When the body is infected with the rubella 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, whether due to a prior infection by the virus or due to vaccination.
IgG levels are tested primarily in women before or in early pregnancy to ensure they are protected against rubella (risk of fetal malformation during the first trimester). A rubella IgG level above 10 units per millilitre of blood (10 IU/mL) provides adequate protection. An inconclusive result (5.00–9.99 IU/mL) does not guarantee sufficient immunity against infection.
The combined anti-rubella 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.