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Hepatitis B is a viral infection that affects the liver and can cause both acute and chronic disease. The hepatitis B surface antigen (HBsAg) test is a blood test used to detect the presence of this antigen, which is a protein found on the surface of the hepatitis B virus. When HBsAg is present in the blood, it indicates that a person is infected with the hepatitis B virus. This test is essential for diagnosing hepatitis B infection and is often one of the first tests performed when hepatitis B is suspected.

The HBsAg test is used for several purposes in medical practice. It helps screen people who may be at risk for hepatitis B, including pregnant women, healthcare workers, people who inject drugs, and individuals with multiple sexual partners. The test is also used to monitor chronic hepatitis B patients and to screen blood and organ donors to prevent transmission. A positive HBsAg result indicates active infection, either acute or chronic, while a negative result generally means the person is not currently infected with hepatitis B.

When interpreting HBsAg test results, timing is important. If HBsAg is detected in the blood for less than six months, it typically indicates an acute infection that may resolve on its own. However, if HBsAg remains detectable for more than six months, this suggests a chronic hepatitis B infection. Additional tests, such as hepatitis B surface antibody (anti-HBs), hepatitis B core antibody (anti-HBc), and hepatitis B e antigen (HBeAg), are often performed alongside the HBsAg test to provide a complete picture of the infection status and immunity.

People should get tested for HBsAg if they have symptoms of hepatitis such as jaundice, fatigue, abdominal pain, or dark urine. Testing is also recommended for those with known exposure to hepatitis B, individuals born in regions where hepatitis B is common, and anyone with risk factors for the infection. The test requires a simple blood draw, and results are typically available within a few days. Early detection through HBsAg testing allows for timely treatment and monitoring, which can prevent serious liver complications such as cirrhosis and liver cancer.