Hepatitis B is a serious liver infection caused by the hepatitis B virus (HBV). For some people, hepatitis B infection becomes chronic, meaning it lasts more than six months. Having chronic hepatitis B increases your risk of developing liver failure, liver cancer or cirrhosis—a condition that causes permanent scarring of the liver. Hepatitis B surface antigen (HBsAg) is a blood test ordered to determine if someone is infected with the hepatitis B virus. If it is found, along with specific antibodies, it means the person has a hepatitis B infection.
Your blood may be screened for HBV for many different reasons. The three tests generally include HBsAg, antibody to HBsAg, and antibody to hepatitis B core antigen. This allows the doctor to know whether you could benefit from vaccination, or if you have active or chronic hepatitis B and need counseling, care, or treatment.