DIGESTIVE HEALTH
Coeliac Blood Test UK: The Full Serological Pathway — and Why a Single tTG Isn’t Enough
Your GP ran a coeliac screen and said it came back normal. You went gluten-free anyway because bread still makes you feel terrible. Six months later, you read that going gluten-free before testing can produce a false negative — and now you don’t know whether you actually have coeliac disease or not.
You’re not alone. Coeliac disease affects roughly 1 in 100 people in the UK, yet only about 36% are diagnosed. That leaves an estimated 500,000 people living with an autoimmune condition they don’t know they have — with an average diagnostic delay of 13 years from first symptoms.
The right blood tests, done in the right order, can collapse that 13-year gap to a single afternoon. But “a coeliac screen” isn’t one test — it’s a NICE NG20-defined pathway of serological markers, each with a specific role. Get the sequence wrong, skip the IgA check, or test after going gluten-free, and the result is meaningless.
This guide covers the full serological pathway, explains why each marker matters, maps the result patterns that GPs sometimes miss, shows you which downstream deficiencies to monitor, and tells you exactly how to prepare so your blood test gives a clear answer.
Why coeliac testing matters more than you think
Coeliac disease is an autoimmune condition triggered by gluten — a protein found in wheat, barley, and rye. When someone with coeliac disease eats gluten, their immune system attacks the lining of the small intestine, flattening the finger-like villi that absorb nutrients. Over time, this causes malabsorption of iron, folate, B12, calcium, and vitamin D, even when diet appears adequate.
The classic image of coeliac disease is a child with a distended belly and chronic diarrhoea. In adults, the presentation is often far more subtle. Fatigue, brain fog, unexplained iron deficiency, mouth ulcers, joint pain, skin rashes (dermatitis herpetiformis), and even osteoporosis can all be the only symptom. The BMJ estimates that for every diagnosed case, there are 3–5 undiagnosed adults walking around with non-specific symptoms attributed to stress, IBS, or “just getting older”.
Untreated coeliac disease increases risk of osteoporosis (from calcium and vitamin D malabsorption), iron-deficiency anaemia, peripheral neuropathy (from B12 depletion), infertility and recurrent miscarriage, and in rare cases small-bowel lymphoma. The good news: a strict gluten-free diet reverses the villous damage in most people within 6–18 months. But you need a diagnosis first — and that starts with blood tests.
Who should be tested?
NICE NG20 recommends serological testing for anyone with:
- Persistent, unexplained gastrointestinal symptoms (bloating, diarrhoea, constipation, abdominal pain)
- Prolonged fatigue without clear cause
- Unexpected weight loss
- Recurrent mouth ulcers (aphthous stomatitis)
- Unexplained iron, B12, or folate deficiency
- Autoimmune thyroid disease or type 1 diabetes (shared HLA-DQ2/DQ8 genetics)
- A first-degree relative with coeliac disease (10–15% risk)
- Dermatitis herpetiformis (intensely itchy blistering rash, typically on elbows, knees, buttocks)
- Unexplained osteoporosis or osteopenia
- Dental enamel defects
- A previous IBS diagnosis — NICE recommends coeliac screening for all IBS patients
The NICE NG20 serological pathway
NICE guideline NG20 (2015, last updated 2022) defines the first-line serological test as IgA tissue transglutaminase (tTG-IgA) alongside a total IgA level. This two-test approach catches the ~2% of coeliac patients who are IgA-deficient and would get a false-negative tTG result.
| Step | Test | Purpose | If positive |
|---|---|---|---|
| 1 | IgA tTG + total IgA | First-line screen — high sensitivity (95%+) for villous atrophy | Refer gastroenterology for duodenal biopsy |
| 2a | IgA EMA (endomysial antibody) | Confirmation if tTG is weakly positive (borderline range) | Strengthens case for biopsy referral |
| 2b | IgG tTG or IgG DGP or IgG EMA | Alternative pathway if total IgA is low (<0.07 g/L) | Refer gastroenterology — IgA-based tests are unreliable |
| 3 | Duodenal biopsy | Gold standard — confirms villous atrophy (Marsh classification) | Diagnosis confirmed → lifelong gluten-free diet |
| 4 | HLA DQ2/DQ8 typing | Specialist use — negative result virtually rules out coeliac disease | Positive ≠ coeliac (40% of the population carry these genes) |
Many GP practices only run tTG-IgA without checking total IgA. If you happen to be IgA-deficient (about 1 in 50 people in the general population, higher among coeliac patients), your tTG-IgA can come back perfectly normal even with active villous damage. The two-test approach catches this.
