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Hepatitis B surface antigen (HBsAg) is a protein 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 the hepatitis B virus, this antigen appears in the blood before symptoms develop and can be detected as early as one to ten weeks after exposure to the virus. The presence of HBsAg is one of the earliest and most reliable indicators of hepatitis B infection, making it an essential screening tool for diagnosing this potentially serious liver disease.

The HBsAg test is commonly used for several purposes in medical diagnostics. Healthcare providers order this test to screen people who may have been exposed to hepatitis B, to diagnose acute or chronic hepatitis B infection, to screen pregnant women since the virus can be transmitted to newborns during childbirth, and to screen blood donors to ensure blood supply safety. The test is also important for monitoring people with known hepatitis B infection and for screening individuals in high-risk groups, including healthcare workers, people who inject drugs, and those with multiple sexual partners.

The test is performed using a blood sample drawn from a vein, typically from the arm. No special preparation is usually required before the test, and results are generally available within a few days. A positive HBsAg result indicates that the person currently has a hepatitis B infection, which could be either acute (recent infection) or chronic (long-term infection lasting more than six months). A negative result typically means the person is not currently infected with hepatitis B, though it cannot rule out a very recent exposure where the antigen has not yet appeared in detectable levels.

If the HBsAg test is positive, additional tests are usually performed to determine whether the infection is acute or chronic and to assess liver function. These may include tests for hepatitis B e antigen, hepatitis B core antibody, hepatitis B surface antibody, and liver enzyme tests. Understanding the full hepatitis B status helps healthcare providers determine the appropriate treatment and management strategy. People with chronic hepatitis B infection require ongoing monitoring and may need antiviral treatment to prevent liver damage, cirrhosis, or liver cancer.