QuantiFERON-TB (interferon-gamma release assay)
A blood test that checks whether your immune system has encountered the tuberculosis bacterium. It is mainly used to detect latent (dormant) TB infection — for example before starting immune-suppressing treatment — and, unlike the old skin test, is not affected by the BCG vaccine.
What it measures
The QuantiFERON-TB test (an 'interferon-gamma release assay', or IGRA; T-SPOT.TB is a similar test) measures how strongly your immune cells react to proteins specific to Mycobacterium tuberculosis. Blood is mixed with these TB-specific proteins in the lab; if your T-cells have previously met the TB bacterium, they release a signalling chemical called interferon-gamma, which the test measures. A positive result means your immune system has been sensitised to TB — in other words, you carry the infection. Crucially, in most people this is latent TB: the bacteria are present but dormant and controlled by the immune system, causing no symptoms and not contagious. The test cannot by itself tell latent from active TB disease; that distinction needs symptoms, a chest X-ray and sometimes sputum tests. Its main advantages over the older tuberculin skin test are that it needs only one visit and, importantly, is not falsely raised by the BCG vaccine. It is widely used to screen for latent TB in people at higher risk or before treatments that suppress the immune system (which can wake dormant TB), so that latent infection can be treated first. An 'indeterminate' result means the test could not be reliably interpreted and usually needs repeating.
What a high value can mean
- A positive result — the immune system has met the TB bacterium; you carry the infection.
- Latent TB infection — the commonest meaning; dormant bacteria, no symptoms, not contagious, but treatable to prevent future active disease.
- Active TB disease — possible if there are symptoms; needs a chest X-ray and further tests to confirm, as the blood test alone cannot distinguish it.
- An indeterminate result — the test could not be interpreted (sometimes with a weak immune system); usually repeated.
High QuantiFERON-TB (interferon-gamma release assay): full guide
What a low value can mean
- Negative — the expected result; no evidence of TB infection.
- A recent exposure — the test can be negative very soon after contact, before the immune response develops, so it may be repeated after a window.
- A weakened immune system — can occasionally give a false-negative or indeterminate result.
Low QuantiFERON-TB (interferon-gamma release assay): full guide
When to discuss with a doctor
This TB blood test is used to detect latent TB infection — commonly when screening people at higher risk (close contacts of someone with TB, people from higher-incidence regions, healthcare workers) and, importantly, before starting treatments that suppress the immune system, which can reactivate dormant TB. A positive result usually means latent, dormant infection and prompts a chest X-ray and clinical review to rule out active disease; if latent, a course of treatment may be offered to prevent it becoming active later. A positive test with symptoms such as a persistent cough, fevers, night sweats or weight loss needs prompt assessment for active TB, which is contagious and treatable. Mediora.AI shows the TB test result alongside relevant markers so it is read in context; interpreting a positive result and deciding on treatment belong with your doctor.