A 6-month-old infant presents to the immunisation clinic for routine vaccination. The mother reports that the child received BCG at birth and DPT dose 1 at 6 weeks of age. On examination, the child is afebrile and clinically well. The healthcare worker suspects a documentation error regarding previous doses. Which investigation is most appropriate to confirm the immune status of this child against diphtheria, tetanus, and pertussis before proceeding with the next scheduled dose?

Explanation

## Investigation of Choice for Assessing Immune Status to DPT ### Clinical Context When there is uncertainty about prior DPT vaccination history or documentation is unclear, serological confirmation of immunity is the gold standard before administering additional doses. This prevents both under-immunisation and unnecessary re-vaccination. ### Why Serum IgG Antibody Titres? **Key Point:** Serum IgG antibodies against diphtheria and tetanus toxoids are the most specific and sensitive markers of prior vaccination response. These antibodies persist for years and directly reflect immune protection. **High-Yield:** - **Protective IgG levels:** Diphtheria ≥0.1 IU/mL, Tetanus ≥0.1 IU/mL - These are internationally standardized cutoffs for seroprotection - IgG persists long-term; IgM is transient and not useful for assessing prior immunity ### Rationale for This Investigation 1. **Specificity:** Directly measures antibodies to the vaccine antigens (toxoids) 2. **Sensitivity:** Detects prior vaccination even if documentation is lost 3. **Clinical utility:** Guides decision on whether to repeat the dose or proceed to next dose 4. **Cost-effective:** Prevents unnecessary re-vaccination if immunity is already established ### Interpretation | Finding | Interpretation | Action | |---------|---|---| | IgG ≥0.1 IU/mL (both toxoids) | Seroprotected from prior vaccination | Proceed to next scheduled dose | | IgG <0.1 IU/mL (either toxoid) | Seronegative; prior dose may not have induced immunity | Repeat the dose or complete series | **Clinical Pearl:** In India, under the National Immunisation Schedule, if vaccination history is doubtful, serology is preferred over repeating the entire series blindly, as it guides rational decision-making and avoids over-vaccination. [cite:Park 26e Ch 7]