Suspected CRS cases can be laboratory confirmed in new-borns and young infants by detection of rubella-specific IgM (See Figure 4.2).
A suspected CRS case should be tested as close to birth as possible and again at 1 month of age if the initial IgM test is negative. At 6 months of age, only about 50% of cases will continue to have detectable levels of rubella IgM in serum [17]. Among infants that are clinically suspicious for CRS but were not identified within the timeframe to measure rubella IgM after birth (1-3 months), serial testing of serum after 9 months of age (when maternal antibody has waned) for IgG can confirm CRS. Maternal antibody normally diminishes at a rate of a 2-fold decline per month. It is necessary to demonstrate sustained levels of rubella IgG from the infant in order to confirm CRS.