HBsAg (Hepatitis B surface antigen) - A "positive" or "reactive" HBsAg test result means that the person is infected with hepatitis B. This test can detect the actual presence of the hepatitis B virus (called the “surface antigen”) in your blood. If a person tests “positive,” then further testing is needed to determine if this is a new “acute” infection or a “chronic” hepatitis B infection. A positive HBsAg test result means that you are infected and can spread the hepatitis B virus to others through your blood.