A 6-month-old infant born to an HBsAg-positive mother presents for routine immunisation review. The child received HBV vaccine at birth and 1 month of age. What is the most appropriate investigation to assess the need for HBIG and confirm protective immunity before the 6-month booster dose?

## Investigation of Choice: Anti-HBs Titre ### Clinical Context Infants born to HBsAg-positive mothers require special management to prevent vertical transmission. The IAP immunisation schedule recommends HBV vaccination at birth (within 12 hours) along with HBIG (hepatitis B immunoglobulin) administration. ### Why Anti-HBs is the Answer **Key Point:** Anti-HBs titre measurement is the gold standard to assess: 1. Response to HBV vaccination (protective immunity) 2. Need for additional HBIG doses 3. Efficacy of the vaccination schedule in high-risk infants **High-Yield:** According to IAP guidelines, anti-HBs titre ≥10 mIU/mL indicates protective immunity. Titres <10 mIU/mL suggest inadequate response and may warrant additional HBIG or modified vaccination schedules. ### Testing Timeline | Investigation | Timing | Purpose | |---|---|---| | Anti-HBs | 1-2 months after primary series completion | Assess protective immunity | | HBsAg | Birth, 6 months, 12 months | Screen for active infection | | Anti-HBc | Indicates past/current infection | Not used for immunity assessment | **Clinical Pearl:** In infants born to HBsAg-positive mothers, anti-HBs should be checked at 6-9 months (after completing the primary series) to confirm seroconversion before the booster dose. ### IAP Recommendation Algorithm ```mermaid flowchart TD A[Infant born to HBsAg+ mother]:::outcome --> B[HBV vaccine + HBIG at birth]:::action B --> C[Complete primary series at 1, 6 months]:::action C --> D[Check Anti-HBs at 6-9 months]:::action D --> E{Anti-HBs ≥10 mIU/mL?}:::decision E -->|Yes| F[Protective immunity confirmed]:::outcome E -->|No| G[Additional HBIG/vaccine doses]:::action ```