Avidities of IgG directed against viral capsid antigen or early antigen: useful markers for significant Epstein-Barr virus serology.

Classical Epstein-Barr virus (EBV) serology can be misleading in some cases due to the variability of the viral capsid antigen (VCA)-IgM response, persistent or reactivated VCA-IgM, or loss of anti-EBNA-1 during suppression of the cellular immune system. Therefore, we studied the usefulness and significance of avidity determinations of VCA-IgG and EA-IgG to achieve unequivocal interpretation of serological results. Avidities of EBV capsid antigen-specific IgG (VCA-IgG) and early antigen-specific IgG (EA-IgG) were determined by indirect immunofluorescence during and after acute EBV infection. Low-avidity antibodies were removed from antigen-antibody complexes by incubation with 6 M urea for 3 minutes. The analysis of 105 sera taken at defined time spans with regard to the onset of clinical symptoms allowed us to determine the kinetics of maturation of avidity of VCA-IgG. All sera had low-avidity antibodies at the onset of disease. More than 90% of the sera showed an avidity index below 0.25 during the first 10 days after the onset of disease. Fifty percent of the sera exhibited an avidity index of 0.25 or above 20-30 days after the onset of clinical symptoms. Sera from past infections uniformly exhibited avidity indices of 0.5 or 1. Avidity of EA-IgG may still be low when avidity of VCA-IgG is already borderline or high, thus allowing further differentiation of acute and recent infections. Avidity determination represents an important additional marker of serology in classical cases and allows diagnosis in aberrant cases, such as acute infections with low or undetectable VCA-IgM.(ABSTRACT TRUNCATED AT 250 WORDS)