Schematic representation of the reactivities of IgM antibodies induced against Gly-Ala repeats of EBNA-1 and against glycine-rich motifs of HCMV antigens, and algorithm for the processing of HCMV IgM-reactive serum samples. Schematic representation of the reactivities of IgM antibodies induced against Gly-Ala repeats of EBNA-1 and against glycine-rich motifs of HCMV antigens, and algorithm for the processing of HCMV IgM-reactive serum samples. (A) EBNA-1 Gly-Ala repeats and HCMV glycine-rich motifs both induce IgM during acute infection with EBV and HCMV. Both populations of IgM antibodies show highly specific reactivity with the antigens from both viruses. (B) HCMV IgM-reactive serum samples should be tested for anti-EBNA-1 IgG antibodies. Detectability of such antibodies indicates past EBV infection and suggests acute HCMV infection, represented by the positive HCMV IgM ELISA (left). Confirmation and differentiation between acute primary and recurrent HCMV infection can be achieved by testing for HCMV glycoprotein-specific antibodies of by antibody avidity testing. Lack of detectability of anti-EBNA-1 IgG requires further EBV IgM serodiagnosis to distinguish between acute EBV and acute HCMV infection. PB, Paul Bunnell test. Dieter Lang et al. Clin. Diagn. Lab. Immunol. 2001; doi:10.1128/CDLI.8.4.747-756.2001