Detection by radioimmunoassay of IgM class antibody to hepatitis B core antigen: a comparison of two methods.

A solid-phase radioimmunoassay (RIA) for IgM anti-HBc is described. The assay (anti-mu RIA) is based on the adsorption of IgM to a tube coated with sheep antibody to the human IgM mu-chain. The adsorbed immunoglobulin is assayed for anti-HBc activity. Positive reactions in the test are shown to be due to IgM antibody, and a confirmation blocking test is described. Reactivity of test sera can be quantitated by comparison with standard sera containing known levels of IgM anti-HBc. Sera assayed for IgM anti-HBc by both the anti-mu RIA and a method employing serum fractionation and competitive RIA gave similar results. Rheumatoid factor (RF) alone did not react in the anti-mu RIA, but reactivity could be generated in the presence of RF and preformed aggregates of IgG anti-HBc.