When the body is infected with the measles virus, it defends itself by producing two types of antibodies in sequence: first, IgM, which appears 3 to 7 days after the onset of symptoms and is present for a few weeks, then IgG, which appears a few days after IgM and lasts for the rest of that person’s life. IgG provides long-term immunity (protection), whether due to a prior infection by the virus or due to vaccination. Testing for anti-measles IgG and/or IgM is therefore used to diagnose the presence of the disease or the status of immunity against infection. The combined anti-measles IgG and IgM test is sometimes used to detect a current or very recent infection.
Negative IgG results accompanied by positive IgM results allow for rapid diagnosis of infection, but they must be confirmed by the appearance (or significant increase) of IgG levels one to three weeks later. A slightly positive result may occur during seroconversion (replacement of IgM with IgG) or indicate old immunity (previous infection). Measles is a reportable disease.