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Hepatitis B surface antigen (HBsAg) is a protein marker found on the surface of the hepatitis B virus. This laboratory test detects the presence of HBsAg in the blood, which indicates an active hepatitis B infection. When someone is infected with hepatitis B, this antigen appears in the blood before symptoms develop, making it one of the earliest markers of infection. The test is commonly used for screening, diagnosis, and monitoring of hepatitis B virus infection.

Testing for HBsAg is recommended for several groups of people. These include pregnant women during prenatal care, individuals with elevated liver enzymes, people who inject drugs, those with multiple sexual partners, healthcare workers exposed to blood, individuals receiving immunosuppressive therapy, and people born in regions where hepatitis B is common. The test is also performed on blood donors and people living with someone who has hepatitis B infection.

The HBsAg test is typically performed using a blood sample drawn from a vein. No special preparation is usually required before the test. A positive result indicates that the person currently has hepatitis B infection, which could be either acute (recent infection) or chronic (long-term infection). If HBsAg remains detectable for more than six months, the infection is considered chronic. A negative result generally means the person does not have hepatitis B infection, though in very early infection the antigen may not yet be detectable.

Follow-up testing is often necessary to determine the stage and activity of the infection. Additional tests may include hepatitis B surface antibody (anti-HBs), hepatitis B core antibody (anti-HBc), and hepatitis B e antigen (HBeAg). These tests help distinguish between acute and chronic infection, determine immunity status, and assess whether the virus is actively replicating. People with chronic hepatitis B require regular monitoring of liver function and may need treatment to prevent liver damage, cirrhosis, or liver cancer.