Modified staphylococcal absorption method used for detecting rubella-specific immunoglobin M antibodies during a rubella epidemic.

A recently described method for detecting rubella-specific immunoglobulin M (IgM) antibody based on absorption of IgG by Staphylococcus aureus strain Cowan I has been applied to 198 sera collected during a recent rubella epidemic in Israel. Modification of the original method introduced for the present study includes treatment with 2-mercaptoethanol of antibody remaining after absorption by staphylococci. This treatment confirms that the residual antibody is IgM (sensitive to 2-mercaptoethanol) rather than IgG (2-mercaptoethanol resistant). None of the 67 control patients (seropositive for rubella but without history of recent illness or contact) had specific IgM when tested by this method, though 15 showed some residual antibody after staphylococcal absorption. A total of 125 of 131 rubella convalescents (95%) were positive 4 to 49 days after onset of the clinical symptoms. Six patients had no IgM antibodies when tested by the method described, and all were convalescents tested late in relation to onset of clinical symptoms (beyond 3 weeks). When density gradient centrifugation was applied to clarify some results, 2 of 3 convalescents classified as IgM negative by the staphylococcal absorption method did in fact possess IgM antibody. None of 10 controls tested by density gradient centrifugation was IgM positive. This combination of staphylococcal absorption and 2-mercaptoethanol treatment is recommended as a screening test for selection of IgM positives, in addition to the use of a more sensitive method (such as density gradient centrifugation) on at least some samples classified as IgM negative.