HBsAg is the surface antigen of the Hepatitis B virus (HBV). HBsAg is among the first serum markers to appear during HBV infection and its presence is indicative of the hepatitis B infection. Ad and Ay are two determinants of HBsAg. They have two subtypes W and R. It is used as a positive control in ELISA test kit for the detection of HBV infection.