Meaning of HBsAg Reactive
A reactive (positive) HBsAg test result indicates current hepatitis B virus infection—either acute or chronic—and the person is infectious. 1
What HBsAg Reactive Means
- HBsAg (Hepatitis B surface antigen) is the hallmark serologic marker of active HBV infection. 2
- A confirmed positive HBsAg result means you have current HBV infection in your bloodstream and can transmit the virus to others. 1
- All HBsAg-positive persons are considered infectious, regardless of other test results. 1
Distinguishing Acute vs. Chronic Infection
To determine whether the infection is acute (recent) or chronic (long-standing), additional testing is required:
- If IgM anti-HBc (IgM antibody to hepatitis B core antigen) is positive: This indicates acute or recently acquired infection. 1
- If IgM anti-HBc is negative but total anti-HBc is positive: This indicates chronic HBV infection. 1
- If HBsAg persists for more than 6 months: The infection is definitively classified as chronic, as spontaneous clearance becomes unlikely after this timeframe. 1
Important Clinical Caveats
False Positives and Confirmatory Testing
- Repeatedly reactive HBsAg results should be confirmed with an FDA-cleared neutralizing confirmatory test to exclude false-positive results. 1
- Transient HBsAg positivity can occur up to 18 days after hepatitis B vaccination (up to 52 days in hemodialysis patients) and is clinically insignificant. 1
Early Infection Window
- HBsAg can be positive before anti-HBc appears (the first 1-2 weeks of infection), representing very early acute infection. 1
- In this scenario, you would see HBsAg positive with all other markers negative. 1
Next Steps After Reactive HBsAg
All HBsAg-positive persons require comprehensive evaluation to determine disease activity and severity: 3
- Test for disease activity: AST, ALT (liver enzymes) 3
- Test for liver function: Bilirubin, albumin, prothrombin time 3
- Test for viral replication status: HBeAg, anti-HBe, HBV DNA quantification 3, 2
- Imaging: Ultrasound to assess for cirrhosis and exclude focal liver lesions 3
- Consider liver biopsy: In patients with elevated liver enzymes to assess fibrosis stage and determine urgency of antiviral therapy 3
Infectivity and Transmission Risk
HBsAg-positive persons can transmit HBV through: 1
- Sexual contact
- Percutaneous exposure (needlestick, sharing needles)
- Perinatal transmission (mother to infant during delivery)
- Household contact with blood exposure
Unvaccinated exposed persons require postexposure prophylaxis with hepatitis B vaccine series plus HBIG (hepatitis B immune globulin) within 24 hours, preferably. 1