The Mantoux test (5 TU PPD intradermally) is performed in a 10-year-old child with BCG vaccination at birth. The induration reads 8 mm at 72 hours. How should this result be interpreted in the Indian context?
- A Positive reaction but induration <10 mm post-BCG may represent BCG-induced sensitisation; IGRA preferred for confirmation ✓
- B Positive; indicates active tuberculosis requiring immediate treatment
- C Negative; BCG vaccination makes all Mantoux tests invalid
- D Positive; induration >5 mm is always diagnostic of TB in any child
Correct answer: A. Positive reaction but induration <10 mm post-BCG may represent BCG-induced sensitisation; IGRA preferred for confirmation
Explanation
In BCG-vaccinated individuals, a Mantoux reaction of 6-10 mm induration is difficult to interpret as it may represent BCG-induced sensitisation rather than M. tuberculosis infection. In India, a cut-off of 10 mm (or 15 mm in some guidelines) is used for BCG-vaccinated children to improve specificity. Interferon-gamma release assays (IGRA — QuantiFERON-TB Gold or T-SPOT.TB) are not affected by BCG vaccination as they use M. tuberculosis-specific antigens (ESAT-6, CFP-10) absent in BCG strains, and are preferred for confirmation when Mantoux is equivocal.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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