Definition: HBsAG stands for hepatitis B surface antigen. When a healthcare provider orders blood tests to determine if someone is infected with the hepatitis B virus, one thing he is looking for is HBsAG in the blood. If it is found, along with other specific antibodies, it means the person has a hepatitis B infection. HBsAG, which is cleared within 4 to 6 months in self-limited infections (infections that resolve by themselves), can be detected in the blood during both acute infections (infections that come on suddenly) and chronic infections (infections that last for longer than 6 months). In addition to the signs and symptoms that a patient has, additional antibodies can be tested to distinguish between acute and chronic infections.
At the center of the hepatitis B virus is DNA, which contains the genes the virus uses to replicate itself. Surrounding the DNA is a protein called HBcAG (hepatitis B core antigen), which cannot be detected with blood tests. Surrounding this is HBsAG, which is actually part of the protective "envelope." This envelope surrounds the virus and protects it from attack by the body's immune system. However, this system is good at getting through this envelope in order to kill the virus. When it does, remnants of surface antigen protein are left in the blood like debris, which lab tests can detect.
