HBsAg Test kit (Colloidal Gold) is used for qualitative detection of Hepatitis B surface Antigen in humanserum or plasma samples. Hepatitis B surface Antigen (HBSAg) is the hepatitis b virus coat protein. HBsAg Test kit (Colloidal Gold) is used for qualitative detection of Hepatitis B surface Antigen in humanserum or plasma samples. Hepatitis B surface Antigen (HBSAg) is the hepatitis b virus coat protein. lt's not contagious, only the antigenicity.But it is usually accompanied by the presence of hepatitis b virus, soit is a sign of the hepatitis b virus infection.