The AD surface antigen of the Hepatitis B virus (HBsAg) is a protein component of the viral envelope that is predominantly expressed in the cytoplasm of infected hepatocytes. HBsAg has antigenic heterogeneity; two pairs of the subtypes d/y and w/r lead to antigenic combinations of adw, ayw, adr, and ayr.

Hepatitis B infection is normally diagnosed using serological tests that detect HBsAg.

If HBsAg is detected in the blood for longer than six months, chronic hepatitis B is diagnosed.

Recombinant Hepatitis B surface antigen AD expressed in Saccharomyces cerevisiae.