Coeliac Blood Test: tTG-IgA, Total IgA and the "Keep Eating Gluten" Rule
The tTG-IgA blood test is the first-line screen for coeliac disease — but only if you are still eating gluten. Learn how to interpret positive, weak positive and negative results.
The first-line coeliac blood test is tTG-IgA plus total IgA. If tTG-IgA is > 10× the upper limit AND anti-endomysial antibody is positive, ESPGHAN and BSG allow diagnosis WITHOUT biopsy in children (and increasingly in adults). If total IgA is low, use IgG-based tests (deamidated gliadin peptide IgG, tTG-IgG). Testing while on a gluten-free diet is worthless — you MUST be eating gluten regularly for at least 6 weeks.
Coeliac disease affects ~1% of Western populations, mostly undiagnosed. tTG-IgA is the highest-sensitivity single test (~95%). Total IgA must be checked in parallel because 2–3% of coeliac patients are IgA-deficient (falsely negative tTG-IgA). Anti-endomysial antibody (EMA) is specific and used to confirm positive tTG-IgA. No-biopsy diagnosis is accepted when tTG-IgA > 10× ULN and EMA-positive. Testing requires ongoing gluten intake (≥ 6 weeks, ≥ 10 g gluten/day) — a gluten-free diet normalises serology within weeks. First-degree relatives of coeliac patients, and patients with unexplained iron deficiency, fatigue, B12/folate deficiency, chronic diarrhoea, IBS, type 1 diabetes, autoimmune thyroid disease, or osteoporosis should be screened.
- First-line test
- tTG-IgA + total IgA
- Sensitivity of tTG-IgA
- ~95%
- IgA deficiency prevalence
- 2–3% in coeliac
- No-biopsy criteria
- tTG-IgA > 10× ULN + EMA positive
- Gluten intake before test
- ≥ 6 weeks, ≥ 10 g/day
The critical "keep eating gluten" rule
Coeliac serology relies on ongoing gluten-driven immune activity. Stop gluten and tTG-IgA falls into the normal range within weeks. Any patient tested on a gluten-free or reduced-gluten diet risks a falsely negative result and a missed diagnosis. If gluten has already been stopped, the standard "gluten challenge" is 10 g/day (equivalent to ~4 slices of bread) for at least 6 weeks before repeat serology.
Coeliac serology decision matrix
| Result | Interpretation | Next step |
|---|---|---|
| tTG-IgA negative + normal total IgA | Coeliac unlikely | No further testing unless high clinical suspicion |
| tTG-IgA weakly positive | Possible coeliac | Endoscopic duodenal biopsy |
| tTG-IgA > 10× ULN + EMA positive | Coeliac (no-biopsy pathway acceptable) | HLA DQ2/DQ8 supportive; start gluten-free diet |
| tTG-IgA negative + low total IgA | IgA deficiency | Use IgG-based tests (DGP-IgG, tTG-IgG) |
Who should be tested
NICE and BSG recommend testing anyone with: unexplained iron deficiency, folate or B12 deficiency, chronic diarrhoea, IBS symptoms, unintentional weight loss, faltering growth in children, elevated liver enzymes, first-degree relative with coeliac, type 1 diabetes, autoimmune thyroid disease, or osteoporosis. Population screening is not recommended, but the threshold to test in the presence of any high-yield symptom is low.
- Confirmed coeliac + persistent symptoms despite strict gluten-free diet — refractory coeliac workup.
- Coeliac + rapid weight loss + abdominal pain — enteropathy-associated T-cell lymphoma risk.
- Coeliac + iron / B12 / folate / vitamin D deficiency — nutritional workup.
- Coeliac + osteoporosis or unexplained fractures — DEXA and bone-health review.
Coeliac serology walk-through
- 1On a gluten-containing diet?Order tTG-IgA + total IgA.
- 2Recently gluten-free?Gluten challenge 10 g/day × 6 weeks then re-test.
- 3tTG-IgA > 10× ULN + EMA positive?No-biopsy diagnosis pathway (adults now included in many guidelines). Start gluten-free diet.
- 4Low IgA?Use DGP-IgG or tTG-IgG.
Related questions people ask
- Do I need to eat gluten before the test?
- What is IgA deficiency?
- Can I diagnose coeliac without biopsy?
- What causes iron deficiency in coeliac?
- Should family members be tested?
- How does coeliac relate to unexplained fatigue?
- What is refractory coeliac disease?
Frequently asked questions
- First-line coeliac test: tTG-IgA + total IgA.
- Must be eating gluten (≥ 10 g/day for ≥ 6 weeks) before testing.
- tTG-IgA > 10× ULN + EMA positive = no-biopsy diagnosis in many guidelines.
- IgA deficiency requires IgG-based tests.
- Test unexplained iron / B12 / folate deficiency and first-degree relatives.
- Serial tTG-IgA monitors gluten-free diet adherence.
References
3 sources- BSGGuidelines for the Diagnosis and Management of Coeliac Disease
UK gastroenterology society.
bsg.org.uk
- NICE NG20Coeliac disease
UK NICE coeliac guideline.
nice.org.uk
- ACGDiagnosis and Management of Celiac Disease
US ACG coeliac guideline.
gi.org
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