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Hepatitis B surface antigen, commonly known as HBsAg, is a protein found on the surface of the hepatitis B virus. When this antigen is present in the blood, it indicates an active hepatitis B infection, whether acute or chronic. The HBsAg test is one of the most important screening tools for detecting hepatitis B virus infection and is routinely used in medical laboratories worldwide. Healthcare providers order this test to diagnose new infections, monitor chronic cases, and screen individuals who may be at risk for hepatitis B.

The test works by detecting the presence of HBsAg in a blood sample. During an active infection, the virus produces this surface protein in large quantities, making it detectable in the bloodstream. If HBsAg is found in the blood for more than six months, it typically indicates a chronic hepatitis B infection. The test is highly sensitive and can detect the antigen even in the early stages of infection, often before symptoms appear. This early detection capability makes it valuable for preventing disease transmission and starting appropriate monitoring or treatment.

Healthcare providers recommend HBsAg testing for several groups of people. Pregnant women are routinely screened to prevent transmission to their babies during childbirth. People with risk factors such as injection drug use, multiple sexual partners, or those living with someone who has hepatitis B should also be tested. Healthcare workers, individuals receiving dialysis, and people born in regions where hepatitis B is common are other groups who benefit from screening. Additionally, anyone with unexplained liver disease or elevated liver enzymes should undergo HBsAg testing.

A positive HBsAg result means that hepatitis B virus is present in the blood and the person can transmit the infection to others. Further testing is usually needed to determine whether the infection is acute or chronic and to assess liver function. A negative result generally means no current infection, though it does not rule out past infection or immunity from vaccination. In some cases, additional hepatitis B tests, including antibody tests, may be performed alongside HBsAg to provide a complete picture of a person's hepatitis B status and immune protection.