Interferon- release assays for the diagnosis of active tuberculosis: sensible or silly?

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Abstract
IGRAs, commercially available as the QuantiFERON-TB-Gold® In Tube (QFT-GIT) test (Cellestis, Carnegie, Australia) and as the T-SPOT.TB® test (Oxford Immunotec, Abingdon, UK), are now established in many countries as advanced tools for the immunodiagnosis of latent TB infection (LTBI). In contrast to the mixture of antigens in the purified protein derivate, which is used in the in vivo TST, the IGRA detects effector memory responses to a limited number of specific RD1 peptides by peripheral blood mononuclear cells (PBMC) ex vivo. The genes that encode for the RD1 proteins are absent in the bacille Calmette-Guérin (BCG) vaccine strain. Consequently, in BCG-vaccinated recent contacts of infectious cases, IGRA responses correlate better to M. tuberculosis exposure than the TST 4–8. Therefore, it is plausible that IGRAs are superior to the TST in predicting the development of active TB in BCG-vaccinated recent contacts. A prospective study that evaluated the predictive value of IGRA for the development of active TB in contacts of patients with TB suggested superiority over the TST 9, although the effect was less pronounced in another study in children 10, and no significant difference in predictive values was found in an investigation with an in-house IGRA in the Gambia 11.