Comparison of interferon-gamma release assay and tuberculin test for screening in healthcare workers

Healthcare workers (HCWs) have an increased risk of tuberculosis (TB). Screening for latent tuberculosis infection and active TB is therefore essential in infection control programs. Tuberculin skin test (TST) and Interferon -gamma Release Assay (IGRA) were used simultaneously in 1686 HCWs between May 2007 and April 2009. A chest X -ray was performed in order to exclude active TB when TST was >or=10mm or IGRA was positive and in HCWs with TB contact or symptoms. IGRA was positive in 33.1% and TST was >10mm in 78.3% of the HCWs. The proportion of positive IGRA results increased with the TST diameter. In those with a TST >15mm, 49.2% were IGRA positive. TST was more than twice as often positive than the IGRA. Therefore, TST+/IGRA- results were more often observed than concordant negative or positive results. In none of the HCWs with a TST+/IGRA- result active TB was diagnosed during the study period. Repeated BCG vaccination increased the number of TST+/IGRA- discordance. The smaller the interval after BCG vaccination, the higher was the TST+/IGRA- discordance. In the screened HCWs population, active TB was diagnosed in 9. At the time of diagnosis TST and IGRA were positive in all active TB cases. The study period covers 24 months, therefore the average annual incidence rate was 268/100 000. TB burden in HCWs in Portugal is high. Considering the limitations that TST and IGRA present, the best solution seems to be the use of both, using the IGRA higher specificity for confirming a positive TST, taking advantage of the best characteristics of each test.