Role Of The QFT-IT Assay For The Diagnosis Of Latent Tuberculosis Infection Among Adult Immigrants

Background. Accurate identification and treatment of contacts with latent tuberculosis infection (LTBI) is a desirable goal to achieve effective tuberculosis (TB) control in areas with low prevalence of disease. In immigrants from high prevalence countries, especially in those who are close contacts of active TB cases, the low specificity of the tuberculin skin test (TST) represents an obstacle to the identification of truly infected BCG-vaccinated individuals. The Interferon-Gamma (IFN-g) Release Assays (IGRAs), based on M. tuberculosis -specific antigens, might improve LTBI diagnosis in this population. Methods. In a retrospective study, we assess the performance of the QuantiFERON-TB Gold In-Tube (QFT-IT, Cellestis Ltd., Victoria, Australia) assay and the TST in 84 adult immigrants from high prevalence countries: 68 (80.9%) of those individuals were close contacts of active TB cases. Results. In 84 immigrants (mean age 37.7 ± 11.6 years, 46.3 % were males, 46.3% were BCG-vaccinated) TST was positive in 68 (80.9%) individuals: among these TST-positive individuals, 26 (38.2%) were negative with QFT-IT. QFT-IT assay tested positive in 44 (52.4%) subjects (TST vs QFT-IT: 80.9% vs 52.4%, p< 0.001). Two (2.4%) subjects tested QFT-IT-indeterminate and was TST-positive. Diagnostic overall agreement between TST and QFT-IT was 63.4% (k=0.23). Conclusions. These preliminary data suggest that the rate of LTBI among adult immigrants from TB endemic countries, in our study most of them also close contacts of active TB cases, is significantly lower when detected by QFT-IT, than by TST. Moreover, our findings suggest that using an IGRA test for LTBI screening in this high risk population might reduce the number of candidates to preventive treatment and can provide potential substantial benefits for TB control.