Direct radioimmunoassay of antibody against hepatitis B core antigen using 32P-labelled core particles

A direct radioimmune assay for antibody against hepatitis B core antigen (HBcAg) was developed. Flexible microtiter plates were coated with HBcAg, incubated with test samples, and thereafter with 32P-labelled HBcAg. Labelling was achieved by endogenous protein kinase of the core particles. This assay was tenfold more sensitive than a conventional inhibition assay employing enzyme-labelled anti-HBc as reagent. The radioimmunoassay detected a large number of positive persons (88/200) in a population with a high prevalence of blood-transmitted hepatitis infections (medical staff, liver and dialysis patients, contact persons, implicated blood donors) which were not detected by the inhibition assay. Such results were rare in healthy blood donors. The weak anti-HBc activity, which was detected only by direct radioimmunoassay, co-purified with IgG and was inhibited by addition of HBcAg to the serum. The activity may be due to a very limited hepatitis B infection or, what is more likely, to cross-reacting antibodies against unknown antigen(s). The factor detected only by direct radioimmune assay appears to be related to viral hepatitis. For detection of anti-HBc as a marker for hepatitis B virus it is, however, preferable to use less sensitive assays.