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Hepatitis B is a viral infection that affects the liver and can lead to serious health complications. The hepatitis B surface antigen (HBsAg) test is a blood test used to detect the presence of this virus in the body. This test looks for a specific protein found on the surface of the hepatitis B virus. When HBsAg is detected in the blood, it indicates that a person is currently infected with hepatitis B, either as an acute (recent) infection or a chronic (long-term) condition.

The HBsAg test is commonly used for several purposes. Healthcare providers order this test when someone shows symptoms of hepatitis such as jaundice (yellowing of skin and eyes), fatigue, abdominal pain, dark urine, or loss of appetite. It is also used as a screening tool for people at higher risk of hepatitis B infection, including healthcare workers, people who inject drugs, individuals with multiple sexual partners, those living with someone who has hepatitis B, and pregnant women. Many countries require HBsAg screening during pregnancy to prevent transmission of the virus from mother to baby.

The test is performed using a simple blood sample drawn from a vein, usually in the arm. No special preparation is needed before the test. Results are typically reported as either positive or negative. A positive result means HBsAg was detected in the blood, indicating current infection with hepatitis B virus. A negative result means no HBsAg was found, suggesting the person is not currently infected. However, a negative result does not rule out past infection or immunity from vaccination.

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. People who test positive for HBsAg should be evaluated by a healthcare provider who can recommend appropriate monitoring and treatment options to prevent liver damage and reduce the risk of transmitting the virus to others.