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Interpretation of Laboratory Tests

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 common serologic test for measuring antibody to CMV.

  • A positive test for CMV IgG indicates that a person was infected with CMV at some time during their life, but does not indicate when a person was infected. This applies for persons ≥12 months of age when maternal antibodies are no longer present.
  • Measurement of CMV IgG in paired samples taken one to three months apart can be used to diagnose primary infection; seroconversion (1st sample IgG negative, 2nd sample IgG positive) is clear evidence for recent primary infection.
  • The presence of CMV IgM cannot be used by itself to diagnose primary CMV infection because IgM can also be present during secondary CMV infection, which includes reinfection with a different strain or reactivation of latent CMV acquired in the past. IgM positive results in combination with low IgG avidity results are considered reliable evidence for primary infection, with limitations of avidity testing described below.
  • IgG avidity testing may provide useful information regarding timing of infection. IgG antibodies produced following primary CMV infection have low avidity (low binding strength). Two to four months following infection, IgG antibodies mature to high-avidity (high binding strength). Therefore, avidity assays can be used to assess low avidity (which indicates recent infection) versus high avidity (past infection). However, not all avidity assays have been validated and, therefore, should be interpreted with caution. The review by Prince and Lapé-Nixon (CVI, 21:1377, 2014) lists commercial tests for CMV IgG avidity that can be purchased in the United States, and describes challenges with interpreting CMV IgG avidity testing results.

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