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Cytomegalovirus Antibody Screening Test

Cytomegalovirus Antibody Screening Test

Definition

Cytomegalovirus (CMV) is a common human virus. Antibodies to CMV are evidence of a current or past infection.

Purpose

Consequences of a CMV infection can be devastating in a pregnant woman, a transplant patient, or a person with human immunodeficiency virus (HIV). Antibody screening helps control the infection risk for these groups.

In a healthy, nonpregnant person, CMV infection is almost never serious. Symptoms, if present, are mild, often resembling infectious mononucleosis due to Epstein-Barr virus. Antibody screening distinguishes between these two infections.

Description

When first exposed to CMV, a person's immune system is triggered and quickly makes antibodies to fight the virus. Antibodies are special proteins designed to attack and destroy foreign material, in this case, the cytomegalovirus.

The test combines a person's serum with a substance to which CMV antibodies attach. This antibody-antigen complex is measured and the amount of original antibody determined. If positive for antibodies, the serum is diluted, or titered, and the test repeated until the serum is so dilute it no longer gives a positive result. The last dilution that gives a positive result is the titer reported.

A test positive for CMV antibodies means the person has been infected with the virus, either currently or in the past; it does not mean the person has lifetime immunity. After an infection, this virus, like all members of the herpes virus group, can stay hidden inside a person and cause infection if the person's immune system later weakens and antibody protection decreases. In fact, reactivation of such hidden (or latent) infection is not at all uncommon and usually occurs without symptoms.

Transplant patients and people with weakened immune systems, including those with HIV, are vulnerable to infection from several routes, including from another person, from a donated organ or transfused blood, or from reactivation of a past infection. Before transplant, both the recipient and donor are usually tested for antibodies. A recipient who has never had CMV (negative for antibodies), should not receive an organ from a donor who has had CMV (positive for antibodies). CVM infection can be associated with organ rejection, or can cause illness such as pneumonia, hepatitis, or death. Similarly, blood is usually screened for CMV antibodies before being transfused into a person with a weakened immune system.

CMV infection is the most common congenital infection (existing at birth). The infection, passed from mother to baby, can cause permanent mental or physical damage, or death. The antibody screening test tells a woman whether or not she has antibody protection against the virus in case she is exposed during pregnancy. Pregnant women 25 years and older who are immune to CMV are much less likely to pass the virus to their babies than younger women who have never been exposed to CMV.

Tests that measure a specific type of antibody help tell the difference between a current and a past infection. Immunoglobulin M (IgM) antibodies appear at the beginning of an infection and last only weeks. Immunoglobulin G (IgG) antibodies appear 10-14 days later and can last a lifetime. A person suspected of having a current infection should be tested at the beginning of the infection and again 10-14 days later.

The CMV antibody screening test is also called the transplant reaction screening test. Results are usually available the following day.

It may be possible to test fetal blood for certain antibodies to CMV virus by drawing a blood sample from the umbilical cord. This may be an important test to add to prenatal care, since newborn babies with CMV often show no symptoms.

Preparation

The adult CMV antibody screening test requires 5 mL of blood. Collection of the sample takes only a few minutes.

Aftercare

Discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs to the puncture site relieve discomfort.

Normal results

A person without previous exposure to CMV will test negative.

Abnormal results

The presence of antibodies means the person has been infected with CMV, either now or in the past. An antibody titer at least four times higher at the end of the illness than at the beginning, or the presence of IgM antibodies, indicates a recent or current first time infection.

People with weak immune systems may not generate antibodies against CMV. A current infection in a transplant patient or a person with HIV is confirmed with other tests, such as viral culture.

Resources

PERIODICALS

Fowler, Karen B., Sergio Stagno, and Robert F. Pass. "Maternal Immunity and Prevention of Congenital Cytomegalovirus Infection." JAMA, The Journal of the American Medical Association February 26, 2003: 1008.

Gerber, Stefan, et al. "Prenatal Diagnosis of Congenital Cytomegalovirus Infection by Detection of Immunoglobulin M Antibodies to the 70-d Heat Shock Protein in Fetal Serum." American Journal of Obstetrics and Gynecology October 2002: 955.

KEY TERMS

Antibody A special protein built by the body as a defense against foreign material entering the body.

Cytomegalovirus (CMV) A common human virus causing mild or no symptoms in healthy people, but permanent damage or death to an infected fetus, a transplant patient, or a person with HIV.

Titer A dilution of a substance with an exact known amount of fluid. For example, one part of serum diluted with four parts of saline is a titer of 1:4.

