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Interpretation of Laboratory Tests

Active infection with CMV can be diagnosed by PCR or viral culture of CMV from urine, saliva, throat swab specimens or other body tissues. Serologic tests that detect CMV antibodies ( IgM and IgG antibody to CMV) are widely available from commercial laboratories. The enzyme-linked immunosorbent assay (ELISA) is the most commonly available serologic test for measuring antibody to CMV. Various fluorescence assays and indirect hemagglutination and latex agglutination tests are also available.

A positive test for CMV IgG indicates that a person was infected with CMV at some time during their life but the IgG test cannot determine when a person was infected. However, if antibody tests of paired acute- and convalescent-phase serum samples show a fourfold rise in IgG antibody and CMV IgM antibody is present or CMV virus is cultured from a urine or throat specimen, an active CMV infection is present.

The presence of CMV IgM is not solely indicative of primary infection. CMV IgM is detectable when a person 1) is newly infected, 2) has been infected in the past but recently re-exposed to CMV, 3) is undergoing reactivation of CMV infection that was acquired in the past, or 4) has a false-positive test result. Thus, the presence of CMV IgM should not be used by itself to diagnose primary CMV infection.

Recently, IgG avidity assays, which measure antibody maturity, have been shown to reliably detect recent primary CMV infection. When a person is infected with CMV for the first time, the body produces low-avidity IgG. After 2-4 months, the body begins to produce high-avidity CMV IgG. Low CMV IgG avidity suggests a primary CMV infection occurred within the past 2-4 months. High CMV IgG avidity suggests that CMV infection occurred at some point in the past. In the United States, CMV IgG avidity tests are not yet widely available commercially.

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