What are Interferon Gamma Release Assays?
Interferon Gamma Release Assays (IGRAs) are blood tests used primarily to detect
latent tuberculosis infection (LTBI). Unlike the traditional
tuberculin skin test (TST), IGRAs measure the immune response in the form of
interferon gamma (IFN-γ) production when blood cells are exposed to specific antigens derived from Mycobacterium tuberculosis.
How do Interferon Gamma Release Assays work?
The principle behind IGRAs is based on the release of
cytokines by T-cells in response to specific antigens. When blood samples from individuals who have been exposed to Mycobacterium tuberculosis are mixed with these antigens, T-cells release interferon gamma. The amount of IFN-γ released can be quantified using enzyme-linked immunosorbent assays (ELISA) or enzyme-linked immunospot (ELISPOT) assays, providing an indication of infection.
What are the advantages of IGRAs over TST?
IGRAs offer several advantages over the traditional TST. Firstly, they are more specific because they use antigens not present in
BCG vaccine strains or most non-tuberculous mycobacteria, reducing false positives. Secondly, IGRAs require only a single patient visit, unlike TST, which requires a follow-up visit to read the results. Moreover, IGRAs have no boosting effect, meaning repeated tests do not affect the results.
What are the limitations of IGRAs?
Despite their advantages, IGRAs have limitations. They are more expensive and require sophisticated laboratory facilities and trained personnel, making them less feasible in low-resource settings. Additionally, IGRAs do not differentiate between latent and active tuberculosis, nor can they predict the risk of progression from latent infection to active disease. Furthermore, immunocompromised individuals may have a reduced IFN-γ response, potentially affecting test sensitivity.In which populations are IGRAs most useful?
IGRAs are particularly beneficial in
BCG-vaccinated individuals, as they reduce the likelihood of false-positive results associated with the TST. They are also preferred in healthcare workers and other professionals at risk of exposure to TB, as well as in settings where follow-up visits for TST readings are impractical. Additionally, IGRAs are useful for testing children over the age of five and adults in low TB prevalence settings.
How do IGRAs fit into the overall TB diagnostic strategy?
IGRAs are an important tool in the
control of tuberculosis, particularly for identifying latent infections. They are used in conjunction with clinical evaluation, radiography, and microbiological testing for comprehensive TB diagnosis. The choice between IGRAs and TST should be informed by resource availability, patient population characteristics, and specific clinical scenarios.
Are there any recent advancements in IGRA technology?
Recent advancements in IGRA technology aim to improve the sensitivity and specificity of these tests. Some
research and development efforts focus on identifying new antigens that could enhance the diagnostic accuracy for both latent and active TB. Additionally, efforts are being made to develop point-of-care tests that are more accessible in resource-limited settings.
Conclusion
Interferon Gamma Release Assays play a crucial role in the detection of latent TB infection, offering significant advantages over traditional methods in certain populations. While they are not without limitations, their specificity and single-visit requirement make them a valuable component of modern TB diagnostic strategies. Ongoing advancements in IGRA technology promise to further refine their utility in the field of
infectious diseases.
