IBS and IBD

Coeliac blood test

Also called: tTG-IgA, anti-tissue transglutaminase, coeliac screening test

The first-line test for coeliac disease is a blood test for anti-tissue transglutaminase IgA antibodies (tTG-IgA). The test is non-invasive, available on the NHS, and accurate when done correctly. The single most important rule: keep eating gluten daily for at least 6 weeks before testing. Cutting gluten before the test gives false negatives. Positive results lead to an endoscopic biopsy for confirmation.

How the test works

Coeliac disease is an autoimmune reaction triggered by gluten. The immune system produces antibodies against tissue transglutaminase, an enzyme involved in gut wall repair. The blood test (tTG-IgA) measures these antibodies. Higher levels strongly correlate with active coeliac disease. The test is usually run alongside a total IgA measurement, because about 2 percent of coeliacs have IgA deficiency and need the IgG version of the test instead.

Why you must keep eating gluten

  • Antibodies fall when gluten is removed from the diet.
  • Antibody levels fall steadily on a gluten-free diet, which is why testing while still eating gluten matters.
  • If you've already gone gluten-free, you need a 'gluten challenge': eat 10 g of gluten daily (about 4 slices of bread) for 6 weeks before testing.
  • Don't reduce gluten 'just in case' before getting tested. Common mistake that delays diagnosis by months.

Reading the result

  • tTG-IgA over 10x upper limit + symptoms: high probability coeliac. In adults, UK practice (BSG no-biopsy pathway) may confirm without biopsy in selected cases, but NICE NG20 still recommends referral for biopsy.
  • tTG-IgA over normal but under 10x: moderate probability. Endoscopic biopsy needed.
  • tTG-IgA negative + total IgA normal: coeliac unlikely.
  • tTG-IgA negative + IgA-deficient: rerun with IgG version.
  • Some labs add EMA (endomysial antibody) as a second-line test for confirmation.

What happens after a positive result

  1. Referral to gastroenterology.
  2. Endoscopy with duodenal biopsy (4-6 samples). Gold standard confirms villous atrophy.
  3. Diagnosis confirmed.
  4. Strict lifelong gluten-free diet under registered dietitian guidance.
  5. Annual follow-up to monitor antibody levels and check for nutrient deficiencies (iron, B12, folate, calcium, vitamin D).
  6. First-degree relatives offered testing (10 percent of immediate relatives also have coeliac).

Common questions

Can I get the test from my GP?
Yes. NICE recommends offering tTG-IgA testing to anyone with persistent unexplained gastrointestinal symptoms, IBS-like symptoms, iron-deficiency anaemia, or a first-degree relative with coeliac. Just ask.
Can a child be tested?
Yes. Same test, same rules. ESPGHAN paediatric criteria allow diagnosis without biopsy in children with tTG-IgA at or above 10x the upper limit of normal plus a positive endomysial antibody on a second sample. NICE NG20 instead refers children to a paediatrician for further investigation.
What if I'm already gluten-free and tested positive in the past but never confirmed?
Talk to a GP. Some people benefit from a formal gluten challenge to get definitive diagnosis (which has lifelong implications for monitoring and family screening). Others continue gluten-free without formal confirmation.
Are home coeliac tests accurate?
The home tTG-IgA tests sold direct-to-consumer have lower sensitivity than lab-based tests. Useful for triage; positive home test should always be confirmed by GP-ordered lab test.

Sources

The Book of Suna is general information, not medical advice. It is not a substitute for talking to a qualified healthcare professional about your own situation.