How does interferon-γ release assay (IGRA) work? Does it show false positive resutls in patietns with baseline elevation of interferon-gamma, for example, in patients with chronic fungal infections, etc.?
Interferon-gamma release assays (IGRAs) are medical tests used in the diagnosis of tuberculosis. Interferon-gamma release assays rely on the fact that T-lymphocytes will release interferon-gamma when exposed to specific antigens.
Interferon gamma. Image source: Wikipedia.
The 3 types of interferons, remembered by the mnemonic ABG: alpha, beta, gamma (click to enlarge the image).
There are currently two interferon-γ release assays available for the diagnosis of tuberculosis:
- QuantiFERON-TB Gold (licensed in US, Europe and Japan)
- T-SPOT.TB, a form of ELISPOT (licensed in Europe)
QuantiFERON-TB Gold test quantitates the amount of gamma interferon produced in response to the ESAT-6 and CFP-10 antigens from Mycobacterium tuberculosis, which are distinguishable from those present in BCG and most other non-tuberculous mycobacteria.
T-SPOT.TB test determines the total number of individual effector T cells expressing gamma interferon.
Blood samples are mixed with one of the following:
- antigens that produce a reaction in the sensitized cells
- negative control (saline)
- positive control (phytohemagglutinin)
The antigens include mixtures of synthetic peptides representing two M. tuberculosis proteins, ESAT-6 and CFP-10. After incubation of the blood with antigens for 16 to 24 hours, the amount of interferon-gamma (IFN-gamma) is measured.
If the patient is infected with M. tuberculosis, their white blood cells will release IFN-gamma in response to contact with the TB antigens. The QFT-G results are based on the amount of IFN-gamma that is released in response to the antigens.
Interferon-Gamma Release Assays (IGRAs) - Blood Tests for TB Infection. CDC.