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Immune Complex Test

Immune Complex Test

Definition

These tests evaluate the immune system, whose function is to defend the body against such invaders as bacteria and viruses. The immune system also plays a role in the control of cancer, and is responsible for the phenomena of allergy, hypersensitivity, and rejection problems when organs or tissue are transplanted.

One of the ways the immune system protects the body is by producing proteins called antibodies. Antibodies are formed in response to another type of protein called an antigen (anything foreign or different from a natural body protein). Immune complex reactions occur when large numbers of antigen-antibody complexes accumulate in the body.

Purpose

The purpose of the immune complex test is to demonstrate circulating immune complexes in the blood, to estimate the severity of immune complex disease, and to monitor response to therapy.

Precautions

Because this test is requested when the physician suspects that a patient's immune system is not functioning properly, special care should be taken during and after blood is drawn. For example, the venipuncture site should be kept clean and dry to avoid any chance of infection.

Description

Immune complexes are normally not detected in the blood. However, when immune complexes are produced faster than they can be cleared by the system, immune complex disease may occur. Examples of such disorders are drug sensitivity, rheumatoid arthritis, and a disease called systemic lupus erythematosus, or SLE.

The method generally used for detecting immune complexes is examination of a tissue obtained by biopsy (removal and examination of tissue sample) and the subsequent use of different staining techniques with specific antibodies. However, since tissue biopsies do not provide information about the level of complexes still in the circulatory system, serum assays obtained from blood samples which indirectly detect circulating immune complexes are useful. However, due to the variability of these complexes, several test methods may be used. Also, as most immune complex assays have not been standardized, more than one test may be required to achieve accurate results.

Preparation

This test requires a blood sample. It is not necessary for the patient to be in a fasting (nothing to eat or drink) state before the test.

Risks

Risks for this test are minimal, but may include slight bleeding from the blood-drawing site, fainting or feeling lightheaded after venipuncture, or hematoma (blood accumulating under the puncture site).

Normal results

Normally, immune complexes are not detected in the blood.

KEY TERMS

Antibody A (immunoglobulin) molecule that interacts with a specific antigen. Antibodies provide protection from microscopic invaders like bacteria.

Antigen Any substance that is capable under certain circumstances of producing an immune response either from antibodies or T-cells; bacteria are often antigens.

Autoimmune disorder A disorder caused by a reaction of an individual's immune system against the organs or tissues of the body. Autoimmune processes can have different results: slow destruction of a particular type of cell or tissue, stimulation of an organ into excessive growth, or interference in function.

Biopsy The removal and examination, usually under a microscope, of tissue from the living body. Used for diagnosis.

Systemic lupus erythematosus A chronic disease of the connective tissues in the body; characterized by involvement of the skin, joints, kidneys, and serosal membranes (membranes that form the outer covering of organs in the abdomen or chest).

Abnormal results

The presence of detectable immune complexes in the blood is important in the diagnosis of autoimmune diseases, such as SLE and rheumatoid arthritis. However, for definitive diagnosis, the results of other studies must be considered with the presence of any immune complex. For example, immune complexes are associated with high numbers of a component called antinuclear antibodies in the diagnosis of systemic lupus erythematosus. A different example are the kidneys. Because of their filtering functions, elements in the kidneys called renal glomeruli can be affected by immune complexes. In such cases, renal biopsy is used to provide conclusive evidence for immune complex.

Resources

BOOKS

Jacobs, David S., et al. Laboratory Test Handbook. 4th ed. New York: Lexi-Comp Inc., 1996.

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Immune Complex Test

Immune complex test

The immune complex test is a test designed to evaluate the status or proper functioning of the immune system . The criterion used to evaluate the operation of the immune system is via the presence of so-called immune complexes.

An immune complex is an association formed between large numbers of antigens and the corresponding antibodies. The latter are produced in a specific response to the presence of the antigen , which has been perceived as being foreign by the body's immune system. The individual antigen-antibody complexes can associate together to form the interlocking network that represents an immune complex.

Normally, immune complexes are removed from the bloodstream by specialized cells of the spleen called macrophages and by other specialized cells located in the liver. However, if this clearance does not occur, the immune complexes will continue to circulate, and will become trapped in the kidneys, lung, skin, joints, or blood vessels. The specific location depends on the composition of the complex. Their presence will cause inflammation and can lead to tissue damage.

