An attempt was made to assess the significance of the relatively frequent low-titre positive reactions in radioimmunoassays for antibody to hepatitis B surface antigen. Serum specimens negative for surface antigen were assayed for antibodies to the surface, core, and e antigens of the Hepatitis B virus. Two populations were studied - one of low hepatitis B incidence (2000 blood donors) and one of high incidence (200 male patients attending a clinic for sexually transmitted diseases). The validity of some singly occurring low-titre anti-HBs and anti-HBc reactions is questioned, and the occurrence of positive reactions unrelated to previous hepatitis B infection inferred.

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.

SUMMARYA solid phase M-antibody capture radioimmunoassay (MACRIA) and a serum fractionation method were used to quantitate the IgM response to the hepatitis B core antigen (IgM anti-HBc) in acute and chronic hepatitis B infections.Antibody to the core antigen was predominantly of the IgM class during the acute phase of hepatitis B. Resolving acute infections remained positive by MACRIA, but at decreasing levels, for as long as 6 months. IgM anti-HBc persisted in HBsAg carriers but at levels very much lower than seen in acute infections. There was no correlation of IgM anti-HBc with severity of chronic liver disease in carriers.Measurement of IgM anti-HBc by MACRIA enabled accurate identification of acute hepatitis B on single serum specimens.