The QuantiFERON-TB Gold In Tube and the T-SPOT are two in-vitro tests for measuring cell-mediated immune responses to peptide antigens from mycobacteria. These antigens, ESAT-6, CFP-10 and TB7.7 (p4) (which is used only in QFT-G) are absent from all BCG strains and from most non tubercular mycobacterial strains (NTMs) with the exception of M. kansasii, M. szulgai and M. marinum. Individuals infected with M. tuberculosis complex organisms (M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti) have mononuclear cells in their blood that recognise these mycobacterial antigens. This recognition process leads to the stimulation and secretion of IFN-γfrom sensitized T-cells. The detection and quantification of IFN-γ, measured by enzyme-linked immunoassay (Quantiferon) or enzyme-linked immunospot (T-SPOT), forms the basis of these tests – detectable as early as two weeks after infection with M. tuberculosis.
Both tests are intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations. QuantiFERON Gold-IT is the laboratory recommended method for routine screening of patients with a normal lymphocyte count. T spots are recommended in specific circumstances (see below).