The Hepatitis B (Core) IgM Antibody test is used to detect antibodies called IgM in your blood. The test is used to find out whether you are actively infected with the hepatitis B virus (HBV). The HBV has an envelope surrounding the central core. During the active stage of infection, your immune system will make IgM antibodies to the core of HBV. These antibodies usually appear in the blood after several weeks after you are first infected with HBV. The core antibodies will not be present in the bodies of people who have had the Hepatitis B vaccine.