IGRAs

IGRAs, or interferon-gamma release assays, are blood tests used to detect infection with Mycobacterium tuberculosis. They measure the release of interferon-gamma from T cells when exposed to TB-specific antigens such as ESAT-6 and CFP-10, antigens absent from most Bacille Calmette-Guérin (BCG) vaccines and many non-tuberculous mycobacteria. The two main commercial formats are QuantiFERON-TB Gold In-Tube (QFT-GIT) and the T-SPOT.TB assay. In QFT-GIT, interferon-gamma is quantified by ELISA after whole blood incubation with TB antigens. In T-SPOT.TB, individual T cells secreting IFN-γ are counted as spot-forming units using an ELISPOT platform. Results are reported as positive or negative, with some tests providing a quantitative IFN-γ value.

IGRAs are used primarily to detect latent TB infection, particularly in people who have been vaccinated with BCG or who have had BCG multiple times, and in healthcare and other groups at increased risk of TB exposure. They are more specific than the traditional tuberculin skin test in BCG-vaccinated populations and require only one visit, unlike the TST which requires a return visit. However, IGRAs do not distinguish latent from active TB disease and cannot definitively diagnose active TB; they should be interpreted with clinical assessment, imaging, and microbiological testing when active disease is suspected. Sensitivity can be reduced in immunocompromised individuals or very young children, and test results can be influenced by assay handling and laboratory conditions. Cost, logistics, and the need for laboratory infrastructure are important considerations in choosing IGRAs for TB screening.