Interferon gamma release assay measures IFN release against which M. TB antigen

Ans. is 'a' i.e., ESAT-6 3 Interferon-gamma release assays (IGRAs) are diagnostic tools for latent tuberculosis infection (LTBI). They are surrogate markers of Mycobacterium tuberculosis infection and indicate a cellular immune response to M. tuberculosis. a IGRAs cannot distinguish between latent infection and active tuberculosis (TB) disease and should not be used for diagnosis of active TB, which is a microbiological diagnosis. A positive IGRA result may not necessarily indicate active TB, and a negative IGRA result may not rule out active TB . 3 Because IGRAs are not affected by Bacille Calmette-Guerin (BCG) vaccination status, IGRAs are useful for evaluation of LTBI in BCG-vaccinated individuals, paicularly in settings where BCG vaccination is administered after infancy or multiple (booster) BCG vaccinations are given. Assay antigens M. tuberculosis-specific antigens include :- Early secreted antigenic target 6 (ESAT-6) and Culture filtrate protein 10 (CFP-10). These are encoded by genes located within the region of difference 1 (RD1) segment of the M. tuberculosis genome. They are more specific for M. tuberculosis than purified protein derivative (PPD) because they are not shared with any BCG vaccine strains or most species of NTM other than M marinum, M. kansasii, M szulgai, and M. flavescens. Types of assays Two IGRAs are available in many countries :- The QuantiFERON-TB Gold In-Tube (QFT-GIT) assay, which has replaced the second-generation QuantiferonTB Gold (QFT-G) assay, and the T-SPOTTB assay. The QFT-GIT assay is an enzyme-linked immunosorbent assay (ELISA)-based, whole-blood test that uses peptides from three TB antigens i.e., CFP-10, and TB7.7) in an in-tube format The result is repoed as quantification of interferon (IFN)-gamma in international units (IU) per mL. A newer assay, the QuantiFERON-TB Gold Plus (QFT-Plus), became available in 2015. This test is available in Europe but not in Noh America. The QFT-Plus assay has two TB antigen tubes, unlike the QFT assay (which has a single TB antigen tube). Sensitivity and specificity IGRAs have specificity >95 percent for diagnosis of latent TB infection. The sensitivity for T-SPOTTB appears to be higher than for QFT-GIT or TST (approximately 90, 80, and 80 percent, respectively) . The higher sensitivity of T-SPOTTB may be useful for evaluating individuals with immunosuppressive conditions. TST specificity is high in populations not vaccinated with BCG (97 percent). Among populations where BCG is administered, it is much lower although variable (approximately 60 percent).