Which statement about hepatitis B surface antigen (HBsAg) is true?

Which statement about hepatitis B surface antigen (HBsAg) is true?

• A. It indicates active infection.
• B. It indicates past infection that has resolved.
• C. It indicates immunity after vaccination.
• D. It indicates non-infectious status.

Answer: (A)

The key idea is that HBsAg shows current infection. If the surface antigen is present in the blood, it means hepatitis B virus is actively circulating, which can occur in acute infection and may persist in chronic infection, signaling ongoing viral replication and potential infectivity to others. This contrasts with immunity from vaccination or recovery, which is indicated by anti-HBs antibodies, not by HBsAg. When someone has resolved the infection, HBsAg becomes negative and anti-HBs is positive. Therefore, a positive HBsAg test does not reflect past infection, vaccination immunity, or a non-infectious status.

The key idea is that HBsAg shows current infection. If the surface antigen is present in the blood, it means hepatitis B virus is actively circulating, which can occur in acute infection and may persist in chronic infection, signaling ongoing viral replication and potential infectivity to others. This contrasts with immunity from vaccination or recovery, which is indicated by anti-HBs antibodies, not by HBsAg. When someone has resolved the infection, HBsAg becomes negative and anti-HBs is positive. Therefore, a positive HBsAg test does not reflect past infection, vaccination immunity, or a non-infectious status.