Detection of rubella-specific IgM antibodies by the staphylococcal absorption method (SAM)

The use of SAM for the detection of rubella-specific IgM was evaluated in the sera of 318 rubella convalescents, 79 vaccinees and 53 infants with congenital rubella syndrome (CRS). The method employed was based on the use of crude inactivated suspension of Staphylococcus aureus strain Cowan I in the hemagglutination inhibition (HI) test and 1-h incubation of antigen and antibody. Among convalescents, 99% were found positive when tested within 30 days, and 84.6% were positive within 90 days following onset of clinical symptoms. Infants with CRS were 50 to 55% positive at the age of 0 to 6 months and 23% at the age of 7 to 11 months. Overnight incubation of antigen and antibody increased the proportion of positive results, particularly among sera of rubella vaccinees. No false-positives were detected among 1,035 healthy controls; nor in 40 patients following cytomegalovirus or Epstein-Barr virus infection, nor in 22 sera containing rheumatoid factor (RF). Comparison of SAM with a commercial enzyme-linked immunosorbent assay (ELISA) kit based on antibody capture showed 84% correlation between the two methods. Discrepancies were observed mainly in low-IgM.antibody-containing sera.