Interferon-gamma release assays in immigrant contacts and effect of remote exposure to Mycobacterium tuberculosis

OBJECTIVE: To assess the association between remote exposure to tuberculosis (TB) and results of the tuberculin skin test (TST), and two interferon-gamma release assays (IGRAs)-QuantiFERON®-TB Gold In-Tube (QFT-GIT) and T-SPOT.TB®-in immigrant contacts of sputum smear-positive TB patients.METHODS: Immigrants aged ≥16 years in close contact with smear-positive TB patients were included. QFT-GIT and T-SPOT.TB were performed if the TST induration size was ≥5 mm. Associations between test results and origin from an endemic country were assessed.RESULTS: Of 433 close contacts, 322 (74%) had TST ≥5 mm, of whom, 282 (88%) had valid test results for all assays. Positive QFT-GIT results were obtained for 152/282 (54%) and positive T-SPOT.TB for 168/282 (60%). After adjustment for age, sex and recent contact, positive IGRA results and TST results ≥10 mm were found to be more frequent among immigrants who originated from Africa, in particular sub-Saharan Africa.CONCLUSION: When IGRAs are used to determine latent TB infection in foreign-born individuals, positive findings not only relate to recent TB infection, but also reflect prior TB exposure in the country of origin. This late reactivity will limit their usefulness in contact investigations among immigrants originating from endemic areas.