Also Known As
Quantiferon TB test (QFT)
TB spot test
TB blood test
Formal Name
Interferon Gamma Release Assay (IGRA)
This article was last reviewed on
This article waslast modified on 15 August 2023.
At a Glance
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.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

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.

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Common Questions
  • 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?

    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 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.

    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.