A hepatitis B surface antigen (HBsAg) blood test is a laboratory test that detects the presence of the hepatitis B virus (HBV) in the blood. HBsAg is a protein found on the surface of the HBV and is often the first sign that a person has been infected with the virus. A positive test result indicates that the person is infected with HBV, while a negative result indicates that the person is not infected or has developed immunity to the virus.
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