Evaluation of a simplified sucrose gradient method for the detection of rubella-specific IgM in routine diagnostic practice.

Rubella-specific IgM was measured in a single fraction of serum from a sucrose density gradient. Haemagglutination inhibition (HAI) tests were performed on paired aliquots of the fraction untreated and after treatment with 2- mercaptoethanol, dilutions of the aliquots being incubated over night with rubella antigen before the addition of red cells. Of 822 sera tested, specific IgM was found in 249, but not in 492. When first tested, the remaining 81 sera gave unsatisfactory results because of contamination of the IgM fraction with IgG (6.0%), probable aggregation of IgG (3.5%), or the persistence of chick red cell agglutinins (0.4%). Tests were performed on 134 patients with rubella confirmed by a rise of HAI antibodies. Rubella-specific IgM was found at a titre of more than eight in the sera taken from 62 of 64 patients between 10 and 29 days after the onset of the rash but in only one of the sera taken between 80 and 119 days, and in none taken later. However, specific IgM was still to be found at lower titre in the sera of 13 patients collected between 80 and 162 days after the onset of the illness. In routine diagnostic tests over three years on the serum from 479 patients with suspected acquired rubella, specific IgM was found at a titre of more than eight in 51 patients and in only 10 instances (2.1%) did a lower level pose a problem in interpretation.