A healthcare worker with a previous history of BCG vaccination has a QuantiFERON-TB Gold In-Tube (QFT-GIT) result of 0.38 IU/mL (reactive/positive; cut-off ≥0.35). TST (Mantoux) is 12 mm. What is the preferred interpretation and clinical decision?

• A QFT-GIT is positive indicating active TB disease; start standard 4-drug ATT immediately
• B QFT borderline positive (0.35–0.70) is uninterpretable; TST of 12 mm post-BCG is diagnostic of active TB; start ATT
• C Both QFT and TST are positive due to BCG vaccination cross-reactivity; no treatment needed as BCG vaccination causes false positivity in both tests
• D QFT-GIT detects interferon-gamma response to ESAT-6 and CFP-10 (absent from BCG and most NTM); positive result indicates latent TB infection (LTBI) rather than active TB; clinical and radiological assessment must exclude active disease before starting preventive therapy (isoniazid 6 months or 3HP regimen) ✓

Explanation

QFT-GIT (IGRA) uses peptides from ESAT-6 (early secreted antigenic target 6) and CFP-10 (culture filtrate protein 10), encoded in the Region of Difference 1 (RD1) of M. tuberculosis genome — absent from BCG strains and most non-tuberculous mycobacteria. A positive QFT is therefore specific for TB exposure and is not confounded by prior BCG vaccination (unlike TST). A positive IGRA indicates latent TB infection (LTBI), not active disease — diagnosis of LTBI requires exclusion of active TB by symptoms, CXR, and sputum examination. For LTBI in HCWs, treatment options include isoniazid for 6 months (6H), or weekly isoniazid + rifapentine for 12 weeks (3HP). The 12 mm TST may be partly BCG-related in this context; IGRA is more specific.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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