Hepatitis B Surface Antigen (HBsAg) test is used to determine the presence of Hepatitis B viral infection by measuring the amount of HBsAg in the blood. Hep B virus spreads mainly through body fluids or unsafe sexual intercourse. Hep B antigen is expressed on the surface of Hepatitis B virus and its detection is a preliminary test to check the infection at an earliest possible stage. A positive Hep B surface antigen test requires further testing to determine the level of infection in the body.