The second type of test, the interferon-gamma release assay (IGRA), is a relatively recent blood test. The IGRA blood test is an immunodiagnostic test measuring an individual's immune reactivity to M. tuberculosis. In these laboratory tests, white blood cells (CD4 and CD8 T-cell lymphocytes) from most individuals infected with M. tuberculosis will release the cytokine interferon-gamma (IFN-g) when mixed with M. tuberculosis antigens. This makes IFN-g a promising biomarker for cell-mediated immunity.
Currently, two commercial IGRAs are readily available for use in the United States (CDC, 2016; Oxford Immunotec, 2020; Qiagen, 2019):
- QuantiFERON®-TB Gold In-Tube test (QFT-Plus) - the newest 4th generation assay utilizes four precoated specimen tubes - to include NIL (negative control), TB-1 and TB-2 ANTIGENs, and MITOGEN (positive control). (Note: This is a recent update from the 3rd generation assay seen in the CDC image). A single whole-blood lithium heparin tube draw is performed and transferred to the four kit tubes. (Alternatively, the four tubes included in the QFT kit may be collected at the time of draw.)
- SPOT-TB® test (T-Spot) - The current assay utilizes a single whole-blood lithium heparin tube drawn from the patient. (The NIL, TB-ANTIGENs, and MITOGEN are included in the procedure using reagents and microplate wells rather than individual coated specimen tubes.)
