Diagnosing TB infection in children: analysis of discordances using in vitro tests and the tuberculin skin test.

The aim of the present study was to compare the performance of the interferon (IFN)-γ tests (QuantiFERON®-TB Gold In-Tube (QFT-G-IT) and T-SPOT®.TB) with the tuberculin skin test (TST) in diagnosing tuberculosis (TB) infection in children, and to analyse discordant results. This was a prospective study including 98 children from contact-tracing studies and 68 children with TST indurations ≥ 5 mm recruited during public health screenings. Positive IFN-γ tests results were associated with risk of exposure (p<0.0001). T-SPOT.TB was positive in 11 (78.6%) out of 14 cases with active TB and QFT-G-IT in nine (64.3%) out of 14 cases. Sensitised T-cells against Mycobacterium avium were detected in six out of 12 children not vaccinated with bacille Calmette-Guérin (BCG), a TST induration 5-9 mm in diameter and both IFN-γ tests negative. In concordant IFN-γ tests results, a positive correlation was found (p = 0.0001) between the number of responding cells and the amount of IFN-γ released. However, in discordant IFN-γ tests results this correlation was negative (p = 0.371): an increase in the number of spot-forming cells correlated with a decrease in the amount of IFN-γ released. The use of IFN-γ tests is helpful for the diagnosis of TB infection, avoiding cross-reactions with BCG immunisation and nontuberculous mycobacterial infections. The analysis of highly discordant results requires further investigation to elucidate possible clinical implications.