Tuberculin skin test and interferon-γ release assay show better correlation after the tuberculin 'window period' in tuberculosis contacts

Background: T-cell interferon-gamma release assays (IGRAs) have been shown to be effective tools for the detection of Mycobacterium tuberculosis infection, offering an enhanced specificity compared to the tuberculin skin test (TST). Most tuberculosis (TB) contact studies have shown a better correlation of IGRA with the intensity of M. tuberculosis exposure than that obtained using the TST. However, the correlation between tests performed before and after the tuberculin 'window period' (time between infection and when the immunological response becomes measurable) remains to be studied. Methods: A longitudinal prospective analysis was performed in TB contacts. We analyzed the correlation between a commercially available IGRA (QuantiFERON (R)-TB Gold in-Tube, QFT) and the TST before and after the tuberculin window period (2 months). Concordance between both tests was assessed using the Kappa coefficient (kappa). Correlation of both tests with the degree of TB exposure was also analyzed. Results: One hundred and fifty-two TB contacts were included in the study. Agreement between the TST and IGRA was better after the window period (kappa = 0.60 at the first visit and kappa = 0.73 after 2 months), especially for non-BCG vaccinated subjects (kappa = 0.81). Both a positive TST and QFT were correlated, after the window period, with the size of place of contact (the smaller the place of contact, the higher the probability of having a positive test) (p = 0.022 and p = 0.02, respectively) and with the total numbers of hours spent with the index case (p = 0.006 for TST and p = 0.007 for QFT). Conclusions: IGRAs are a good alternative to the TST in contact tracing studies, especially after the tuberculin window period.