The blood tests for tuberculosis are called interferon-gamma release assays, or IGRAs.
Understanding IGRAs
IGRAs are blood tests used to determine if someone is infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB). Unlike the traditional skin test (TST), IGRAs only require one patient visit.
How IGRAs Work:
- Blood Sample: A blood sample is taken from the individual being tested.
- TB Protein Exposure: The blood sample is then mixed with specific TB proteins.
- Immune Response Measurement: The test measures how the person's immune system reacts to these TB proteins. Specifically, it measures the amount of interferon-gamma (IFN-γ) released by immune cells in response to the TB proteins.
- Positive Result: A positive result indicates that the person's immune system has previously encountered TB bacteria and has become sensitized to it. This means they are likely infected with TB, but further tests are needed to determine if they have active TB disease.
- Negative Result: A negative result suggests that the person's immune system has not been exposed to TB bacteria, meaning they are likely not infected. However, false negatives can occur, especially in people with weakened immune systems.
Types of IGRAs:
There are several commercially available IGRAs. Two common examples include:
- QuantiFERON-TB Gold In-Tube (QFT-GIT): This test requires only one tube of blood and measures the amount of interferon-gamma released in response to TB antigens.
- T-SPOT. TB test: This test counts the number of T cells that release interferon-gamma in response to TB antigens.
Advantages of IGRAs:
- Single Visit: Requires only one patient visit, unlike the TST which requires a return visit to read the results.
- Not Affected by BCG Vaccination: IGRAs are not affected by prior Bacille Calmette-Guérin (BCG) vaccination, which can cause false-positive results with the TST.
- Objective Results: The results are objective and laboratory-based, eliminating the subjective interpretation required for the TST.
Disadvantages of IGRAs:
- Cost: Generally more expensive than the TST.
- Blood Draw Required: Requires a blood draw, which may be a barrier for some individuals.
- Laboratory Processing: Requires laboratory facilities and trained personnel.
In conclusion, IGRAs are a valuable tool for diagnosing TB infection, offering advantages over the traditional skin test in certain situations. They measure the immune system's response to TB proteins in a blood sample.
