.

Multiple subtypes of E. coli cause diarrheal illness, and they are classified by how they cause the disease. For example, some invade the lining of the intestines, causing inflammation, while others produce toxins.

E. coli that produce poisons called Shiga toxins are generally the only type of E. coli that are tested for in clinical settings from stool specimens. The Shiga toxins associated with these infections are so called because they are related to the toxins produced by another type of disease-causing bacteria, Shigella. Shiga toxin-producing E. coli (STEC) may also be called verocytotoxic E. coli (VTEC) or enterohemorrhagic E. coli (EHEC).

A strain of STEC called O157:H7 is the STEC strain responsible for most gastrointestinal illness outbreaks in the U.S. However, non-O157 strains of STEC are gaining recognition, in part due to increased testing for them by clinical laboratories. For example, a 2011 outbreak of E. coli O104:H4, a non-O157 STEC, was associated with travel to Germany and resulted in 32 deaths related to contaminated sprouts. According to the Centers for Disease Control and Prevention, STEC O157 causes about 36% of STEC infections in the U.S., while non-O157 STEC cause the rest.

Outbreaks have been linked to the consumption of contaminated food, including undercooked ground beef, unpasteurized juice, unpasteurized milk, and raw produce such as leafy greens and alfalfa sprouts. STEC may also be transmitted through contaminated water, contact with farm animals or their environment, and from person to person. Even ingesting small numbers of E. coli can cause an infection.

In addition to symptoms of nausea, severe abdominal cramps, watery diarrhea, fatigue, or possible vomiting and low-grade fever, STEC infections are often associated with bloody stools and, less commonly, can lead to serious complications, specifically hemolytic uremic syndrome (HUS). HUS is a result of the toxin entering the blood and destroying red blood cells (hemolysis). It can lead to kidney failure (uremia or the build up of nitrogen wastes in the blood) and can be life-threatening. Signs and symptoms include decreased frequency of urination (evidence of uremia), fatigue, and pale skin due to hemolytic anemia. HUS usually develops about a week after the onset of diarrhea.

About 5-10% of people who are diagnosed with an O157 STEC infection develop HUS. Children, the elderly, and persons with weakened immune systems are at greatest risk. However, most healthy persons recover from a STEC infection within a week and do not develop HUS. Non-O157 Shiga toxin-producing E. coli can cause the same symptoms and complications and likely account for 20-50% of STEC infections in the U.S. annually. Different testing techniques are required to identify O157 and non-O157 Shiga toxin-producing E. coli.

How is the sample collected for testing?

A fresh liquid or unformed stool sample is collected in a clean, dry container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should be taken to the laboratory immediately or refrigerated and taken to the lab as soon as possible. Some laboratories provide transport media to support the survival of the organism from the time of collection until delivery to the laboratory. STEC becomes difficult to detect in the stool after one week of illness, so the timing of sample collection relative to the onset of illness is important.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Epstein-Barr virus (EBV) is a virus that typically causes a mild to moderate illness. Blood tests for Epstein-Barr virus detect antibodies to EBV in the blood and help establish a diagnosis of EBV infection.

Epstein-Barr virus causes an infection that is very common. According to the Centers for Disease Control and Prevention (CDC), most people in the United States are infected by EBV at some point in their lives. The virus is very contagious and easily passed from person to person. It is present in the saliva of infected individuals and can be spread through close contact such as kissing and through sharing utensils or cups.

After initial exposure to EBV, there is a period of several weeks before associated symptoms may appear, called the incubation period. During the acute primary infection, the virus multiplies in number. This is followed by a decrease in viral numbers and resolution of symptoms, but the virus never completely goes away. Latent EBV remains in the person’s body for the rest of that person’s life and may reactivate but usually causes few problems unless the person’s immune system is significantly weakened.

