Performance of Tuberculin Skin Test Compared to QFT-IT to Detect Latent TB Among High-risk Contacts in Mexico

Background and AimsWe undertook this study to compare the performance of tuberculin skin test (TST) and QuantiFERON-TB-Gold In-tube assay (QFT-IT) to identify latent TB infection (LTBI) among close contacts of pulmonary TB cases.MethodsA cross-sectional study was conducted in north central Mexico. Thirty nine TB index cases diagnosed between 2008 and 2010 and 123 corresponding close contacts were interviewed regarding their exposure time to the index case prior to TB diagnosis and relevant sociodemographic factors. TST (induration ≥5 and ≥10 mm) and QFT-IT (≥0.35 IU/mL) were tested to determine LTBI status. Kappa coefficients were used to assess the agreement between TST and QFT-IT. Multivariate logistic regression modeling using TST and QFT-IT as dependent variables, and cumulative exposure time and sociodemographic variables associated with LTBI were used as independent variables.ResultsLTBI prevalence in adult contacts was 53.6 and 34.1% when TST cut-offs were set at ≥5 mm and ≥10, respectively, and 41.4% according to QFT-IT. Agreement between TST and QFT-IT was 73.1 and 74.8%, and kappa coefficients 0.47 and 0.46, for TST ≥5 and ≥10 mm, respectively, although these were higher when data were stratified by cumulative exposure, reaching 84.9% and 0.70 for TST ≥5 mm if exposure was ≥500 h/month. None of the predictive variables analyzed for LTBI using multivariate regression was significantly associated.ConclusionsTST ≥5 mm appears to be a useful test to identify LTBI among closely exposed contacts in this geographic setting.