hepatitis B surface antigen (HBsAg) a surface coat lipoprotein antigen of the hepatitis B virus, peaking with the first appearance of clinical disease symptoms. Tests for serum HBsAg are used in the diagnosis of acute or chronic hepatitis B and in testing blood products for infectivity.
Cancer antigen 125 (CA-125) measure the amount of this protein in the blood. CA-125 is found on the surface of many ovarian cancer cells.