Most people are infected by EBV in childhood and experience few or no symptoms. However, when the initial infection occurs in adolescence, it can cause infectious mononucleosis, commonly called mono, a condition associated with fatigue, fever, sore throat, swollen lymph nodes, an enlarged spleen, and sometimes an enlarged liver. These symptoms occur in about 25% of infected teens and young adults and usually resolve within a month or two.

People with mono are typically diagnosed by their symptoms and the findings from a complete blood count (CBC) and a mono test (which tests for a heterophile antibody). About 25% of those with mono do not produce heterophile antibodies and will have a negative mono test; this is especially true with children. Tests for EBV antibodies can be used to determine whether or not the symptoms these people are experiencing are due to a current infection with the EBV virus.

EBV is the most common cause of mono. According to the CDC, examples of other causes of mono include cytomegalovirus (CMV), hepatitis A, hepatitis B or hepatitis C, rubella, and toxoplasmosis. Sometimes, it can be important to distinguish EBV from these other illnesses. For instance, it may be important to diagnose the cause of symptoms of a viral illness in a pregnant woman. Testing can help to distinguish a primary EBV infection, which has not been shown to affect a developing baby, from a CMV, herpes simplex virus, or toxoplasmosis infection, as these illnesses can cause complications during the pregnancy and may harm the fetus.

It can also be important to rule out EBV infection and to look for other causes of the symptoms. Those with strep throat, an infection caused by group A streptococcus, for instance, need to be identified and treated with antibiotics. A person may have strep throat instead of mono or may have both conditions at the same time.

Several tests for different types and classes of EBV antibodies are available. The antibodies are proteins produced by the body in an immune response to several different Epstein-Barr virus antigens. During a primary EBV infection, the level of each of these EBV antibodies rises and falls at various times as the infection progresses. Measurement of these antibodies in the blood can aid in diagnosis and typically provides the healthcare practitioner with information about the stage of infection and whether it is a current, recent, or past infection.

Epstein-Barr virus (EBV) is a virus that typically causes a mild to moderate illness. Blood tests for Epstein-Barr virus detect antibodies to EBV in the blood and help establish a diagnosis of EBV infection.

Epstein-Barr virus causes an infection that is very common. According to the Centers for Disease Control and Prevention (CDC), most people in the United States are infected by EBV at some point in their lives. The virus is very contagious and easily passed from person to person. It is present in the saliva of infected individuals and can be spread through close contact such as kissing and through sharing utensils or cups.

After initial exposure to EBV, there is a period of several weeks before associated symptoms may appear, called the incubation period. During the acute primary infection, the virus multiplies in number. This is followed by a decrease in viral numbers and resolution of symptoms, but the virus never completely goes away. Latent EBV remains in the person’s body for the rest of that person’s life and may reactivate but usually causes few problems unless the person’s immune system is significantly weakened.

Most people are infected by EBV in childhood and experience few or no symptoms. However, when the initial infection occurs in adolescence, it can cause infectious mononucleosis, commonly called mono, a condition associated with fatigue, fever, sore throat, swollen lymph nodes, an enlarged spleen, and sometimes an enlarged liver. These symptoms occur in about 25% of infected teens and young adults and usually resolve within a month or two.

People with mono are typically diagnosed by their symptoms and the findings from a complete blood count (CBC) and a mono test (which tests for a heterophile antibody). About 25% of those with mono do not produce heterophile antibodies and will have a negative mono test; this is especially true with children. Tests for EBV antibodies can be used to determine whether or not the symptoms these people are experiencing are due to a current infection with the EBV virus.

EBV is the most common cause of mono. According to the CDC, examples of other causes of mono include cytomegalovirus (CMV), hepatitis A, hepatitis B or hepatitis C, rubella, and toxoplasmosis. Sometimes, it can be important to distinguish EBV from these other illnesses. For instance, it may be important to diagnose the cause of symptoms of a viral illness in a pregnant woman. Testing can help to distinguish a primary EBV infection, which has not been shown to affect a developing baby, from a CMV, herpes simplex virus, or toxoplasmosis infection, as these illnesses can cause complications during the pregnancy and may harm the fetus.

