Comparison of interferon gamma release assays and tuberculin skin test performance for diagnosing Mycobacterium tuberculosis infection in the Pakistani population

Data on clinical utility of IGRAs in BCG-vaccinated populations is limited. This cross-sectional study evaluated diagnostic performance of ELISPOT and ELISA-based IGRAs and compared them with conventional Tuberculin Skin Test (TST) for detecting Mtb infection in adult Pakistani population. Subjects (n = 325, age 18-64 years) recruited included active TB cases (n = 150), non-TB other pulmonary diseases patients (n = 50) and healthy individuals (n = 125). Two different IGRAs, X-DOT-TB (ELISPOT-based) and QFT-Plus (ELISA-based) were used to screen Mtb infection. Samples from 112/150 (74.7%) active TB patients, 10/50 (20.0%) non-TB other pulmonary diseases cases and 16/125 (12.8%) healthy individuals were X-DOT-TB positive, while 52.7% (29/55) active TB patients, 20.0% (4/20) non-TB other pulmonary diseases cases and 22.6% (12/53) healthy individuals tested by QFT-Plus had positive results. Sensitivity of X-DOT-TB and QFT-Plus was 79.5% (95%CI 77.4-81.5), and 55.7% (95%CI 52.9-58.5), respectively, significantly higher than TST 35.8% (95%CI 34.4-37.1, p < 0.05). Specificity of X-DOT-TB was 85.1% (95%CI 83.2-87.0), that of QFT-Plus was 78.1% (95%CI 75.3-80.9), comparable with TST specificity of 82.2% (95%CI 80.3-84.1, p > 0.05). IGRAs, particularly X-DOT-TB assay found to have greater sensitivity for detecting Mtb infection than TST. Our study suggests that IGRAs can be used as an adjunct diagnostic tool for detection of Mtb infection in BCG-vaccinated populations.