Enzyme immunoassay for anti-hepatitis B core (HBc) immunoglobulin G1 and significance of low-level results in competitive assays for anti-HBc.

An enzyme immunoassay (EIA) for anti-hepatitis B core (HBc) immunoglobulin G1 (IgG1) was compared with a commercial radioimmunoassay (RIA) for anti-HBc antibody (Corab: Abbott Laboratories, North Chicago, Ill.). In parallel tests of 445 consecutive samples, discrepant results were obtained with 2 samples, 1 of which was positive only by the RIA and the other of which was positive only by the EIA for anti-HBc IgG1. In tests of another 192 samples with low blocking activity in the RIA (inhibition range, 90 to 30%), 10 samples gave discrepant results, 5 of which were positive only by the RIA and the other 5 of which were positive only by the EIA for anti-HBc IgG1. Of 12 samples with discrepant results, 11 samples were tested further for anti-HBc IgG3, IgM, and IgA1 by the EIA. Of these, seven samples were positive for anti-HBc IgG1, anti-HBc IgG3, or both. All seven samples were also positive for anti-hepatitis B surface (HBs) antigen. Three samples were negative for anti-HBc IgG1, anti-HBc IgG3, or both but were positive for anti-HBc IgM, anti-HBc IgA1, or both; and one sample was reactive only in the RIA. These four samples were all negative for anti-HBs. Thus, low-level results in the RIA caused by anti-HBc IgM, anti-HBc IgA, or both reflect the unspecific activation of immature B lymphocytes that is not related to previous exposure to hepatitis B virus (HBV). In contrast, the presence of anti-HBc IgG1, anti-HBc IgG3, or both indicates differentiated anti-HBc IgG-producing plasma cells and previous exposure to HBV, as was also shown by the presence of anti-HBs. On class and subclass determination for confirmation of positivity for anti-HBc in 19 serum samples, which was identified by screening of blood from 1,343 donors by a competitive EIA (Hepanostika; Organon), 9 samples with positive results, all low level, did not indicate previous exposure to HBV. It was concluded that determination of classes and subclasses of anti-HBc provides a tool for discriminating positive anti-HBc results not caused by HBV exposure.