Cytomegalovirus (CMV) is a common virus. In North America, between 50% and 85% of the population has had a CMV infection, usually during childhood, without experiencing any symptoms (influenza- or mononucleosis-like symptoms). The body defends against a CMV infection by first producing IgM antibodies, which gradually disappear and are replaced after a few months by IgG antibodies, which remain detectable for the rest of that person’s life. For its part, the CMV virus is still present, but in a latent form that can, however, be reactivated.
Positive CMV IgG and IgM test results in a symptomatic individual likely indicate a recent first-time exposure to CMV or a reactivation of the virus. IgG retesting two to three weeks later is done to confirm an active infection. If the exposure is very recent, the results might be positive for IgM only. When the IgG and IgM results are both negative in a symptomatic person, these symptoms are probably not due to the CMV, and the person’s immune system probably has an abnormality and is not producing enough antibodies.