Cytomegalovirus

Definition

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

Purpose

Consequences of 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 doesn't mean the person has lifetime immunity. After an infection, this virus, like all members of the herpesvirus 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.

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.

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. 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.

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.

Further Reading

For Your Information

Books

• American Association of Blood Banks. Technical Manual, 12th ed. Bethesda: American Association of Blood Banks, 1996, pp. 579, 581.
• Fischback, Francis. A Manual of Laboratory and Diagnostic Tests, 5th ed. Philadelphia: Lippincott, 1996, pp. 539-540.
• Isada, Carlos M., et al. Infectious Diseases Handbook. Hudson: Lexi-Comp Inc., 1995, pp. 321-322.
• Mayo Medical Laboratories. Interpretive Handbook. Rochester, Minnesota: Mayo Medical Laboratories, 1997, pp. 189-191.

  Periodicals

• Laliberte, Kimberly A. "CMV Screening: Interpreting the Test Result." Advance for Medical Laboratory Professionals, (July 14, 1997): 12-13, 17.

Gale Encyclopedia of Medicine. Gale Research, 1999.