It can also be important to rule out EBV infection and to look for other causes of the symptoms. Those with strep throat, an infection caused by group A streptococcus, for instance, need to be identified and treated with antibiotics. A person may have strep throat instead of mono or may have both conditions at the same time.

Several tests for different types and classes of EBV antibodies are available. The antibodies are proteins produced by the body in an immune response to several different Epstein-Barr virus antigens. During a primary EBV infection, the level of each of these EBV antibodies rises and falls at various times as the infection progresses. Measurement of these antibodies in the blood can aid in diagnosis and typically provides the healthcare practitioner with information about the stage of infection and whether it is a current, recent, or past infection.

Red blood cells (RBCs), also called erythrocytes, are cells that circulate in the blood and carry oxygen throughout the body. The RBC count totals the number of red blood cells that are present in your sample of blood. It is one test among several that is included in a complete blood count (CBC) and is often used in the general evaluation of a person’s health.

Blood is made up of a few different types of cells suspended in fluid called plasma. In addition to RBCs, there are white blood cells (WBCs) and platelets. These cells are produced in the bone marrow and are released into the bloodstream as they mature. RBCs typically make up about 40% of the blood volume. RBCs contain hemoglobin, a protein that binds to oxygen and enables RBCs to carry oxygen from the lungs to the tissues and organs of the body. RBCs also help transport a small portion of carbon dioxide, a waste product of cell metabolism, from those tissues and organs back to the lungs, where it is expelled.

The typical lifespan of an RBC is 120 days. Thus the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect RBC production and some conditions may result in significant bleeding. Other disorders may affect the lifespan of RBCs in circulation, especially if the RBCs are deformed due to an inherited or acquired defect or abnormality. These conditions may lead to a rise or drop in the RBC count. Changes in the RBC count usually mirror changes in other RBC tests, including the hematocrit and hemoglobin level.

• If RBCs are lost or destroyed faster than they can be replaced, if bone marrow production is disrupted, or if the RBCs produced do not function normally, or do not contain enough hemoglobin, then you may develop anemia, which affects the amount of oxygen reaching tissues.
• If too many RBCs are produced and released, then you can develop polycythemia. This can cause thicker blood, decreased blood flow and related problems, such as headache, dizziness, problems with vision, and even excessive clotting or heart attack.

A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and the blood cells on the slide are examined and evaluated. Traditionally, trained laboratorians have examined blood smears manually using a microscope. More recently, automated digital systems have become available to help analyze blood smears more efficiently.

A blood smear is a snapshot of the cells that are present in the blood at the time the sample is obtained. The blood smear allows for the evaluation of these cells:

• White blood cells (WBCs, leukocytes) — help fight infections or participate in immune responses
• Red blood cells (RBCs, erythrocytes) — carry oxygen to tissues
• Platelets (thrombocytes) — small cell fragments that are vital to proper blood clotting

These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood is dynamic but is generally maintained by the body within specific ranges.

The drop of blood on the slide used for a blood smear contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. A blood smear examination:

• Compares the WBCs’ size, shape, and general appearance to the established appearance of “normal” cells. It also determines the five different types of WBCs and their relative percentages (manual WBC differential).
• Evaluates the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology)
• Estimates the number of platelets present

A variety of diseases and conditions can affect the number and appearance of blood cells. Examination of the blood smear can be used to support findings from other tests and examinations. For example, RBCs that appear smaller and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs that are not fully mature may add to information from other tests to help make a diagnosis of infection, malignancy, or other conditions.

Erythrocyte sedimentation rate (ESR or sed rate) is a test that indirectly measures the degree of inflammation present in the body. The test actually measures the rate of fall (sedimentation) of erythrocytes (red blood cells) in a sample of blood that has been placed into a tall, thin, vertical tube. Results are reported as the millimeters of clear fluid (plasma) that are present at the top portion of the tube after one hour.

