TTG / Tissue Transglutaminase

The blood test that first raises the flag for coeliac disease has a name that sounds like a car part: tTG-IgA, or tissue transglutaminase IgA antibody test. It’s the most widely used initial screening test for coeliac disease, and understanding what it measures — and what its limitations are — helps make sense of the diagnostic process.

What Is the tTG-IgA Test?

Tissue transglutaminase (tTG) is an enzyme produced by cells in the body, including those in the gut lining. When someone with coeliac disease eats gluten, the immune system generates antibodies against this enzyme. The tTG-IgA test detects those antibodies in the blood.

In people with coeliac disease who are actively eating gluten, the test has a high sensitivity — typically 90–95% — and high specificity. A strongly positive result, combined with symptoms and the right clinical picture, is a strong indicator of coeliac disease.

However, the test has limitations that are worth knowing:

IgA deficiency. Around 2–3% of coeliacs have selective IgA deficiency — meaning their bodies don’t produce normal levels of IgA antibodies. In these people, the tTG-IgA test returns a false negative, because the antibodies being tested for aren’t present in meaningful amounts regardless of gluten intake. This is why a total IgA level is usually measured alongside the tTG-IgA — if IgA is low, alternative testing (tTG-IgG or DGP-IgG) is used instead.

Gluten-free diet before testing. If someone has already cut out gluten before the test, antibody levels drop — often into the normal range — and the test may return a false negative. This is one of the most common reasons the diagnosis process stalls: people feel better without gluten, stop eating it, and then test negative. Re-testing requires a gluten challenge — eating a meaningful amount of gluten daily for at least 6–8 weeks.

Positive tTG-IgA is not a diagnosis on its own. A raised tTG-IgA indicates coeliac disease is likely and should be followed by a duodenal biopsy for confirmation in most adults. There are conditions other than coeliac disease that can cause mildly elevated tTG-IgA — type 1 diabetes, liver disease, heart failure — so the biopsy remains the confirmatory step.

What This Means for You

If your doctor runs a coeliac screen, the tTG-IgA is likely the test they’ll use. Keep eating gluten until the tests are complete. A negative result with low total IgA needs follow-up testing. A positive result needs an endoscopy to confirm the diagnosis properly.

Related in the Coeliac Alphabet: Biopsy · Undiagnosed · Diagnosis FAQ · back to the full A–Z.