Interferon-gamma release assay

Interferon-gamma Release Assay

TheInterferon-gamma release assay (IGRA) is a medical diagnostic test used to detect latent Mycobacterium tuberculosis infection. It is an in vitro blood test that measures the immune response to specific antigens associated with the tuberculosis (TB) bacterium. Unlike the traditional tuberculin skin test (TST), IGRA does not require the patient to return for a reading, and it is not affected by prior Bacillus Calmette–Guérin (BCG) vaccination.

Principle of the Test

IGRAs are based on the principle that T-cells from individuals infected with M. tuberculosis will release interferon-gamma (IFN-γ) when re-exposed to specific antigens. These antigens, such as ESAT-6 and CFP-10, are absent from BCG strains and most non-tuberculous mycobacteria, making the test more specific for M. tuberculosis.

Procedure

1.Blood Collection: A blood sample is drawn from the patient. 2.Incubation: The blood is mixed with antigens specific to M. tuberculosis and incubated. 3.Detection: The amount of IFN-γ released in response to the antigens is measured using an enzyme-linked immunosorbent assay (ELISA). 4.Interpretation: The results are interpreted based on the amount of IFN-γ released compared to control samples.

Advantages

-Specificity: IGRAs are more specific than the TST, especially in BCG-vaccinated individuals. -Convenience: Only one patient visit is required, as opposed to the two visits needed for the TST. -No Booster Effect: Repeated testing does not boost the immune response, unlike the TST.

Limitations

-Cost: IGRAs are generally more expensive than the TST. -Indeterminate Results: Some tests may yield indeterminate results, requiring retesting. -Limited Data in Certain Populations: There is limited data on the use of IGRAs in children under 5 years of age and in immunocompromised individuals.

Clinical Use

IGRAs are used primarily for: - Screening for latent TB infection in individuals at risk, such as healthcare workers and those with close contact with TB patients. - Diagnosing latent TB infection in individuals who have received the BCG vaccine.

Comparison with Tuberculin Skin Test

While both IGRAs and the TST are used to detect latent TB infection, IGRAs offer several advantages in terms of specificity and convenience. However, the choice between the two tests may depend on the clinical setting, availability, and cost considerations.

Also see

- Tuberculosis - Interferon - Enzyme-linked immunosorbent assay - Bacillus Calmette–Guérin