As shown in the accompanying figure, the hepatitis B virus has several antigens that are assessed individually by the laboratory. Specific assays for antibodies against the envelope antigen ('e'), the core ('c') antigen, and the surface antigen ('s') are available. Laboratories also have the ability to measure the HepB surface antigen and the e antigen itself. With regard to the acute viral hepatitis panel, the assays of interest are the hepatitis B core IgM Ab and the hepatitis B surface Ag.
The hepatitis B core IgM Ab (HBc IgM) appears in the serum early in the course of HBV infection. It is usually present when symptom onset occurs. HBc IgM typically becomes undetectable after six to nine months. After this period, Hepatitis B core IgG Ab will be present. The Hepatitis B surface antigen (HBsAg) is a protein on the surface of HBV. With this assay we are not detecting a patient antibody but instead, the viral protein itself. HBsAg is the first marker to appear in a new acute infection. Since it is detecting a viral protein and not a patient antibody, we do not have to wait for a humoral immune response, and thus, it is the first detectable marker in HBV infection. It can be detected as early as one week after exposure, though sensitivity will be highest after about 20-30 days. HBsAg becomes undetectable around seven to fifteen weeks after symptoms subside.
HBsAg can also be seen in chronic HBV, often when symptoms return and the disease 'flares up'. The body normally produces antibodies to HBsAg so this protein is used to make the HBV vaccine. Antibodies to HBsAg are often measured as a verification of vaccination.