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Hepatitis B surface antigen, commonly known as HBsAg, is a protein found on the surface of the hepatitis B virus. This antigen appears in the blood when someone has an active hepatitis B infection, whether acute or chronic. A laboratory test for HBsAg is one of the most important screening tools used to detect hepatitis B virus infection. The presence of this antigen in a blood sample indicates that the person is currently infected with the virus and can potentially transmit it to others.

The HBsAg test is typically performed using a blood sample drawn from a vein. Healthcare providers often order this test when screening individuals at higher risk for hepatitis B, such as pregnant women, people with multiple sexual partners, healthcare workers, individuals born in regions where hepatitis B is common, or those who share needles. The test is also used to monitor people already diagnosed with hepatitis B to determine if they have cleared the infection or if it has become chronic.

When interpreting HBsAg test results, a positive result means that hepatitis B surface antigen has been detected in the blood, indicating an active infection. If HBsAg remains positive for more than six months, the infection is considered chronic. A negative result generally means the person does not have an active hepatitis B infection, though additional tests may be needed to determine if someone had a past infection that has resolved or if they have been successfully vaccinated against hepatitis B.

The HBsAg test is often performed alongside other hepatitis B markers to provide a complete picture of infection status. These may include tests for hepatitis B surface antibody, which indicates immunity from vaccination or past infection, and hepatitis B core antibody, which helps distinguish between acute and chronic infection. Together, these tests help healthcare providers determine the stage of infection, whether treatment is needed, and if the person has developed immunity to the virus. Regular monitoring of HBsAg levels is important for people with chronic hepatitis B to assess disease progression and response to treatment.