How does interferon-gamma release assay (IGRA) (QuantiFERON) work?

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. (PDF)

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