Immune complexes can develop as a result of what is termed a low-grade persistent infection. Examples include Streptococcus viridans infection of the blood, Staphylococcus heart infections, and viral hepatitis . Second, immune complexes can form in response to the continued exposure to an antigen, such as the repeated inhalation of mold in a farming or animal care facility. Finally, immune complexes are often a hallmark of autoimmune diseases. The continual response of the body's immune system overloads the ability of the body to remove the immune complexes that form. Examples of autoimmune diseases for which an immune test is beneficial in terms of diagnosis are systemic lupus erythematosus, rheumatoid arthritis, Lyme disease and human immunodeficiency virus infection.

Being able to test for the presence of abnormal levels of immune complexes can alert the physician to an abnormal function of the immune system, such as an autoimmune disorder.

Immune complexes can be detected by the application of special stains to tissue that has been obtained from a patient. The stains contain antibodies that bind to the complexes and this binding is highlighted by the presence of the staining agent. This test is useful because it directly detects the presence of the immune complexes. However, for routine clinical use, this method is cumbersome and invasive. This has stimulated the development of blood tests that indirectly detect the complexes in the blood serum.

There are several methods available. Often more than one will be used to test the same sample. This is because the test methods are not yet uniformly standardized across the medical community. But, if the results from several tests are positive for immune complexes, the validity of the diagnosis is ensured.

Immune complex tests include the Raji cell, C1q binding, conglutinin, and anti-C3 assays. The Raji cell assay, for example, detects the immune complexes following the binding of the complexes with an immune molecule called complement . In addition, the complement has been labeled with a compound known as fluorescein isothiocyanate. The latter compound is able to fluoresce when light of a certain wavelength is shone on it. The detection occurs in a machine called a flow cytometer, in which fluid moves past a detector that is programmed to detect certain chemical aspects. In the Raji cell assay, detection of the fluorescent isothiocyanate indicates the presence of the immune complex.

A normal result in an immune complex test is a negative result. In other words, immune complexes are normally absent.

See also Antibody-antigen, biochemical and molecular reactions; Laboratory techniques in immunology

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Cytomegalovirus Antibody Screening Test

Cytomegalovirus Antibody Screening Test

Definition

Cytomegalovirus (CMV) is a common human virus. When first exposed to CMV, a healthy person's immune system is triggered and quickly makes antibodies to fight the virus. Specialized screening tests can be done to document the presence or absence of antibodies specific to the cytomegalovirus. Antibodies to CMV can be evidence of a current or a past infection.

Purpose

Up to 85% of people have antibodies to CMV by the time they are 40 years old. In a healthy, nonpregnant person, CMV infection is almost never serious. Symptoms, if present, are mild, often resembling infectious mononucleosis caused by the Epstein-Barr virus. However, consequences of CMV infection can be devastating to the fetus, transplant patients, patients with human immunodeficiency virus (HIV) and other patients with suppressed immune systems. People with weakened immune systems are vulnerable to infection from several routes, including from another person, from a donated organ or transfused blood, or from reactivation of a past infection. CMV is related to the herpes simplex and varicella (chickenpox) viruses in that it remains dormant in the body but can resurface with or without symptoms.

Antibody screening helps control the infection risk for high risk groups. For instance, before a transplant, both the recipient and donor are usually tested for antibodies. A recipient who has never had CMV (negative for antibodies) should not receive an organ from a donor who has had CMV (positive for antibodies) because active infection could be acquired by the recipient. CVM infection in this context can be associated with organ rejection, or can cause illness such as pneumonia, hepatitis, or death. In some transplant patients, particularly after bone marrow transplantation, testing may be done as often as every week to screen for new or recurrent infection. Reports have shown that by regularly testing transplant recipients for presence of CMV antigens, clinicians can predict human CMV disease and begin treatment before symptoms begin.

Women who do not have evidence of previous CMV infection (i.e., antibody screen negative) should try to avoid exposure to CMV in pregnancy. CMV infection is the most common congenital infection. The infection, passed from mother to baby, can cause permanent mental or physical damage, or death and newborns often have no symptoms. However, CMV antibody screening is not routinely ordered as part of prenatal care.

Blood is usually screened for CMV antibodies before being transfused into a person with a weakened immune system. CMV infection can be very similar to illness due to Epstein-Barr virus. If there is a need to know the specific source of a patient's symptoms, antibody screening distinguishes between these two infections.

Precautions

Tests for CMV are performed on serum or biopsied tissue. Blood is collected by venipuncture by a nurse or phlebotomist. When collecting blood or handling tissue the health care provider should observe universal precautions to prevent transmission of bloodborne pathogens. Tests for immunoglobulin M (IgM) antibodies to CMV may be falsely positive in persons with rheumatoid arthritis. It is important to remember that an antibody screening test may not be useful in a person without an intact immune system. People with weak immune systems may not generate antibodies against CMV. A suspected infection in a transplant patient or a person with AIDS is confirmed with other tests, such as a viral culture, examination of biopsied material for cytopathic effects, or a viral DNA assay such as the polymerase chain reaction.

