March 12, 2026 Author : Mafalda Teixeira (Internal Medicine) The IGRA test is a screening test that reveals whether the body has had contact with the mycobacterium that most commonly causes tuberculosis. It may be prescribed for people at higher risk of having a latent infection or of it developing into tuberculosis. It is not the same as a tuberculin skin test and has several advantages over it. What is the IGRA test and what is its purpose? IGRA stands for interferon gamma release assay. It is a blood test that reveals whether the body has had previous contact with Mycobacterium tuberculosis . Mycobacterium tuberculosis is the mycobacterium that most frequently causes tuberculosis. IGRA is generally used as a screening test for latent Mycobacterium tuberculosis infection in people who have had contact with an active tuberculosis case or who belong to risk groups like immunosuppressed patients or candidates for biological therapies, among others. Having a latent infection means that Mycobacterium tuberculosis exists in the body in an inactive form. However, a latent infection can turn into an active infection. It is also an alternative to the tuberculin skin test (or Mantoux test) when this one is inconclusive, impractical, or in people previously vaccinated with BCG, reducing false positive results. What is the difference between a latent and an active Mycobacterium tuberculosis infection? A latent infection means that the mycobacterium is present in the body, but in an inactive form , with very low replication and contained by the organism immune response. The person has no symptoms, feels well, has usually normal imaging tests, and does not transmit the infection to others (is not contagious). Having an active Mycobacterium tuberculosis infection means that the mycobacterium is active , with significant replication and tissue damage, such as in lung tissue. The disease caused by this infection is tuberculosis and is associated with: Symptoms, for example cough (sometimes with blood), fever, night sweats, fatigue, weight loss; Changes in diagnostic tests, such as chest imaging; Risk of contagion to other people; Need for treatment. Is the IGRA test enough to diagnose a latent infection? No, the IGRA test result (positive / negative / indeterminate) must be interpreted in the context of the other data of the person to whom it was prescribed . An indeterminate result means that a reliable interpretation was not possible and it may be necessary to repeat the test. A positive IGRA test, by itself, does not distinguish between latent and active infection. It does not mean that one has tuberculosis, but it also does not rule out its occurrence. That is, when the IGRA test result is positive : If there are no symptoms or changes in diagnostic tests that suggest tuberculosis, it is probably a latent infection. If there are symptoms, diagnostic tests are necessary to confirm that it is an active infection. On the other hand, a negative IGRA test makes previous contact with Mycobacterium tuberculosis less likely, although it does not completely rule it out. Who may have indication for an IGRA test? This test may be prescribed for people who have a higher risk of having a latent infection and/or that it will progress to an active infection , for example: Those who have infection with the human immunodeficiency virus (HIV) or other diseases that interfere with the proper functioning of the immune system. Those undergoing treatment with medications that have immunosuppressive effects, both intentional and secondary, for example: Chemotherapy; Treatment with corticosteroids in autoimmune diseases, asthma, inflammatory diseases, among other inflammatory diseases; Biological medications; Treatments after transplants. In the screening of risk groups and close contacts of tuberculosis cases. Healthcare professionals in the context of occupational health. Is the IGRA test and the tuberculin skin test the same? No. The IGRA test is much more recent, it began to be used in clinical practice at the beginning of the 21st century. In many situations has replaced the tuberculin skin test, which for many years was used in the screening of latent tuberculosis. IGRA test measures in a laboratory a specific substance: interferon-gamma released by T lymphocytes in the blood after stimulation by specific antigens of Mycobacterium tuberculosis . The tuberculin skin test is based on the evaluation of a skin reaction that reflects the immune response to tuberculin antigens injected intradermally. Does the IGRA test have advantages over the tuberculin skin test? Yes, it does. The main advantages of the IGRA test are: It only requires a blood sample and is a quick test (a few hours), while the tuberculin skin test is performed in two stages: intradermal injection and, after 2-3 days, reading the reaction. The IGRA test result does not depend on an evaluation that can always introduce some subjectivity, even when the person performing it is very well trained. It is a test with high specificity and good sensitivity , resulting in fewer false positives than the tuberculin test. It is not influenced by BCG vaccination , while the tuberculin test cannot distinguish whether the skin reaction is due to infection or vaccination. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
