ROSH Gastrointestinal Practice Exam

Which serological marker indicates active hepatitis B infection?

1. Hepatitis B surface antibody (anti-HBs)

2. Hepatitis B surface antigen (HBsAg)

3. Hepatitis B core antibody (anti-HBc)

4. Hepatitis D antigen

The correct answer is: Hepatitis B surface antigen (HBsAg)

The presence of Hepatitis B surface antigen (HBsAg) is the key serological marker that indicates active hepatitis B infection. HBsAg is a protein on the surface of the hepatitis B virus. Its detection in the blood suggests that an individual is currently infected with the virus, as it is produced during the active replication of the hepatitis B virus in the liver. In the context of monitoring and diagnosing hepatitis B, HBsAg is the initial marker used to evaluate infection status. When HBsAg is present, it indicates that the virus is detectable in the patient's bloodstream, which is a hallmark of an active infection—either an acute or chronic phase of hepatitis B. Other markers, such as hepatitis B surface antibody (anti-HBs) and hepatitis B core antibody (anti-HBc), serve different purposes. Anti-HBs indicates recovery from infection and immunity, while anti-HBc can indicate past or current infection but is not definitive for active disease by itself. Hepatitis D antigen is not a marker for hepatitis B directly and is only relevant in cases where a co-infection with hepatitis D virus occurs. Thus, understanding HBsAg's role is crucial in identifying and managing hepatitis B infections.