What it measures
An interferon-gamma release assay (IGRA) mixes a blood sample with proteins from the tuberculosis (TB) bacterium. If the white cells have met TB before, they release a substance called interferon-gamma, which the lab measures.
Why people test it
- Used to check for latent TB, an infection with no symptoms, for example after contact with someone with TB.
- Asked for in health checks for some jobs, such as in healthcare.
- Checked before starting some medicines that suppress the immune system.
Typical range
Reported as positive, negative or indeterminate; a positive result shows past contact with TB but cannot tell latent from active TB on its own. Methods and cut-offs vary between labs, and your report shows how the lab reads the result.
Finger-prick or vein?
Offered as a vein draw only. The blood goes into special tubes that need to reach the lab within a set time.
Unlike the TB skin test, the result is not affected by a past BCG vaccination.