Performance of Tuberculin Skin Testing Compared With Interferon‐Gamma Release Assays Foreign‐Born Individuals

Objectives To assess the prevalence of Mycobacterium tuberculosis infection among foreign‐born individuals entering prison and to evaluate the performance of the tuberculin skin test (TST) in vaccinated individuals (Bacillus Calmette‐Guérin vaccine, BCG), using interferon gamma release assays (IGRAS) as the reference standard. Methods We conducted a prospective observational study including foreign‐born individuals admitted to prisons in Catalonia (Spain) between 1 March 2023 and 30 June 2023. A TST was performed if there was no prior positive TST or prior TB infection, or if the TST was negative for more than 1 year. Current TB was excluded by clinical and radiological evaluation. BCG vaccination status was assessed using medical records, identification of a vaccination scar, and the BCG World Atlas. TST positivity was analysed according to BCG vaccination status. Among BCG‐vaccinated individuals with a positive TST, IGRA results were used to estimate the positive predictive value (PPV) of the TST overall and by TST induration diameter. Results Among 1075 foreign‐born individuals admitted during the study period, 32 (3.0%) had a previous diagnosis of active TB. Of the remaining 1043, 86.4% were classified as BCG‐vaccinated. Overall, 45.2% had a positive TST, with no significant difference between vaccinated individuals and unvaccinated individuals (46.1% vs. 39.9%; p = 0.08). Among BCG‐vaccinated individuals with a positive TST, IGRA testing was performed in 71%, yielding an overall TST PPV of 65%. The PPV increased with induration diameter, from 51.6% (10–14 mm) to 81.8% (≥ 20 mm). Conclusions Foreign‐born individuals entering prison show a high prevalence of M. tuberculosis infection. In BCG‐vaccinated individuals, the TST shows limited predictive value, particularly at lower induration thresholds. While further studies are needed, these findings support the preferential use of IGRA‐based strategies for LTBI screening in this setting.