When a sample of blood is placed in a tube, the red blood cells normally settle out relatively slowly, leaving little clear plasma. The red cells settle at a faster rate in the presence of an increased level of proteins, particularly proteins called acute phase reactants. The level of acute phase reactants such as C-reactive protein (CRP) and fibrinogen increases in the blood in response to inflammation.

Inflammation is part of the body’s immune response. It can be acute, developing rapidly after trauma, injury or infection, for example, or can occur over an extended time (chronic) with conditions such as autoimmune diseases or cancer.

The ESR is not diagnostic; it is a non-specific test that may be elevated in a number of these different conditions. It provides general information about the presence or absence of an inflammatory condition.

There have been questions about the usefulness of the ESR in light of newer tests that have come into use that are more specific. However, ESR test is typically indicated for the diagnosis and monitoring of temporal arteritis, systemic vasculitis and polymyalgia rheumatica. Extremely elevated ESR is useful in developing a rheumatic disease differential diagnosis. In addition, ESR may still be a good option in some situations, when, for example, the newer tests are not available in areas with limited resources or when monitoring the course of a disease.

Erythropoietin (EPO) is a hormone produced primarily by the kidneys, with small amounts made by the liver. EPO plays a key role in the production of red blood cells (RBCs), which carry oxygen from the lungs to the rest of the body. This test measures the amount of erythropoietin in the blood.

The body uses a dynamic feedback system to help maintain sufficient oxygen levels and a relatively stable number of RBCs in the blood.

Erythropoietin is produced and released into the blood by the kidneys in response to low blood oxygen levels (hypoxemia). The amount of erythropoietin released depends on how low the oxygen level is and the ability of the kidneys to produce erythropoietin.
EPO is carried to the bone marrow, where it stimulates production of red blood cells. The hormone is active for a short period of time and then eliminated from the body in the urine.
As oxygen levels in the blood rise to normal or near normal levels, the kidneys slow production of EPO.
However, if your kidneys are damaged and do not produce enough erythropoietin, then too few RBCs are produced and you can becomes anemic. Similarly, if your bone marrow is unable to respond to the stimulation from EPO, then you may become anemic. This can occur with some bone marrow disorders or with chronic diseases, such as rheumatoid arthritis. (Read Anemia of Chronic Diseases to learn more.)

If you have a condition that affects the amount of oxygen you breathe in, such as a lung disease, you may produce more EPO to try to compensate for the low oxygen level. People who live at high altitudes may also have higher levels of EPO and so do chronic tobacco smokers.

If you produce too much erythropoietin, which can happen with some benign or malignant kidney tumors and with a variety of other cancers, you may produce too many RBCs (polycythemia or erythrocytosis). This can lead to an increase in the blood’s thickness (viscosity) and sometimes to high blood pressure (hypertension), blood clots (thrombosis), heart attack, or stroke. Rarely, polycythemia is caused by a bone marrow disorder called polycythemia vera, not by increased erythropoietin.

Erythrocyte sedimentation rate (ESR or sed rate) is a test that indirectly measures the degree of inflammation present in the body. The test actually measures the rate of fall (sedimentation) of erythrocytes (red blood cells) in a sample of blood that has been placed into a tall, thin, vertical tube. Results are reported as the millimeters of clear fluid (plasma) that are present at the top portion of the tube after one hour.

When a sample of blood is placed in a tube, the red blood cells normally settle out relatively slowly, leaving little clear plasma. The red cells settle at a faster rate in the presence of an increased level of proteins, particularly proteins called acute phase reactants. The level of acute phase reactants such as C-reactive protein (CRP) and fibrinogen increases in the blood in response to inflammation.

