The IGRA (Interferon-Gamma Release Assay) is a blood test used to determine if a person has been infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB). It specifically measures a person's immune response to TB bacteria. An IGRA is often used to help diagnose latent TB infection.
How the IGRA Test Works:
The IGRA test works by assessing the body's immune system response to TB bacteria. Here's a breakdown:
- Blood Sample Collection: A blood sample is taken from the individual being tested.
- Exposure to TB Antigens: The blood sample is then exposed to synthetic peptides representing proteins present in Mycobacterium tuberculosis.
- Interferon-Gamma Measurement: If the person has been infected with TB, their immune cells (specifically T cells) will recognize these antigens and release interferon-gamma (IFN-γ). The test measures the amount of IFN-γ released.
- Interpretation of Results: The amount of IFN-γ released is used to determine whether the test is positive (suggesting TB infection), negative (suggesting no TB infection), or indeterminate (inconclusive).
Types of IGRA Tests:
Two main IGRA tests are commonly used:
- QuantiFERON-TB Gold In-Tube (QFT-GIT): This test involves collecting blood directly into tubes containing the TB antigens.
- T-SPOT.TB: This test requires processing the blood sample to isolate specific immune cells before exposure to the TB antigens.
Advantages of IGRA Tests:
- Single Visit: IGRAs require only one patient visit, unlike the TST (Tuberculin Skin Test), which requires a return visit for reading.
- No Boosting Phenomenon: IGRAs are not affected by prior BCG (Bacillus Calmette-Guérin) vaccination, which can sometimes cause false-positive results with the TST.
- Objective Results: The results are read in a laboratory, eliminating subjective interpretation.
Disadvantages of IGRA Tests:
- Blood Draw Required: Requires a blood draw, which may be a barrier for some individuals.
- Laboratory Processing: Requires laboratory processing, which may delay results.
- Cost: Generally more expensive than the TST.
- Limited Data in Children: Less data is available on the use of IGRAs in young children compared to adults.
When is an IGRA Test Recommended?
IGRA tests are recommended for:
- Individuals who have received the BCG vaccine.
- Individuals who are unlikely to return for a TST reading.
- Individuals at higher risk of TB infection, such as those who have been in contact with someone with active TB, healthcare workers, and people with weakened immune systems.
In summary, the IGRA test is a valuable tool for detecting Mycobacterium tuberculosis infection by measuring the immune response to TB antigens in a blood sample.
