Interferon Gamma Release Assay
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
An interferon gamma release assay (IGRA) measures the release of interferon gamma from white blood cells in response to tuberculosis (TB) bacteria using a blood sample taken from a vein in the arm. It is used to help detect latent or active tuberculosis infection, particularly in screening and contact tracing.
Why get tested?
To look for latent or active infection from Mycobacterium tuberculosis.
When to get tested?
You may receive an IGRA test if:
- You are suspected to have tuberculosis.
- You have been in close contact with a known tuberculosis patient.
- You live in or have recently travelled to a country with a high prevalence of tuberculosis.
- You have a compromised immune system meaning you are more susceptible to infection (such as HIV).
- Work or live in close knit community settings such as care homes, prisons, homeless shelters or hospitals where there is close contact between members meaning it is more likely for infection to spread.
- Have been to an area with a known TB outbreak.
Sample required?
A sample of blood will be taken to perform this test.
Test preparation needed?
No test preparation is needed.
Common questions
IGRAs are blood tests for cell-mediated immune response. They measure T‑cell release of IFN‑g following stimulation by antigens specific to the M. tuberculosis complex, such as ESAT‑6 and CFP10. Because of this, they are more specific than the TB skin test as the antigens that trigger IFN‑g release are not present in the strains used in the BCG vaccines or many non-tuberculosis Mycobacteria (NTM). The result indicates that the body has produced an immune response specifically to M. Tuberculosis, which will lead to further testing. This method is becoming preferred over the TB skin test as it will give a quantitative value of IFN‑g as opposed to the visual response from the skin test, which may have subjective interpretation.
- When there is clinical suspicion of tuberculosis or latent tuberculosis.
- After a positive or inconclusive TB skin test.
- When a patient has a compromised immune system (such as having HIV) meaning they are more susceptible to infection.
- When a patient is joining or is part of a high risk community, such as prisons and hospitals.
- When a patient has been exposed to a known tuberculosis patient.
- When a patient has travelled to areas with a higher prevalence of TB.
A positive result means that your body is likely to be producing an immunological response to M. Tuberculosis. Doctors may then perform further tests, such as an AFB sputum culture, which will confirm this. A negative result means that it is unlikely that you have latent or active M. Tuberculosis infection.
The IGRA test may also be called by its commercial names. These are most commonly the T‑spot test and the Quantiferon test, but these both assess the levels of M. Tuberculosis specific IFN‑g in the blood.
Benefits of the test include:
- More specific towards M. Tuberculosis.
- Requires only a single laboratory test compared to the TB skin test, where you require a separate appointment for a medical professional to read the results.
- Analyser interpretation reduces human error.
- IGRA tests typically have a fast turnaround time.
Limitations include:
- Can have false positives and negatives.
- Cannot distinguish between active and latent infection therefore further tests such as AFB sputum culture, CT scans and X‑rays must be performed to confirm this.
- Cross reactivity is recognised between the antigens of M. Tuberculosis and M. leprae, the cause of leprosy.
- Not all non-Tuberculosis Mycobacteria have been assessed for cross reactivity.