8 biomarkers for coeliac investigation
A thorough coeliac workup goes beyond the tTG screen. These 8 markers cover the serological diagnosis and the downstream nutritional damage coeliac disease causes before anyone notices.
| Biomarker | What it measures | Why it matters for coeliac |
|---|---|---|
| tTG-IgA | IgA antibodies against tissue transglutaminase | First-line coeliac marker; 95%+ sensitivity when IgA is adequate |
| Total IgA | Baseline immunoglobulin A level | Rules out IgA deficiency — without this, a negative tTG means nothing |
| Ferritin | Iron storage protein | Iron malabsorption is the most common single presentation of adult coeliac; often the only abnormality |
| Vitamin B12 | Active B12 in circulation | Ileal damage (less common) depletes B12; neurological symptoms may precede gut symptoms |
| Folate | Active folate (vitamin B9) | Absorbed in the proximal jejunum — exactly where coeliac damage begins; depletion is common and early |
| Vitamin D | 25-hydroxyvitamin D | Fat-soluble vitamin malabsorption; low vitamin D accelerates osteoporosis risk already elevated in coeliac |
| FBC (full blood count) | Haemoglobin, MCV, MCH, platelets, WBC | Microcytic anaemia (low MCV) from iron loss; macrocytic anaemia (high MCV) from B12/folate loss; both can coexist |
| hs-CRP | High-sensitivity C-reactive protein | Helps differentiate coeliac (usually normal CRP) from IBD (usually elevated); red flag if CRP is markedly raised |
NHS ranges vs clinically meaningful thresholds
The NHS “normal range” for each marker tells you whether you fall within 95% of the population. It does not tell you whether you’re absorbing nutrients properly. The grey zone between “technically normal” and “optimal” is where undiagnosed coeliac damage hides for years.
| Biomarker | NHS “normal” | Optimal / clinical target | Grey zone (investigate) |
|---|---|---|---|
| tTG-IgA | <7 U/mL | <4 U/mL | 4–10 U/mL — weakly positive, warrants EMA confirmation |
| Total IgA | 0.8–4.0 g/L | ≥0.8 g/L | <0.07 g/L = IgA deficiency → switch to IgG pathway |
| Ferritin | 15–300 µg/L (men), 15–200 (women) | 50–150 µg/L | 15–50 µg/L — depleted stores, symptoms likely |
| Vitamin B12 | 180–900 ng/L | ≥500 ng/L | 180–350 ng/L — functional deficiency possible, neurological risk |
| Folate | ≥3.0 µg/L | ≥10.0 µg/L | 3.0–7.0 µg/L — early depletion, impaired DNA synthesis |
| Vitamin D | ≥25 nmol/L | 75–125 nmol/L | 25–50 nmol/L — insufficient; accelerates bone loss |
| Haemoglobin | 130–170 g/L (men), 120–150 (women) | 140+ (men), 130+ (women) | Low-normal with low ferritin = iron restriction even if “not anaemic” |
| hs-CRP | <5 mg/L | <1.0 mg/L | 1.0–3.0 mg/L — low-grade inflammation; above 10 mg/L suggests IBD rather than coeliac |
The IgA deficiency trap: why 2% of coeliac patients get a false negative
IgA tissue transglutaminase (tTG-IgA) is the workhorse of coeliac screening. It has a sensitivity above 95% in most studies —provided the patient actually makes enough IgA. Around 2–3% of coeliac patients have selective IgA deficiency, a harmless immunological variant in which total serum IgA is below 0.07 g/L. When this happens, all IgA-based tests — tTG-IgA and EMA-IgA — can come back completely normal even with severe villous atrophy.
The fix is simple but often missed: always run total IgA alongside tTG-IgA. If total IgA is low, the lab switches to IgG-class tests (IgG tTG, IgG DGP, or IgG EMA). NICE NG20 is explicit about this, yet many practices still request tTG-IgA alone.
If you’ve previously tested negative for coeliac disease but don’t know whether your total IgA was checked, the result may not be reliable. A repeat test including total IgA — while eating gluten — is the only way to be sure.
