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Significance of isolated anti‐HBc seropositivity by ELISA: Implications and the role of radioimmunoassay

Hepatitis B virus (HBV) surface antigen (HBsAg) and antibody to HBsAg (anti‐HBs) are excellent markers for HBV infection and its immunity. The significance of isolated antibody to HBV core antigen (anti‐HBc) seropositivity is not certain. To elucidate this, sera from 638 Chinese adult subjects, aged 18‐52 years, seronegative for both HBsAg and anti‐HBs, were tested for anti‐HBc. Fifty‐one (8%) were found to have an isolated anti‐HBc seropositivity by ELISA, and all were negative for IgM‐anti‐HBc. The anti‐HBc persisted in all subjects who attended follow‐up for hepatitis B vaccination (n = 48) for a period of 8 months. These 48 subjects received 3 doses of hepatitis B vaccine (HB‐VAX, 10 μg or 20 pg) a t 0, 1, and 6 months: 72.9% developed a primary anti‐HBs response (suggestive of a false‐positive anti‐HBc seropositivity), 4.2% developed an anamnestic or secondary anti‐HBs response, and 22.9% did not develop an anti‐HBs response. Increasing the cutoff point of the ELISA or reconfirmation with radioimmunoassay (RIA) reduced only a minor half of the false positives. This low specificity of anti‐HBc ELISNRIA, together with the high rate of anti‐HBs response to hepatitis B vaccine, indicates that subjects with isolated anti‐HBc seropositivity should be included in vaccination programs.