Comparative agreement between tuberculin skin test and QuantiFERON-TB Gold In-Tube across different levels of exposure in a college-based tuberculosis outbreak

Background Optimal intervention thresholds for students in college TB outbreaks remain debated. Tuberculosis preventive treatment (TPT) is recommended for tuberculin skin test (TST) induration greater than 15 mm. The possibility of TPT recommendation for individuals with TST medium or weak positivity in the context of recent exposure, particularly with close contact with TB, is not entirely clear. Methods The study used a cross-sectional design and included a 1-year follow-up of participants after preventive treatment. TST and QuantiFERON-TB Gold In-Tube test (QFT) were used among close contacts of the tuberculosis cases from a college. Since individuals with a strongly positive TST result undergo TPT directly, close contacts with a TST less than 15 mm were further assayed with QFT. TST and QFT were compared based on TB exposure, which was classified as inside or outside classes with TB cases. Student’s t -tests, Chi-square tests, Kappa statistics for agreement, and the Cochran–Armitage test for trend analysis were performed. Results A total of 1,182 close contacts of TB cases during an outbreak were administered TST. A total of 175 (14.8%, 175/1,182) students had a strongly positive test result (≥15 mm). QFT was performed on the remaining 1,007 students, of whom 85 (8.4%, 85/1,007) tested positive. Among individuals with TST negative indurations (<5 mm), low positivity (5–9 mm), and medium positivity (10–14 mm), QFT positivity was 2.9% (7/237), 5.4% (3/55), and 10.5% (75/715), respectively. The rates of QFT positivity varied with increased TST levels ( χ 2 = 13.751, p = 0.001). The latent tuberculosis infection rate, as determined by QFT, was significantly higher among students in classes with TB cases (14.11%, 24/149) than among those in classes without TB cases (7.11%, 61/858; p < 0.001). Notably, in classes with TB cases, TST and QFT demonstrated moderate agreement (Kappa = 0.52, p < 0.001), supporting the use of the 10 mm cutoff to represent infection status in high-exposure settings. Conclusion TST showed better concordance with QFT when close contacts were under intense and frequent TB exposure, and a 10 mm result for TST would be a potential recommendation for preventive treatment in close contacts with high exposure to TB transmission.