HBsAg: Hepatitis B Surface Antigen
HBsAg stands for Hepatitis B surface antigen, which is a protein found on the surface of the hepatitis B virus and serves as the primary serologic marker for detecting both acute and chronic HBV infection. 1
Clinical Significance
HBsAg is the principal screening test for identifying persons with hepatitis B virus infection, with immunoassays demonstrating sensitivity and specificity greater than 98%. 1
What HBsAg Indicates
- A positive HBsAg test indicates active HBV infection, either acute or chronic. 1
- HBsAg appears in serum during both acute and chronic infection, making it present in the bloodstream of infected individuals regardless of infection stage. 1
- The presence of HBsAg with total anti-HBc (antibody to hepatitis B core antigen) but negative IgM anti-HBc indicates chronic HBV infection. 1
- HBsAg alone without other markers can indicate early acute infection or transient presence (up to 18 days) after vaccination. 1
Distinguishing from Other Hepatitis B Markers
It is critical not to confuse HBsAg with anti-HBs (antibody to hepatitis B surface antigen), as they represent opposite clinical states. 2, 3
- Anti-HBs indicates immunity from either resolved infection or successful vaccination, with levels ≥10 mIU/mL considered protective. 1, 2
- HBsAg indicates active infection and infectivity to others. 1
- These two markers are mutually exclusive in typical infections—you cannot have both positive simultaneously except during rare transition periods. 1
Clinical Applications
Screening Populations
Universal HBsAg screening is recommended for all pregnant women at the first prenatal visit, regardless of previous vaccination or negative tests, to prevent perinatal transmission. 1
Routine HBsAg testing is recommended for persons born in geographic regions with HBsAg prevalence >2%, men who have sex with men, and injection-drug users. 1
Postexposure Management
HBsAg status of the source patient determines postexposure prophylaxis decisions. 1
- For HBsAg-positive source exposures, unvaccinated persons require both hepatitis B vaccine series and HBIG (hepatitis B immune globulin). 1
- Previously vaccinated persons exposed to HBsAg-positive sources should receive a hepatitis B vaccine booster dose. 1
Important Caveats
To ensure an HBsAg-positive result is not false-positive, samples with repeatedly reactive HBsAg results should be tested with a neutralizing confirmatory test. 1
HBsAg-negative results do not always guarantee absence of HBV infection—occult hepatitis B infection (OBI) can occur where HBV DNA is present in the liver but HBsAg is undetectable in serum. 4