Application of low-avidity immunoglobulin G studies to diagnosis of Epstein-Barr virus infectious mononucleosis.

Single serum samples from 121 patients suffering from clinical infectious mononucleosis were tested by an indirect immunofluorescence assay for avidity of Epstein-Barr virus (EBV) capsid antigen immunoglobulin G (IgG), involving a wash step with phosphate-buffered saline-8 M urea. Ninety-four samples showed serological markers of recent EBV infection (presence of viral-capsid antigen-specific IgM [87 cases] and/or presence of IgG in the absence of EBV nuclear antigen antibodies [85 cases]). The remaining 27 cases had serological evidence of prior infection (presence of viral-capsid IgG and EBV nuclear antibodies and absence of IgM). In the avidity assay, 89 samples from patients with recent infection showed low-avidity IgG and 25 samples from patients with prior infection had high-avidity IgG. The avidity assay showed a sensitivity that was at least equal to those of classical serological procedures for diagnosing EBV infectious mononucleosis. Further studies are necessary, however, to establish the specificity of the assay and the duration of low-avidity antibodies.