Passively acquired antibody to hepatitis B surface antigen. Pitfall in evaluating immunity to hepatitis B viral infections.

Antibody to hepatitis B surface antigen (anti-HBs) has been used clinically to indicate an immune response to hepatitis B virus (HBV) and a protection against reinfection with the virus. We describe a child with hemophilia who had high-titer IgG anti-HBs in his serum and who subsequently developed viral B hepatitis. The child had received a unit of fresh frozen plasma 17 days prior to the determination of anti-HBs. The fresh frozen plasma donor was later found to be anti-HBs positive. The patient's anti-HBs was most likely passively acquired and therefore did not signify immunity to HBV. Various tests, including hepatitis B surface antigen group-specific and subtype determinants, ratio units of anti-HBs, and antibody class, have been used to determine whether or not anti-HBs will confer immunity. Although these tests have been thought to accurately predict immune status against infection with HBV, our case shows this may not be true, especially in patients who have been recently transfused. Anti-HBs testing may be predictive of immunity to HBV in the absence of a source of passively acquired anti-HBs.