LTBI among migrants by Mediterranean Sea: assessing prevalence and its variations according with different thresholds and diagnostic tools. A 10-month on-field experience.

Background: In 2016, 362 753 migrants reached Europe by sea. Most of migrants come from high tuberculosis (TB) burden countries and travel in conditions that increase the risk for communicable diseases. The goal of WHO End TB Strategy is to end global epidemic by 2035. Management of latent TB infection (LTBI) in low TB incidence countries is thus essential. Nevertheless, a lack of uniformity in policies and procedures for LTBI screening in Europe is perceived. The aim of this study was to estimate the LTBI prevalence in migrants by Mediterranean Sea. Methods: A cross-sectional study was conducted, involving 1038 migrants. Since a gold standard method is not available, LTBI prevalence was assessed in four alternative scenarios with different thresholds and diagnostic tools: (i) TST >= 5 mm; (ii) TST >= 10 mm; (iii) TST >= 5mm plus IGRA; and (iv) TST >= 10mm plus IGRA. TST = tuberculin skin test; IGRA = interferon-gamma release assay. Results: The four scenarios returned the following prevalence: (i) TST >= 5 mm: 40%; (ii) TST >= 10 mm: 33%; (iii) TST >= 5mm plus IGRA: 27%; and (iv) TST >= 10mm plus IGRA: 25%. Moreover, a positive association was found between the proportion of IGRA positive patients and the size of TST induration site. No patient who reported TST >= 18mm tested IGRA negative. Conclusions: Prevalence varied substantially in the investigated scenarios. Significant differences were noted according with the nationality of migrants, probably attributable to different Bacillus Calmette-Guerin vaccination coverage rates in the countries of origin or different exposition to non-tuberculous mycobacteria infection. Data about the nationality can suggest the need of a tailored approach according to migrants' area of origin.