The Hepatitis B surface antigen (HBsAg) test is a laboratory test used to detect the presence of the Hepatitis B virus in the blood. HBsAg is a protein on the surface of the virus that can be detected in the blood as early as 1-2 weeks after infection. The test involves taking a blood sample from the arm and sending it to a laboratory for analysis. The results of the test are reported as positive or negative for the presence of HBsAg. A positive test result for HBsAg indicates that a person is infected with the Hepatitis B virus. Symptoms of Hepatitis B infection can include fever, fatigue, abdominal pain, loss of appetite, and jaundice. Most people with Hepatitis B recover without treatment within a few weeks to months, but some may develop chronic Hepatitis B infection, which can lead to liver damage, cirrhosis, or liver cancer. If a person tests positive for HBsAg, the doctor may recommend additional testing to evaluate liver function and determine the course of treatment. Treatment for chronic Hepatitis B infection may include antiviral medications or other therapies to prevent or manage complications. Note: HBsAg test is not a vaccine, and a negative test result does not indicate immunity to Hepatitis B. The Hepatitis B vaccine is recommended for all infants, children, and adults who are at risk of infection.