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

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.

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.

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).

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.