What does a positive HBsAg (Hepatitis B surface antigen) test result indicate, a current or past hepatitis B infection?

Interpretation of Positive HBsAg Test Result

A positive HBsAg (Hepatitis B surface antigen) test indicates a current active hepatitis B virus infection, not a past resolved infection. 1

Understanding HBsAg

HBsAg is the hallmark serological marker of hepatitis B virus (HBV) infection. Its presence in serum has specific clinical implications:

• Current infection: HBsAg positivity indicates ongoing HBV infection 1
• Timing: HBsAg is the first serological marker to appear during acute HBV infection, typically detectable 30 days after exposure (range: 6-60 days) 1
• Chronicity: Persistence of HBsAg for more than 6 months defines chronic HBV infection 1

Differentiating Current vs. Past Infection

To properly interpret HBV serological markers:

Current Infection Markers:

• HBsAg positive: Indicates active infection (acute or chronic)
• HBV DNA: Usually detectable, confirming viral replication
• IgM anti-HBc: If positive, suggests acute infection within past 6 months
• Anti-HBs: Typically negative during active infection

Past Resolved Infection Markers:

• HBsAg negative: Cleared from blood after recovery
• Anti-HBs positive: Indicates immunity from past infection
• Anti-HBc positive: Persists for life after infection
• HBV DNA negative: No active viral replication

Clinical Implications of Positive HBsAg

A positive HBsAg result requires:

1. Further testing to determine disease phase:

   • Complete HBV serological panel (HBeAg, anti-HBe, anti-HBc IgM)
   • HBV DNA quantification
   • Liver function tests (ALT/AST)

2. Disease phase assessment:

   • Acute infection: HBsAg+, IgM anti-HBc+
   • Chronic infection: HBsAg+ for >6 months, with variable HBeAg/anti-HBe status 1

3. Monitoring for complications:

   • Risk of progression to cirrhosis and hepatocellular carcinoma
   • Need for antiviral therapy assessment

Common Pitfalls in Interpretation

• False assumption: Many incorrectly believe HBsAg indicates past infection when it actually indicates current infection
• Transient positivity: HBsAg can be transiently positive for up to 18 days after hepatitis B vaccination (clinically insignificant) 1
• Isolated anti-HBc: Some patients have only anti-HBc positive results, which can represent:
  • Resolved infection with waned anti-HBs
  • Low-level chronic infection with undetectable HBsAg
  • False-positive reaction 1

Next Steps After Positive HBsAg

1. Confirm chronicity: Repeat HBsAg testing after 6 months if acute infection is suspected
2. Complete serological workup: Test for HBeAg, anti-HBe, anti-HBc (total and IgM), and HBV DNA
3. Assess liver health: Liver function tests, fibrosis assessment
4. Screen for complications: Ultrasound and alpha-fetoprotein for hepatocellular carcinoma surveillance if chronic
5. Evaluate for treatment: Based on disease phase, viral load, and liver damage

Remember that all HBsAg-positive individuals should be considered infectious 1, and appropriate counseling about preventing transmission should be provided.