Digestive health
tTG Blood Test UK: What Tissue Transglutaminase Is Telling You
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Last reviewed July 2026
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The tTG blood test measures IgA antibodies against tissue transglutaminase (tTG-IgA). It is the first-line NHS screen for coeliac disease. A raised result suggests coeliac disease and needs a gut biopsy to confirm it; a normal result is only reliable if you were eating gluten and your total IgA is not low. It is an NHS and GP-ordered test, not a self-diagnosis.
tTG stands for tissue transglutaminase. When someone with coeliac disease eats gluten, their immune system produces antibodies against this enzyme, and the blood test picks them up. That is why the tTG-IgA test sits at the front of every coeliac investigation in the UK.
This guide explains what the number means, why a single tTG is never the whole story, and how to make sure your result actually answers the question you asked.
What the tTG test measures
Tissue transglutaminase is an enzyme found throughout the body. In coeliac disease, gluten triggers the immune system to make IgA antibodies against it. The tTG-IgA test counts those antibodies. The more active the coeliac process, the higher the number tends to be, which is why the test is such a useful first screen.
NICE guideline NG20 names IgA tissue transglutaminase (tTG-IgA) as the first-choice test when investigating a possible diagnosis of coeliac disease, alongside a total IgA level.
— NICE guideline NG20, Coeliac disease
The test is highly sensitive: when someone is eating gluten and makes enough IgA, a raised tTG catches the great majority of coeliac cases. But those two conditions, gluten intake and adequate IgA, are exactly what can quietly break the result.
Why total IgA is checked at the same time
Around 1 in 50 people do not make much IgA at all, a harmless variant called selective IgA deficiency. If your total IgA is very low, an IgA-based test like tTG-IgA can read normal even with active coeliac disease. This is the classic false negative.
NICE NG20 is explicit that total IgA should be measured alongside tTG-IgA for exactly this reason. If total IgA is low, the lab switches to IgG-based tests instead. A tTG result without a total IgA beside it is only half a test, so it is worth asking whether yours was checked.
You must be eating gluten for the test to work
The tTG test detects antibodies your body makes in response to gluten. Cut gluten out and the antibodies fade, so the test can read normal even if you are coeliac. This is the single most common reason a coeliac diagnosis gets missed.
“You should eat some gluten in more than one meal every day for 6 weeks before you have the blood tests.”
— NICE guideline NG20, Coeliac disease
If you have already gone gluten-free and feel better, do not simply start eating it again to test. Speak to your GP first. A supervised gluten challenge, with the right follow-up, is the safe way to get a reliable answer.
How to read a tTG result
Labs report tTG-IgA in units per millilitre (U/mL), and each lab sets its own cut-off, so always read the range printed on your own report rather than a figure from the internet. In broad terms:
- Normal (negative): coeliac disease is unlikely, provided you were eating gluten and your total IgA is adequate.
- Weakly positive (borderline): a smaller rise that a GP usually confirms with a second antibody test (endomysial antibody, EMA) before referral.
- Strongly positive: a high result, often ten times the upper limit or more, that points firmly towards coeliac disease and prompts referral.
A positive tTG is not the diagnosis by itself. In adults, the UK pathway confirms coeliac disease with a gut biopsy taken during a gastroscopy. That is why you should not start a gluten-free diet on the strength of a blood test alone; doing so can blur the biopsy.
This is an NHS and GP test
The tTG test is a diagnostic step for a specific autoimmune condition, so it belongs with your GP. If you have symptoms that could fit coeliac disease, unexplained tiredness, bloating, persistent diarrhoea, unexplained iron deficiency or a close relative with coeliac disease, your GP can request tTG-IgA and total IgA, and arrange onward referral if needed. Helvy does not offer the coeliac antibody test, and this guide is not a substitute for that pathway.
Where private testing can help is the wider nutritional picture. Coeliac disease damages the gut lining and depletes markers such as ferritin, folate and vitamin B12. Tracking those alongside general wellbeing is a reasonable thing to do privately, while the coeliac diagnosis itself stays with your GP.
Frequently asked questions
What does a positive tTG result mean?
A positive (raised) tTG-IgA suggests coeliac disease, especially when it is strongly positive. It is not a final diagnosis: in adults the UK pathway confirms coeliac disease with a gut biopsy. Your GP arranges the next step, so do not start a gluten-free diet until you have spoken to them.
What is a normal tTG level?
Each lab sets its own cut-off in U/mL, so the range on your own report is the one that counts. A result below that cut-off is normal, but it is only reliable if you were eating gluten regularly and your total IgA is not low.
Can a tTG test be wrong?
It can read falsely normal if you have cut out gluten before testing or have selective IgA deficiency. That is why NICE recommends measuring total IgA alongside tTG-IgA and testing while you are still eating gluten.
Is tTG the same as a coeliac test?
tTG-IgA is the main first-line part of the coeliac screen, but a full coeliac pathway also includes total IgA, sometimes endomysial antibody (EMA), and a confirming biopsy. Our coeliac blood test guide walks through the whole sequence.
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