A simplified method for the detection of rubella-specific IgM in sera was developed involving HAI tests on only one fraction from a sucrose density gradient with and without treatment with 2-mercaptoethanol (2 ME). Slight trailing of IgG into the IgM fraction was shown to occur in patients with high titres of both IgG and IgM, but this could be detected by the 2ME treatment and did not affect the results.
By this test, rubella-specific IgM was found in all of 54 patients with a rash and a rise of rubella HAI antibodies. When the antigen and serum were incubated for 1 hr. before adding the RBC, specific IgM was found in all 51 sera taken between 5 and 40 days after onset, and in about half the sera taken between 42 and 77 days, but in none of 16 people known to have possessed rubella antibodies at least a year previously. When antigen and serum were incubated for 18 hr. before adding the RBC, specific IgM was found in all of 21 sera taken between 2 and 49 days after onset, and in 6 of 11 sera taken between 50 and 117 days, but not in 9 people known to have possessed rubella antibodies at least a year before. The method has been found to be very useful in detecting recent rubella among patients in early pregnancy.