Improved test for IgM antibody to Epstein-Barr virus using an absorption step with Staphylococcus aureus

The determination of levels of IgM antibody to Epstein-Barr virus (EBV) capsid antigen was improved by absorbing unfractionated test serum with Staphylococcus aureus cells containing protein A. Nonspecific background fluorescence was significantly decreased, thus facilitating the reading of slides and the detection of specific fluorescent reactions in serum with low levels of this IgM antibody. Moreover, the inclusion of the absorption step eliminated false-positive reactions caused by the presence of rheumatoid factor in serum and shortened the time needed for the incubation of test serum with the EBV-infected cell smear. This improved antibody test should facilitate the laboratory diagnosis of acute EBV infections, including unaccompanied by a heterophil antibody response.