When the body is infected with the mumps virus, it defends itself by producing two types of antibodies in sequence: first, IgM, which appears 3 to 28 days after the onset of symptoms and is present for a few weeks, then IgG, which appears a few days after IgM and lasts 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 (MMR or MMRV vaccine). Anti-mumps IgG and/or IgM testing is used to diagnose the presence of or immunity against the disease.
The combined anti-mumps 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 low (inconclusive) positive result may occur during seroconversion (replacement of IgM with IgG) or may indicate old immunity (previous infection). Mumps are a reportable disease.