Interferon-γ release assays (IGRA) are medical tests used in the diagnosis of some infectious diseases, especially tuberculosis.
T-lymphocytes will release interferon-γ when exposed to specific antigens.
Currently three interferon-γ release assay available for the diagnosis of tuberculosis:
IFN-γ tests include the following three:
T-SPOT.TB
QuantiFERON-TB Gold
QuantiFERON-TB Gold In-Tube
QuantiFERON-TB Gold test quantitates the amount of gamma interferon produced in response to the ESAT-6 and CFP-10 antigens from Mycobacterium tuberculosis.
The test can distinguish findings from those present in BCG and most other non-tuberculous mycobacteria, and determines the total number of individual effector T cells expressing gamma interferon.
Useful test for the diagnosis of latent tuberculosis in HIV or other immunosuppressed patients.
IGRAs such as the QFT test and the enzyme linked immunospot assay use enzyme-linked immunoabsorbence tp measure antigen specific production of interferon by circulating T cells in whole blood.
IGRAs have been shown in a meta-analysis of 59 meta-analysis studies to have a sensitivity of 97.7% for latent tuberculosis infection, similar to that of the sensitivity of tuberculin skin test.
The specificity of IGRAs is 97%, which is greater than that tuberculin skin test in patients with BCG vaccination.
IGRA targets antigens which are 2 ranking present in M tuberculosis, early secretory antigenic target 6 (ESAT-6), and culture filtrate protein 10 (CFP-10).
The above proteins are present in other mycobacteria, but diagnostic usefulness has not been validated for these other agents.
Under appropriate conditions mycobacterial cultures are positive and approximately 80% of cases.
Smears for acid fast bacteria are negative and a 95% of tissue biopsy specimens.
IGRAs nor tuberculin skin test can distinguish between active tuberculosis and latent infection.
The main advantage of IGRAs is that the test requires only a single patient visit.
The use of an IGRA test is recommended in any situation in which a tuberculin skin test is used.
IGRA refers to interferon-gamma-release assay.
QuantiFERON
Interferon-gamma release assays (IGRAs) are diagnostic tools for latent tuberculosis infection (LTBI).
IGRAs are surrogate markers of Mycobacterium tuberculosis infection and indicate a cellular immune response to M. tuberculosis.
IGRAs cannot distinguish between latent infection and active tuberculosis (TB) disease.
IGRAs should not be used as a sole method for diagnosis of active TB, which is a microbiological diagnosis.
A positive IGRA result may not necessarily indicate active TB; however, a negative IGRA result rules out the possibility of both active and latent tuberculosis.
IGRAs are not affected by Bacille Calmette-Guérin (BCG) vaccination status.
The specificity of tuberculin skin test (TST) varies depending on timing of BCG and whether repeated vaccinations are given.
An interferon-γ release assay (IGRA) is used in tuberculosis diagnosis.
The result is reported as quantification of IFN-gamma in international units (IU) per mL.
An individual is considered positive for M. tuberculosis infection if the IFN-gamma response to TB antigens is above the test cut-off.
QuantiFERON-TB Gold In-Tube (QFT-GIT), is the third generation test.
Approved by the Food and Drug Administration (FDA) as an aid for detecting latent Mycobacterium tuberculosis infection.
This test is an in vitro diagnostic aid that measures a component of cell-mediated immune reactivity to M. tuberculosis and is based on the quantification of interferon-gamma (IFN-γ) released from sensitized lymphocytes in whole blood incubated overnight with purified protein derivative (PPD) from M. tuberculosis and control antigens.
Tuberculin skin testing (TST) has been used for years as an aid in diagnosing latent tuberculosis infection (LTBI) and includes measurement of the delayed type hypersensitivity response 48–72 hours after intradermal injection of PPD.
TST and QFT do not measure the same components of the immunologic response and are not interchangeable.
Assessment of the accuracy of these tests is limited.by lack of a standard for confirming LTBI.
QFT testing requires phlebotomy, and assesses responses to multiple antigens simultaneously and does not boost anamnestic immune responses.
Compared with TST, QFT results are less subject to reader bias and error.
The level of concordance of the tests is affected by prior bacille Calmette-Guérin (BCG) vaccination, immune reactivity to nontuberculous mycobacteria (NTM), and a prior positive TST.
A positive IGRA result is predictive of future active TB risk.
IGRA is at least as sensitive and was more specific compared to traditional TST.
The progression rate to active disease among untreated QFT positive individuals is significantly greater than for untreated TST positives (14.6% versus 2.3%).
In a small study of close contacts who went on to develop active TB, all were QFT positive, but only 83% were TST positive.
Prior BCG vaccination can produce false positive TST results.
Evidence suggests false positive TST results are common and that QFT testing could guide more targeted treatment and alleviate unnecessary anti-tuberculous treatment.
The FDA’s cutpoint for a positive result was established at >0.34 International Units/millilitre (IU/ml).
In areas of low risk and low prevalence, the positive predictive value of any test is diminished.
Limitations of QFT include the need to draw blood and process it within 12 hours after collection and limited laboratory and clinical experience with the assay.
Blood samples are mixed with antigens that 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.
Clinical evaluation and tests such as a chest radiograph, sputum smear, and culture are needed to differentiate between a diagnosis of latent TB or active TB.
Results can be available within 24 hours.
Does not boost responses measured by subsequent tests, which can happen with tuberculin skin tests (TST), and is not subject to reader bias that can occur with TST, and is not affected by prior BCG (bacille Calmette-Guérin) vaccination.
Blood samples must be processed within 12 hours after collection while white blood cells are still viable.
There is limited data on its use in children younger than 17 years of age, among persons recently exposed to M. tuberculosis, and in immunocompromised persons.
False positive results can occur with Mycobacterium szulgai, Mycobacterium kansasii, and Mycobacterium marinum.
Has a consistent specificity of >99% in low risk individuals and a sensitivity as high as 92% in individuals with active disease, depending on setting and extent of disease.