Cytomegalovirus Antibody Screen

After primary infection, cytomegalovirus (CMV) remains latent in white blood cells. The presence of CMV antibodies indicates past infection with this virus. In an immunocompromised patient, CMV can be reactivated to cause active infection. Administration of blood or tissue from a seropositive donor may cause active CMV infection in CMV-seronegative organ transplant recipients or neonates, especially those born prematurely.

Antibodies to CMV can be detected by several methods, including passive hemagglutination, latex agglutination, enzyme immunoassay, and indirect immunofluorescence. The complement fixation test is only 60% sensitive compared with other assays and should not be used to screen for CMV antibodies. Screening tests for CMV antibodies are qualitative; they detect the presence of antibody at a single low dilution. In quantitative methods, several dilutions of the serum sample are tested to indicate acute infection with CMV.

Purpose

• To detect past CMV infection in donors and recipients of organs and blood
• To screen for CMV infection in infants who require blood transfusions or tissue transplants
• To detect past CMV infection in immunocompromised patients

Patient preparation

• As appropriate, explain the purpose of the test to the patient or the parents of an infant.
• Tell the patient that the test requires a blood sample and who will perform the venipuncture and when.
• Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the sample takes less than 3 minutes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 5-ml red-top tube.
• Allow the blood to clot for at least 1 hour at room temperature.

Precautions

• Handle the sample gently to prevent hemolysis.
• Transfer the serum to a sterile tube or vial, and send it to the laboratory.
• If transfer must be delayed, store the serum at 39.2° F (4° C) for 1 to 2 days or at -4° F (-20° C) for longer periods to avoid contamination.

Reference values

Patients who have never been infected with CMV have no detectable antibodies to the virus. Immunoglobulin G (IgG) and IgM are normally negative.

Abnormal findings

A serum sample collected early during the acute phase or late in the convalescent stage may not contain detectable IgG or IgM antibodies to CMV. Therefore, a negative result doesn't preclude recent infection. More than a single sample is needed to ensure accurate results.

A serum sample that tests positive for antibodies at this single dilution indicates that the patient has been infected with CMV and that his white blood cells contain latent virus capable of being reactivated in an immunocompromised host. Immunosuppressed patients who lack antibodies to CMV should receive blood products or organtransplants from donors who are also seronegative. Patients with CMV antibodies don't require seronegative blood products.

Interfering factors

• Hemolysis due to rough handling of the sample.
• Failure to store serum at the proper temperature.