Blood test that measures Hepatitis B Surface Antigen levels.
Hepatitis B Surface Antigen is the earliest indicator of the presence of a Hepatitis B infection
The HBsAg test is often used in STD screening as well as health screening for employment reasons
a negative HBsAg means that the person does not have Hepatitis B. There are rare windows of time in which the HBsAg antibodies will have disappeared while long-term IgG antibodies have not yet appeared.