Clinical evaluation of a monoclonal assay for hepatitis B surface antigen:Identification of “HBsAg-like” polypeptides non-reactive in conventional radioimmunoassays

An immunoradiometric assay for hepatitis B surface antigen (HBsAg) that employs monoclonal antibodies directed against the common epitope(s) of HBsAg was used to analyse 3,694 samples of human serum. Further analysis of those sera identified as HBsAg-positive in this assay demonstrated that the findings with the monoclonal-antibody-based assay correlated with the presence of HBsAg as determined by Ausria II. A small proportion of apparently false-positive reactions were observed, in that some sera, although reactive with the monoclonal antibodies, were not positive in conventional immunoassays using polyclonal antisera, nor were they neutralisable with polyclonal anti-HBs. The material purified by monoclonal immunoabsorbants from representative “true” and “false-positive” sera was run on polyacrylamide gels and examined under the electron microscope. The antigen in the apparently false-positive sera contained some polypeptides of similar size to those found in HBsAg, but no virus particles were seen by electron microscopy. The majority of patients with this monoclonal-antibody-reactive antigen gave either a history of hepatitis B virus (HBV) contact or had signs of liver disease.