Additional value of interferon-γ release assay to tuberculin skin test for diagnosis of latent tuberculosis infection in Greek health care workers

Background: Increased emphasis is given lately on the need for effective screening for latent tb infection (LTBI) and active tb, among health care workers (HCWs). Interferon-γ release assays (IGRAs) are more specific than tuberculin skin test (TST) for LTBI screening, mainly because they remain negative following BCG vaccination or infection with non-tuberculous mycobacteria (NTMB). Aims and objectives: The majority of Greeks receive BCG vaccination between 6-10 yo while NTMBs are present, mainly in coastal areas. Therefore, the combination of TST + IGRA seems more suitable for LTBI detection than TST alone. Methods: 785 HCWs in a University Hospital had TST. 537 of them (68.1%) were BCG vaccinated. TST ≥10mm was considered positive and was followed by QuantiFERON-TB® Gold In-Tube Assay (QFT-GIT). Analyses were performed in SPSS 16.0. Results: From the 785 HCWs, 286 (36.4%) were TST positive (TST +). 209 of them had IGRA. Only 30/209 were positive (TST+/IGRA+ = 14.3%) and LTBI treatment was suggested. The remaining 179 were followed for 9-24mo and none developed active tb. TST+/IGRA+ increased with TST diameter from 5.7% (10-14mm) to 48.8% (>20mm). TST+/IGRA- were most likely in younger, recently BCG vaccinated HCWs(84.6% in 20-29 yo) and less likely in older HCWs (45 %, in 50-59 yo). Conclusion: A poor overall agreement between TST and QFT-GIT was found, especially among young, BCG-vaccinated HCWs. Use of IGRAs as a second step for the diagnosis of LTBI (after positive TST) offers a more specific screening tool resulting in a significant reduction of number of HCWs in need of LTBI treatment, without increasing the risk of active tb.