A 6-month-old infant born to an HBsAg-positive mother presents for routine immunisation review. The child received HBV vaccine at birth and at 6 weeks. Which investigation is most appropriate to confirm adequate immune response and guide further HBV prophylaxis decisions?

Explanation

## Serological Assessment in Infants Born to HBsAg-Positive Mothers **Key Point:** Anti-HBs titre (antibody to hepatitis B surface antigen) is the gold standard investigation to assess immune response and protective immunity following HBV vaccination in infants born to HBsAg-positive mothers. ### Rationale for Anti-HBs Testing According to IAP immunisation guidelines, infants born to HBsAg-positive mothers require: 1. HBV vaccine at birth (within 12 hours) 2. HBIG (Hepatitis B Immunoglobulin) at birth 3. HBV vaccine at 6 weeks and 14 weeks 4. **Anti-HBs titre testing at 9–12 months** to confirm seroconversion **High-Yield:** A protective anti-HBs titre is ≥10 mIU/mL. If titre is <10 mIU/mL at 9–12 months, the infant is considered a non-responder and requires: - Revaccination with a complete 3-dose series, OR - Testing for HBsAg and anti-HBc to rule out chronic infection ### Interpretation Table | Test Result | Interpretation | Action | | --- | --- | --- | | Anti-HBs ≥10 mIU/mL | Adequate immune response | Protective immunity; routine booster at 5 years | | Anti-HBs <10 mIU/mL | Non-responder | Revaccinate; retest after 1–2 months | | HBsAg positive | Chronic infection | Refer for specialist management | **Clinical Pearl:** Anti-HBs alone is tested at 9–12 months; HBsAg and anti-HBc are tested only if anti-HBs is <10 mIU/mL to rule out occult or chronic infection. [cite:IAP Immunisation Guidelines 2023]