Given the difficulty of diagnosing rubella clinically, serologic testing (for the presence of IgM or a ≥4-fold rise in IgG titer) is generally used for diagnosis.

• If the IgM sample taken within the first 4 days of rash is negative but clinical suspicion remains, testing should be repeated; IgM antibody titers are generally positive for up to 6 weeks.
• Congenital rubella can be diagnosed by detection of IgM antibodies, although titers may be negative during the first month; by isolation of the virus from throat swabs, urine, or CSF; and/or by an IgG titer that does not decline at the expected rate of a twofold dilution per month.
• In the United States, screening of pregnant women for rubella IgG antibodies is part of routine prenatal care; seronegative women should be vaccinated postpartum.