Which blood test indicates infection with Hep B?

Which blood test indicates infection with Hep B?

• A. HBsAb
• B. HBcAb
• C. HBsAg
• D. HBeAg

Answer: (C)

The key idea here is identifying active infection by detecting the virus itself in the blood. The surface antigen of hepatitis B virus, HBsAg, is present when the virus is circulating, so its detection means the person currently has an HBV infection. This marker can appear early after exposure and may persist in chronic infection, signaling ongoing disease. Other markers tell different things. HBsAb, the surface antibody, indicates immunity—either from vaccination or from recovery after infection—so it does not mark an active infection. HBcAb, the core antibody, shows exposure to the virus, but by itself it doesn’t prove that the infection is active. HBeAg reflects viral replication and higher infectivity in someone with HBV, but it’s a sign of how actively the virus is replicating rather than a standalone indicator that infection is present.

The key idea here is identifying active infection by detecting the virus itself in the blood. The surface antigen of hepatitis B virus, HBsAg, is present when the virus is circulating, so its detection means the person currently has an HBV infection. This marker can appear early after exposure and may persist in chronic infection, signaling ongoing disease.

Other markers tell different things. HBsAb, the surface antibody, indicates immunity—either from vaccination or from recovery after infection—so it does not mark an active infection. HBcAb, the core antibody, shows exposure to the virus, but by itself it doesn’t prove that the infection is active. HBeAg reflects viral replication and higher infectivity in someone with HBV, but it’s a sign of how actively the virus is replicating rather than a standalone indicator that infection is present.