The gluten challenge: why you must eat gluten before testing
Coeliac blood tests detect antibodies produced in response to gluten exposure. If you’ve reduced or eliminated gluten from your diet, your antibody levels drop — even if you have coeliac disease. This produces a false-negative result that neither confirms nor rules out the condition.
NICE NG20 and Coeliac UK recommend eating gluten in more than one meal every day for at least 6 weeks before serological testing. The equivalent is roughly 2 slices of wheat bread daily, or an equivalent portion of pasta, cereal, or crackers.
This is one of the most common errors in coeliac investigation. A patient goes gluten-free, feels better, then asks their GP to “check for coeliac”. The tTG comes back negative — not because they don’t have coeliac disease, but because the test had nothing to detect. The patient is told they “don’t have it”, and a genuine autoimmune condition goes unmanaged.
Already gluten-free? Do not reintroduce gluten without speaking to your GP or gastroenterologist first. A supervised gluten challenge with follow-up testing and possible biopsy is the safest approach. Reintroducing gluten can cause significant symptoms in people who are genuinely coeliac.
Coeliac disease vs gluten sensitivity vs wheat allergy
These three conditions are frequently confused. They share one symptom trigger (foods containing wheat/gluten) but differ fundamentally in mechanism, risk, and management.
| Condition | Mechanism | Blood test | Intestinal damage | Long-term risk |
|---|---|---|---|---|
| Coeliac disease | Autoimmune (T-cell mediated) | tTG-IgA positive, confirmed by biopsy | Yes — villous atrophy (Marsh 3) | Osteoporosis, anaemia, lymphoma, infertility |
| Non-coeliac gluten sensitivity (NCGS) | Unknown (innate immune? gut permeability?) | Negative coeliac serology, diagnosis of exclusion | No villous atrophy | No known long-term complications; quality of life impact |
| Wheat allergy | IgE-mediated (classic allergy pathway) | Wheat-specific IgE, skin prick test | No | Anaphylaxis risk in severe cases |
The distinction matters because coeliac disease requires lifelong strict avoidance (even trace amounts of gluten cause villous damage, whether or not you feel symptoms), whereas NCGS can often tolerate small amounts without structural harm. Blood testing for coeliac is the essential first step — if coeliac is ruled out properly (with adequate gluten intake and total IgA checked), then NCGS becomes a reasonable working diagnosis based on symptoms.
6 downstream deficiencies to monitor after diagnosis
Even after starting a gluten-free diet, nutritional recovery takes time. Villous healing is typically 6–18 months, and until absorption normalises, deficiencies persist. These 6 markers should be monitored at diagnosis, at 6 months, and annually thereafter.
| Deficiency | Why coeliac causes it | Symptoms | Target on GFD |
|---|---|---|---|
| Iron (ferritin) | Duodenal absorption site is the primary damage zone | Fatigue, breathlessness, hair loss, restless legs, pica | Ferritin ≥50 µg/L |
| Folate (B9) | Absorbed in proximal jejunum, first area damaged | Fatigue, macrocytic anaemia, glossitis, cognitive fog | Folate ≥10 µg/L |
| Vitamin B12 | Ileal involvement (less common but serious) | Peripheral neuropathy, memory problems, balance issues | B12 ≥500 ng/L |
| Vitamin D | Fat-soluble vitamin; malabsorption due to damaged villi | Bone pain, muscle weakness, low mood, increased fracture risk | 25(OH)D 75–125 nmol/L |
| Calcium | Duodenal absorption site damaged + vitamin D depletion reduces calcium uptake | Muscle cramps, tingling, osteoporosis over time | Corrected calcium 2.2–2.6 mmol/L |
| Bone density (DEXA) | Dual malabsorption (calcium + vitamin D) accelerates bone loss | Asymptomatic until fracture — screening is the only detection | DEXA at diagnosis if osteoporosis risk; repeat at 2–3 years |
No competitor coeliac page maps these downstream deficiencies with clinical targets. Most stop at the tTG result. But for someone already diagnosed, the deficiency monitoring is arguably more important than the original screen — it’s what determines whether you’re actually healing.
5 result patterns and what they mean
Coeliac investigation rarely produces a single isolated abnormality. These are the patterns we see most often, each pointing to a different clinical picture.
1. Classic coeliac picture
tTG-IgA elevated + low ferritin + low folate + normal CRP. The hallmark: autoimmune inflammation with downstream nutrient malabsorption. CRP stays normal because coeliac is a localised autoimmune process, not a systemic inflammatory one. This pattern strongly predicts villous atrophy on biopsy. Refer to gastroenterology — do not start a gluten-free diet until biopsy is complete.
