Interferon, in the context of tuberculosis (TB), primarily refers to its role in TB blood tests, known as interferon-gamma release assays (IGRAs), which help determine if a person is infected with TB bacteria. These tests do not use interferon as a treatment or preventative medication. Instead, they measure the immune system's response to TB proteins.
Understanding TB Blood Tests (IGRAs)
IGRAs are diagnostic tools that detect Mycobacterium tuberculosis infection, the bacteria that causes TB. These tests work by measuring the amount of interferon-gamma (IFN-γ) released by a person's white blood cells when they are exposed to TB-specific antigens (proteins).
How IGRAs Work:
- Blood Sample Collection: A blood sample is taken from the individual being tested.
- Antigen Exposure: The blood sample is exposed to synthetic peptides representing Mycobacterium tuberculosis antigens.
- IFN-γ Measurement: If the person is infected with TB bacteria (latent or active), their immune system will recognize these antigens and release IFN-γ. The amount of IFN-γ released is then measured.
- Result Interpretation: The results are interpreted to determine if the individual has a TB infection. A positive result generally indicates TB infection (latent or active), requiring further evaluation to rule out active TB disease.
Types of TB Blood Tests (IGRAs):
There are two main IGRAs approved by the U.S. Food and Drug Administration:
- QuantiFERON-TB Gold In-Tube (QFT-GIT): This test involves collecting blood directly into tubes coated with the TB antigens.
- T-SPOT.TB test: This test counts the number of T cells that release interferon-gamma when stimulated by TB antigens.
Comparison with Tuberculin Skin Test (TST):
While IGRAs are blood tests, the tuberculin skin test (TST), also known as the Mantoux test, is another method for detecting TB infection. Here's a brief comparison:
| Feature | IGRA | TST (Mantoux) |
|---|---|---|
| Type of Test | Blood test | Skin test |
| Method | Measures IFN-γ release | Measures skin reaction to injected antigen |
| Requires return visit | Typically not | Required to have the test read within 48-72 hours |
| BCG Vaccination Interference | Not affected | Can be affected |
| Preferred in | People who have received BCG vaccine |
Important Considerations:
- A positive IGRA or TST result only indicates TB infection; it does not distinguish between latent TB infection (LTBI) and active TB disease. Further testing (e.g., chest X-ray, sputum cultures) is needed to determine if a person has active TB disease.
- IGRAs are generally preferred over the TST for people who have received the BCG vaccine, as the BCG vaccine can cause false-positive TST results.
In summary, interferon's role in the context of TB is within the diagnostic process. Interferon-gamma release assays (IGRAs) measure the release of interferon-gamma to detect TB infection. They are not used as a treatment or preventative medication for TB.