Description

The most frequently used antibody test for CMV is an enzyme linked immunosorbent assay (ELISA). In this test, CMV antigens are attached to the bottom of the wells of a microtiter plate. An initial dilution of the patient's serum is added to a well of the plate and incubated. If antibodies to the CMV virus are present they attach to the CMV antigens in the well. The plate is washed, and antihuman immunoglobulin G (IgG) or IgM conjugated to alkaline phosphatase is added. This antibody will attach to any antigen-antibody complexes in the wells. The plate is washed again, and substrate (e.g., p-nitrophenylphosphate) is added. The plate is incubated to allow the enzyme to split the phosphate from the substrate, which results in production of a yellow color. After incubating, a reagent (strong alkali) is added to stop the enzyme reaction and the optical density of the wells is measured. The test is interpreted as positive if the optical density of the patient's sample is equal to or higher than the low-level standard. Multiple calibrators can be used to quantify the amount of anti-CMV in the patient's sample. Tests that measure a specific type of antibody help differentiate between a current and a past infection. IgM antibodies appear at the beginning of an infection and last only weeks. IgG antibodies appear 10-14 days later and can last a lifetime. A person suspected of having a current infection should be tested at the beginning of the infection and again 10-14 days later. CMV also can be detected from DNA samples. New techniques allow for sampling of newborn blood taken from the umbilical cord.

The CMV antibody screening test is also called the transplant reaction screening test. In 2004, new data began to show that real-time polymerase chain reaction (PCR) assays were the new standard for assessing transplant patients for CMV.

Preparation

This test requires 5 mL of blood. Collection of the sample takes only a few minutes.

Aftercare

Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Applying warm packs to the puncture site relieves discomfort.

Complications

The most common complication is a bruise at the site of the puncture or excessive bleeding. The patient can apply moist warm compresses if there is any discomfort.

Results

A person without previous exposure to CMV will test negative. A positive test for CMV antibodies means the person is infected or has been infected previously with the virus. An antibody titre that is significantly higher at the end of the illness than at the beginning, or the presence of IgM antibodies, indicates a recent or current first-time infection. A positive antibody test does not mean that the person has lifetime immunity. After an infection, this virus, like all members of the herpesvirus group, can stay dormant inside a person and cause infection if the person's immune system later weakens and antibody protection decreases. In fact, reactivation of such latent infection is not uncommon and usually occurs without symptoms.

KEY TERMS

Antibodies— Proteins made by the body to attach to foreign molecules called antigens and aid in their destruction.

Antigen— A marker on the surface of a molecule that stimulates an immune response and can be identified using the specific antibody formed against it.

Congenital— Existing at birth.

Titre— A term denoting the concentration of an antibody. The titre is defined as the reciprocal of the highest dilution of the sample that gives a positive reaction.

Health care team roles

CMV antibody tests are ordered and interpreted by a physician. The sample is usually collected by a nurse or phlebotomist. Testing is performed by clinical laboratory scientists/medical technologists. Nurses, nurse practitioners, and physician assistants will educate patients about the purpose of the test, the nature of CMV infection, and the consequences specific to an individual's situation.

Resources

BOOKS

Fischbach, Frances Talaska. A Manual of Laboratory and Diagnostic Tests, 5th ed. Lippincott, 2000: 607-608.

Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2001. Lange Medical Books/McGraw-Hill, 2001: 1316-1317.

PERIODICALS

"CMV-DNA and pp65-antigen Tests Predict Human Cytomegalovirus Symptom Onset." Genomics & Genetics Weekly (Oct. 1, 2004):38.

Gerber, Stefan, et al. "Prenatal Diagnosis of Congenital Cytomegalovirus Infection by Detection of Immunoglobulin M Antibodies to the 70-kd Heat Shock Protein in Fetal Serum." American Journal of Obstetrics and Gynecology (Oct. 2002):955-960.

"Real-time PCR is Standard for Quantitative CMV Viremia Assessment in Transplants." Medical Devices & Surgical technology Week (Oct. 24, 2004):111.

OTHER

"CMV Screening." Centers for Disease Control. 2001. 〈http://www.cdc.gov/ncidod/diseases/cmv.htm〉.

MedLine Plus. 2001. 〈http://www.nlm.nih.gov/medlineplus/ency/article/000663.htm〉.

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