Rubella Antibodies, IgM

The Rubella Antibodies, IgM test (rubella IgM; German measles specific IgM) assists in identifying acute primary rubella and can help differentiate primary infection from re-exposure. A single acute-phase specimen that is IgM reactive supports recent infection. Rubella-specific IgM is typically detectable in most infected individuals within three weeks of rash onset and in many vaccine recipients by three weeks. In infants with congenital infection, IgM responses generally appear between two and 12 weeks after birth.

A negative IgM result at birth does not exclude congenital rubella; the likelihood of detecting IgM in cord blood declines as the interval between conception and fetal infection increases. Postnatally acquired rubella is uncommon in infants, yet a positive IgM result outside the immediate neonatal period warrants consideration of acquired infection. Rubella-specific IgM can persist for months after acute illness and may be present after vaccination. False-positive IgM reactivity occurs and should be interpreted with clinical correlation and, when indicated, follow-up rubella IgG testing and/or paired sera.