HBsAg (Hepatitis B virus surface antigen), also known as the Australian antigen, is the earliest test (one week after infection) that screens for hepatitis B virus (HBV) infection. The test detects a protein (antigen) present in the structure of the virus. Test results must be interpreted with other hepatitis B profile tests (anti-HB antibodies and, if necessary, HB core antibodies, HBe antigen, etc.).
A confirmed positive HBsAg result is consistent with an acute infection, or if positive for more than 6 months, a chronic HBV infection. The test is sometimes positive in infected individuals even before symptoms appear. A positive test alone does not indicate whether there is a risk of transmission. The test becomes negative again during the healing phase of the infection (typically within 12 weeks). A negative result therefore indicates that an individual is not infected with HBV or is cured (has eliminated the virus). The HBsAg test is the best test for identifying individuals with chronic infections, including “healthy” HBV carriers.