Most CMV infections are rarely diagnosed because the virus usually produces few, if any, symptoms. However, people who have had CMV develop antibodies to the virus which remain in their body for the rest of their life. Two types of CMV antibodies may be found in the blood: IgM and IgG.
IgM antibodies are the first to be produced by the body in response to a CMV infection. They are present in most individuals within a week or two after the initial exposure. After several months, the level of CMV IgM antibody usually falls below detectable levels. IgG antibodies are produced by the body several weeks after the initial CMV infection to provide long-term protection. Levels of IgG rise during the active infection, then stabilize as the CMV infection resolves and the virus becomes inactive.
Once a person has been exposed to CMV, they will have some measurable amount of CMV IgG antibody in their blood for the rest of their life. CMV IgG antibody testing can be used, along with IgM testing, to help confirm the presence of a recent or previous CMV infection. If both CMV IgG and IgM are present in a symptomatic patient, then it is likely that he or she has either recently been exposed to CMV for the first time or that a previous CMV infection has been reactivated. This can be confirmed by measuring IgG levels again two or three weeks later.
A high level of IgG is not as important as an actively rising level. If there is a four-fold increase in IgG between the first and second sample, then the patient has an active CMV infection (primary or reactivated).