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Hepatitis B surface antigen, commonly abbreviated as HBsAg, is a protein found on the surface of the hepatitis B virus. When this antigen is present in the blood, it indicates that a person is currently infected with the hepatitis B virus. The HBsAg test is one of the most important laboratory tests for diagnosing hepatitis B infection and is often the first test performed when hepatitis B is suspected. This test detects the presence of the antigen in blood samples and helps healthcare providers determine if someone has an active infection.

The HBsAg test is typically ordered when a person shows symptoms of hepatitis, such as jaundice, dark urine, fatigue, abdominal pain, or nausea. It is also commonly used as a screening test for people at higher risk of hepatitis B infection, including healthcare workers, people who inject drugs, individuals with multiple sexual partners, pregnant women, and those born in regions where hepatitis B is common. Additionally, the test may be performed before blood donation or as part of routine health screenings.

A positive HBsAg result means that the hepatitis B virus is present in the blood and the person is infectious. If HBsAg remains detectable for more than six months, the infection is considered chronic. A negative result generally indicates that the person is not currently infected with hepatitis B, though additional tests may be needed to determine if someone has recovered from a past infection or has been successfully vaccinated. In some cases, the test may need to be repeated or combined with other hepatitis B markers to get a complete picture of infection status.

The test is performed using a blood sample, typically drawn from a vein in the arm. No special preparation is usually required before the test. Results are generally available within a few days. If HBsAg is detected, additional tests such as hepatitis B e antigen, hepatitis B core antibody, and liver function tests may be ordered to assess the stage of infection and liver health. Early detection through HBsAg testing is crucial for managing hepatitis B infection and preventing transmission to others.