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Latent TB — SCE Infectious Diseases MCQ

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EasyTuberculosisLatent TBSCE Infectious Diseases

A 35-year-old woman has a positive Mantoux test (18 mm induration) during contact tracing for a household TB case. She had BCG vaccination as a neonate. Her IGRA (QuantiFERON) is positive. Chest X-ray is normal. She is asymptomatic. What is the significance of the IGRA result in a BCG-vaccinated individual?

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Correct answer: D — IGRA is not affected by prior BCG; a positive result indicates true TB infection

IGRAs (QuantiFERON-TB Gold Plus, T-SPOT.TB) use antigens (ESAT-6, CFP-10) that are specific to M. tuberculosis complex and absent from BCG (M. bovis BCG) and most NTM. Therefore, unlike the Mantoux test, IGRA results are not confounded by prior BCG vaccination. A positive IGRA in a BCG-vaccinated person indicates true latent TB infection and should prompt treatment. This is a major advantage of IGRA over Mantoux in BCG-vaccinated populations.

Reference: NICE NG33 2016 – TB; UKHSA 2024 – IGRA guidance