Inflammation is part of the body’s immune response. It can be acute, developing rapidly after trauma, injury or infection, for example, or can occur over an extended time (chronic) with conditions such as autoimmune diseases or cancer.

The ESR is not diagnostic; it is a non-specific test that may be elevated in a number of these different conditions. It provides general information about the presence or absence of an inflammatory condition.

There have been questions about the usefulness of the ESR in light of newer tests that have come into use that are more specific. However, ESR test is typically indicated for the diagnosis and monitoring of temporal arteritis, systemic vasculitis and polymyalgia rheumatica. Extremely elevated ESR is useful in developing a rheumatic disease differential diagnosis. In addition, ESR may still be a good option in some situations, when, for example, the newer tests are not available in areas with limited resources or when monitoring the course of a disease.

Glomerular filtration rate (GFR) is a measure of how well your kidneys are working. The kidney’s primary function is to filter blood. Waste and excess water gets removed and turned into urine. The levels of salts and minerals in blood are adjusted to maintain a healthy balance. In addition, kidneys produce hormones that regulate blood pressure, maintain bone health, and control production of red blood cells.

Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important substances, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to build up in the blood.

Chronic kidney disease (CKD) is a common disease associated with a slow and progressive decrease in kidney function. Diabetes and high blood pressure are the two main causes of CKD. Aging is another risk factor for CKD that is often overlooked. As we age, so do our kidneys, losing about 1% of kidney filtration per year after age of 40.

Kidney diseases tend to progress silently and most cases of early stages of CKD go undiagnosed. Most people have no symptoms until 30-40% of kidney function is lost. Early detection of kidney dysfunction is crucial because it can help to minimize the damage by starting a suitable treatment. Screening for CKD is recommended for high risk groups, such as people with diabetes, high blood pressure (hypertension), heart disease, family history of kidney failure, and the elderly (age 60 and older).

Simple laboratory tests used to evaluate kidney function include the blood creatinine test and/or blood cystatin C test to estimate GFR and urine albumin and urine creatinine clearance.

Measuring glomerular filtration rate (GFR) directly is considered the most accurate way to detect changes in kidney function, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. For this reason, the estimated GFR (eGFR), which represents the best routinely available measurement of kidney function, is usually used. See “How can my actual GFR be determined?” under Common Questions for more information.

The eGFR is calculated according to the formulae recommended by the National Kidney Foundation using measured test results of blood creatinine levels and in special circumstances blood cystatin C levels.

• The most common approach is the eGFR based on blood creatinine level along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and levels increase in the blood. The variations in muscle metabolism and muscle mass can lead to significant differences in measured creatinine levels. The eGFR may not be reliable in amputees, body builders, vegetarians, patients at the extremes of weight and age, and those with rapidly changing kidney function (e.g., acute kidney injury). In addition, some medications can increase creatinine.
• The eGFR calculated based on blood cystatin C level is recommended for confirmatory testing when eGFR based on serum creatinine is less accurate (e.g., elderly, overweight or people with abnormal muscle mass). Cystatin C is a small protein produced at a constant rate by all nucleated cells in the body and is found in various body fluids, including blood. It is filtered by the kidneys and broken down at a constant rate. It does not reenter the blood circulation or get eliminated in the urine. Unlike creatinine, which is affected by muscle mass, cystatin C is affected by age, inflammation, obesity, and diabetes. Cystatin C is a relatively new biomarker and extensive research is ongoing to define its optimal use. See “Is there anything else I should know?” under Common Questions for more information.

Different equations may be used to calculate eGFR. The following two are most common and require a blood creatinine result, age, and assigned values based upon sex and race.

• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (2009)—recommended by the National Kidney Foundation for calculating eGFR in adults.
• Modification of Diet in Renal Disease Study (MDRD) equation—some laboratories continue to use this equation.

