How reliable is TST for LTBI detection in migrants? Evidence from an Italian three-year study

Background

In 2025 approximately 1,100 migrants were hosted across more than 50 CAS (Centri di Accoglienza Straordinaria-Extraordinary Reception Centers) in the province of Siena (Italy). Upon admission residents undergo diagnostic screening for infectious diseases, with Latent Tuberculosis Infection (LTBI) being of particular epidemiological relevance. As most of this population comes from high TB-incidence countries where BCG vaccination is routinely administered, the reliability of the Tuberculin Skin Test (TST) as a screening tool warrants careful evaluation. Aim of the study is to estimate the prevalence of LTBI among CAS residents in the Amiata Senese-Val d’Orcia-Valdichiana Senese Health District, and to assess the Positive Predictive Value (PPV) of TST in the study population.

Methods

Between March 2023 and March 2026, the local Public Health Unit implemented a sequential screening protocol: TST for all residents, followed by Interferon-Gamma Release Assay (IGRA/QuantiFERON) in TST-positive subjects. IGRA-positive cases proceeded to chest X-ray and infectious disease specialist consultation.

Results

A total of 798 migrants underwent TST (mean age 27 years). Pakistan was the most represented country of origin (279 subjects, 35%), followed by Bangladesh (25.7%). Overall, 251 subjects (31.5%) tested TST-positive. Of these, 196 (78.1%) underwent IGRA testing: 109 (55.6%) were positive, 30 (15.3%) indeterminate, and 57 (29.1%) negative. IGRA-confirmed LTBI prevalence was 13.7% of the total screened population. Excluding indeterminate results, the PPV of TST was 65.7%.

Conclusions

Approximately one in three TST-positive results represents a false positive, likely attributable to prior BCG vaccination. IGRA performance is influenced by the subject’s immune status; accordingly, an indeterminate result may indicate an underlying immunodeficiency. In migrants moving from high TB-incidence countries, IGRA should be preferred as the standard of care for detecting LTBI.

Key messages

• LTBI affects 13.7% of migrants screened in the enrolled CAS facilities, confirming the epidemiological relevance of TB surveillance among migrants from high TB-incidence countries.

• TST shows a PPV of 65.7% in LTBI detection in migrants: IGRA confirmation is essential and should become the standard of care in this population.