Which laboratory test assesses immunity to hepatitis B after vaccination?

Laboratory Test for Hepatitis B Immunity After Vaccination

The laboratory test to check for immunity to hepatitis B after vaccination is anti-HBs (antibody to hepatitis B surface antigen), measured using a quantitative method that can detect protective concentrations ≥10 mIU/mL, typically performed via enzyme-linked immunosorbent assay (ELISA). 1

Timing of Testing

Testing should be performed 1-2 months after administration of the final dose of the vaccine series 1. This timing is critical because:

• Testing too early may not capture the full immune response
• Testing too late may miss the opportunity to identify non-responders who need revaccination
• If HBIG was administered (post-exposure scenarios), testing may need to be deferred for up to 6 months after HBIG administration to avoid detecting passively acquired antibodies 1

Interpretation of Results

Protective immunity is defined as anti-HBs ≥10 mIU/mL 1. The guidelines provide clear thresholds:

• ≥10 mIU/mL: Person is considered immune and protected
• <10 mIU/mL: Person is not adequately protected and should be revaccinated

Important Caveat About Assay Methods

Different assays have different cutoff values, so anti-HBs levels may vary depending on the specific test used 1. Always refer to the package insert of the specific test being used for accurate interpretation. Research suggests that ELISA methods correlate better with protective antibody levels compared to radioimmunoassay, despite RIA being more sensitive 2.

Who Should Be Tested

According to the 2018 ACIP guidelines, postvaccination serologic testing is NOT routine for all vaccinated persons. Testing is specifically recommended for 1:

• Healthcare personnel and public safety workers at risk for blood or body fluid exposure
• Hemodialysis patients and those who might require outpatient hemodialysis
• HIV-infected persons and other immunocompromised persons (e.g., stem cell transplant recipients, chemotherapy patients)
• Sex partners of HBsAg-positive persons
• Infants born to HBsAg-positive mothers (testing includes both anti-HBs AND HBsAg)

For these high-risk groups, documentation of immunity is essential to guide future management and post-exposure prophylaxis decisions.

What Happens If Anti-HBs Is <10 mIU/mL

The guidelines provide a clear algorithm 1:

1. Revaccinate with a complete second series (all 3 doses)
2. Retest 1-2 months after the final dose of the second series
3. If still <10 mIU/mL after 6 total doses:
   • Test for HBsAg to rule out chronic infection
   • If HBsAg-negative: person is a non-responder and susceptible to HBV
   • Counsel about prevention measures and need for HBIG after any HBV exposure

Long-Term Monitoring

Immunocompetent persons with anti-HBs ≥10 mIU/mL have long-term protection and do NOT need further periodic testing 1, 3. This is a critical point that prevents unnecessary repeat testing.

However, immunocompromised persons may need annual testing to assess whether anti-HBs concentrations remain protective 1.

Common Pitfalls to Avoid

• Don't use qualitative tests: Use quantitative methods that can measure the actual concentration, not just positive/negative results
• Don't test too early: Wait the full 1-2 months after the final vaccine dose
• Don't confuse anti-HBs with anti-HBc: Anti-HBc indicates past or current infection, NOT vaccine-induced immunity
• Don't assume all positive results are protective: Some assays may detect antibodies below the protective threshold of 10 mIU/mL 2
• Don't routinely test low-risk individuals: Testing is not cost-effective for those without occupational or other high-risk exposures 3