Additional sets of CKD-EPI equations recommended by the National Kidney Foundation for adults that use cystatin C level to calculate eGFR, along with age and gender, include:

• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation (2012)
• Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine and cystatin C equation (2012)

The results reported using one of the above equations versus the other will not be identical but should give a healthcare practitioner similar information.

Separate equations, based on serum creatinine levels and height, are recommended for use in youth age 18 years and under. One is the updated Schwartz equation that takes the blood urea nitrogen (BUN) level into consideration.

The eGFR equations are not valid for those who are 70 years of age or older because muscle mass normally decreases with age.

See “Could I calculate my own eGFR?” under Common Questions for more information about these eGFR equations.

Glomerular filtration rate (GFR) is a measure of how well your kidneys are working. The kidney’s primary function is to filter blood. Waste and excess water gets removed and turned into urine. The levels of salts and minerals in blood are adjusted to maintain a healthy balance. In addition, kidneys produce hormones that regulate blood pressure, maintain bone health, and control production of red blood cells.

Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important substances, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to build up in the blood.

Chronic kidney disease (CKD) is a common disease associated with a slow and progressive decrease in kidney function. Diabetes and high blood pressure are the two main causes of CKD. Aging is another risk factor for CKD that is often overlooked. As we age, so do our kidneys, losing about 1% of kidney filtration per year after age of 40.

Kidney diseases tend to progress silently and most cases of early stages of CKD go undiagnosed. Most people have no symptoms until 30-40% of kidney function is lost. Early detection of kidney dysfunction is crucial because it can help to minimize the damage by starting a suitable treatment. Screening for CKD is recommended for high risk groups, such as people with diabetes, high blood pressure (hypertension), heart disease, family history of kidney failure, and the elderly (age 60 and older).

Simple laboratory tests used to evaluate kidney function include the blood creatinine test and/or blood cystatin C test to estimate GFR and urine albumin and urine creatinine clearance.

Measuring glomerular filtration rate (GFR) directly is considered the most accurate way to detect changes in kidney function, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. For this reason, the estimated GFR (eGFR), which represents the best routinely available measurement of kidney function, is usually used. See “How can my actual GFR be determined?” under Common Questions for more information.

The eGFR is calculated according to the formulae recommended by the National Kidney Foundation using measured test results of blood creatinine levels and in special circumstances blood cystatin C levels.

The most common approach is the eGFR based on blood creatinine level along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and levels increase in the blood. The variations in muscle metabolism and muscle mass can lead to significant differences in measured creatinine levels. The eGFR may not be reliable in amputees, body builders, vegetarians, patients at the extremes of weight and age, and those with rapidly changing kidney function (e.g., acute kidney injury). In addition, some medications can increase creatinine.
The eGFR calculated based on blood cystatin C level is recommended for confirmatory testing when eGFR based on serum creatinine is less accurate (e.g., elderly, overweight or people with abnormal muscle mass). Cystatin C is a small protein produced at a constant rate by all nucleated cells in the body and is found in various body fluids, including blood. It is filtered by the kidneys and broken down at a constant rate. It does not reenter the blood circulation or get eliminated in the urine. Unlike creatinine, which is affected by muscle mass, cystatin C is affected by age, inflammation, obesity, and diabetes. Cystatin C is a relatively new biomarker and extensive research is ongoing to define its optimal use. See “Is there anything else I should know?” under Common Questions for more information.
Different equations may be used to calculate eGFR. The following two are most common and require a blood creatinine result, age, and assigned values based upon sex and race.

Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (2009)—recommended by the National Kidney Foundation for calculating eGFR in adults.
Modification of Diet in Renal Disease Study (MDRD) equation—some laboratories continue to use this equation.
Additional sets of CKD-EPI equations recommended by the National Kidney Foundation for adults that use cystatin C level to calculate eGFR, along with age and gender, include:

Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) cystatin C equation (2012)
Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine and cystatin C equation (2012)
The results reported using one of the above equations versus the other will not be identical but should give a healthcare practitioner similar information.