2. Silent coeliac (no GI symptoms)
tTG-IgA elevated + isolated iron deficiency or osteoporosis + no bloating or diarrhoea. Up to 50% of adult coeliac diagnoses present without classic GI symptoms. The patient comes in for fatigue, unexplained anaemia, or a DEXA showing osteoporosis — and the coeliac screen is an afterthought that turns positive. This pattern is why NICE recommends screening anyone with unexplained iron deficiency, regardless of gut symptoms.
3. IgA-deficient false negative
tTG-IgA negative + total IgA <0.07 g/L + persistent GI symptoms + low ferritin/folate. The tTG result is meaningless because the patient doesn’t produce enough IgA to generate the antibody. The nutrient depletion pattern is the clue. This needs IgG-class testing (IgG tTG or IgG DGP) and probable biopsy referral. This is the pattern that a tTG-only screen misses entirely.
4. Borderline tTG — coeliac or not?
tTG-IgA 4–10 U/mL (weakly positive) + adequate total IgA + normal nutrients. A weakly positive tTG can reflect early coeliac disease, another autoimmune condition cross-reacting, or a laboratory artefact. NICE NG20 recommends EMA testing to confirm: EMA is less sensitive but highly specific (99%+). If EMA is positive, proceed to biopsy. If negative, retest in 3–6 months while continuing to eat gluten.
5. Not coeliac — but something else
tTG-IgA negative + adequate IgA + normal nutrients + persistent GI symptoms. Coeliac is excluded (assuming adequate gluten intake). Differentials include non-coeliac gluten sensitivity, IBS, lactose intolerance, SIBO, and inflammatory bowel disease. If hs-CRP is raised or symptoms include blood in stool, weight loss, or nocturnal symptoms, IBD investigation (faecal calprotectin) is the next step. See our IBS blood test guide for the full IBD vs IBS differentiation pathway.
Red flags: when symptoms need urgent investigation
Most coeliac presentations are not urgent. But the following features should prompt same-week GP review or A&E attendance regardless of blood test results:
| Red flag | Why it matters | Action |
|---|---|---|
| Unintentional weight loss >5% | May indicate severe malabsorption, lymphoma, or other malignancy (NICE NG12) | Urgent GP referral — cancer pathway if >60 or other risk factors |
| Rectal bleeding or melaena | Not a coeliac feature — suggests IBD, colorectal pathology, or GI bleed | Same-week GP; A&E if significant bleed |
| Iron-deficiency anaemia in men or post-menopausal women | Coeliac can cause this, but GI malignancy must be excluded first (NICE NG12) | Urgent 2-week-wait referral for lower GI investigation |
| Persistent vomiting | Uncommon in coeliac — may indicate obstruction or other upper GI pathology | Same-week GP assessment |
| Haemoglobin <70 g/L | Severe anaemia — may need transfusion regardless of cause | Same-day GP or A&E |
| Neurological symptoms (numbness, tingling, ataxia) | B12 deficiency from coeliac can cause irreversible peripheral neuropathy if untreated | Urgent B12/folate testing + GP review |
| New onset over 60 | Late-onset coeliac is real, but GI malignancy must be excluded | GP referral for full GI investigation |
| Dermatitis herpetiformis with systemic features | DH itself confirms coeliac, but widespread skin disease may need dermatology + GI input together | GP referral for dual specialist pathway |
What the NHS tests vs what Helvy tests
Most NHS coeliac screens include tTG-IgA and sometimes total IgA. If positive, you’re referred for biopsy. If negative, the investigation usually stops — even if downstream deficiencies are already accumulating.
| Marker | NHS coeliac screen | Helvy Essential | Helvy Nutrition |
|---|---|---|---|
| tTG-IgA | Usually | Yes | — |
| Total IgA | Sometimes | — | — |
| Ferritin | If anaemia suspected | Yes | Yes |
| Full blood count | If anaemia suspected | Yes | — |
| Vitamin B12 | Rarely | Yes | Yes |
| Folate | Rarely | Yes | Yes |
| Vitamin D | Rarely | Yes | Yes |
| hs-CRP | If IBD suspected | Yes | — |
| TSH | Rarely | Yes | — |
| FT4 | Rarely | Yes | — |
| HbA1c | Only if T1DM screening | