How Is Coeliac Disease Diagnosed?
Getting a coeliac diagnosis is a specific, ordered process — and doing it in the wrong order, or after you’ve already cut out gluten, is the single most common way people end up without a clear answer.
Short answer
Diagnosis is a two-step process: a blood test (tTG IgA) first, then usually an endoscopy with a small-bowel biopsy to confirm. The one rule that matters above all others — keep eating gluten throughout, until every test is finished. Cutting gluten early lets the gut start healing and can produce a false negative.
Step one: the blood test
The first-line test is tTG IgA (tissue transglutaminase immunoglobulin A), a blood test that measures antibodies your immune system produces in response to gluten. A high result is a strong signal. Your doctor should also check your total IgA level at the same time, because roughly 2–3% of people with coeliac disease are IgA deficient — which would make the standard tTG IgA test read falsely negative. If you’re IgA deficient, alternative antibody tests (such as DGP IgG or EMA) are used instead. We cover what the numbers mean in the tTG IgA blood test explained.
Step two: the endoscopy and biopsy
A positive blood test is normally followed by an upper endoscopy with a small-bowel biopsy, performed by a gastroenterologist. Under light sedation in most countries, a thin camera is passed to your duodenum and several tiny tissue samples are taken. A pathologist looks for villous atrophy — the flattening of the finger-like projections (villi) that line the small intestine and do the work of absorbing nutrients. The degree of damage is graded on the Marsh scale, from Marsh 1 through Marsh 3c. The biopsy remains the reference standard for confirming coeliac disease in adults, because it shows the actual damage rather than only the immune signal.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Blood test | tTG IgA plus total IgA | First-line screen; total IgA rules out a false negative |
| 2. Endoscopy + biopsy | Tissue samples from the duodenum, graded on the Marsh scale | Confirms villous atrophy — the reference standard in adults |
| Throughout | Keep eating gluten | Removing it early can hide the disease |
The no-biopsy route (and who it applies to)
Some guidelines — notably ESPGHAN for children, and increasingly some adult pathways — allow a diagnosis without biopsy when tTG IgA is very high (typically more than ten times the upper limit of normal), a second antibody test agrees, and other conditions have been excluded. Whether this applies to you depends on your age, your results and your country’s guidelines. Ask your gastroenterologist directly rather than assuming.
The one rule that trips people up
Do not start a gluten-free diet before your tests are complete. Reducing or removing gluten causes antibody levels to fall and the gut lining to begin healing — which can turn a true positive into a false negative and leave you without a formal diagnosis. That formal diagnosis matters: it unlocks follow-up care, dietitian referrals, bone-density monitoring and family screening. If you’ve already gone gluten-free before testing, don’t quietly restart on your own — talk to your doctor about a monitored gluten challenge first. There’s more on that in testing after going gluten-free.
The bottom line
Coeliac diagnosis runs blood test first, biopsy second, with a no-biopsy route for some high-antibody cases — and it only works while you’re still eating gluten. If you’re already gluten-free, ask your doctor before changing anything, because a formal diagnosis is what unlocks the follow-up care that comes next.
Where to go next
- Back to every question: Coeliac Diagnosis & Medical FAQ.
- Previous question: What are the symptoms of coeliac disease?
- Next question: What does the tTG IgA blood test tell you?
- Already gluten-free? Can you test after going gluten-free?
Sources: Coeliac UK (getting diagnosed); NICE guideline NG20; ESPGHAN diagnostic guidelines (paediatric no-biopsy criteria); Marsh classification. General information, not individual medical advice — your diagnosis and monitoring are led by your GP and gastroenterologist.
Last verified: 2026-07-20.
Reviewed against the HTGF methodology — every claim sourced, every listing tiered and dated. This article is practical guidance, not medical advice.
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