To look for latent or active infection from Mycobacterium tuberculosis.
Interferon Gamma Release Assay
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.
A sample of blood will be taken to perform this test.
No test preparation is needed.
The IGRA is a blood test that looks for the levels of interferon gamma (IFN-g) released by white blood cells as a response to the presence of Mycobacterium tuberculosis, the causative agent of Tuberculosis (TB). This is most commonly acquired through respiratory transmission and it is able to live in an inactive state (latency) or cause disease. Disease is most commonly affects the pulmonary system with symptoms of weight loss, coughing up blood, a persistent cough and night sweats, however it is not limited to the pulmonary system can affect multiple systems within the body. This test can provide doctors with a quick indication of infection from this organism, however it should not be used in isolation and should be supported by acid fast bacillus (AFB) culture or polymerase chain reaction (PCR) results as well as imaging (such as CT scans or X-rays) to confirm diagnosis.
How is it used?
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 is it requested?
- 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.
What does the test result mean?
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.
Is there anything else I should know?
Benefits and Limitations of the IGRA test
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.
- 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.