To diagnose and follow the course of an infection with hepatitis B or to determine if the vaccine against hepatitis B has produced the desired level of immunity. This test is useful in the differential diagnosis of hepatitis. Hepatitis B spreads by contact with an infected person's blood, semen or other body fluid. You may feel like you have the flu if you contract this virus or you may not show any symptoms at all. If you suspect that you may have come in contact with an infected person, you should have this lab performed.