interferon-gamma release assay
An interferon-gamma release assay is a blood test that asks the immune system, in a test tube, whether it remembers TB. A blood sample is mixed with proteins specific to the TB bacterium; if the person's white cells have met TB before, they react by releasing a signalling chemical called interferon-gamma, which the test measures.
Its advantage over the older skin test is selectivity. The proteins it uses are found in the TB bacterium but not in the BCG vaccine and not in most environmental mycobacteria, so a prior BCG jab does not cause a false positive. It also needs only a single visit, since the answer comes from the laboratory rather than from a delayed skin reaction.
Like the skin test, an IGRA detects exposure and immune memory, not active disease, so it cannot tell latent infection apart from active TB and cannot confirm or exclude TB in the lungs. Results can occasionally be indeterminate, and the test does not perform reliably enough to be used as a stand-alone rule-out in seriously ill or immunosuppressed patients.
Before starting a powerful immune-suppressing drug, a patient has an IGRA to check for latent TB so it can be treated first and reactivation avoided.
Screening for latent TB before immune suppression.
IGRA and the tuberculin skin test answer the same question by different means; neither distinguishes latent from active TB, so a positive result still requires clinical and imaging assessment.