The hepatitis B virus, which mainly infects the liver, has an inner core and an outer envelope (the surface). The HBsAg test detects the outer envelope that would identify an individual infected with the hepatitis B virus. Hepatitis B can cause inflammation of the liver, and in the earliest stage of the disease, infected people may feel ill or even have yellow discoloration of the skin or eyes, known as jaundice. Fortunately, most patients recover completely and test negative for HBsAg approximately four months after the illness. A small percentage of people may become chronic carriers of the virus, and in these cases, the test may remain positive for months or even years.
A part of the hepatitis B virus that, when in the blood, is one of the earliest markers of infection with hepatitis B virus, appearing even before symptoms. Also found in blood of a carrier. HBsAg is tested for in the differential diagnosis of hepatitis.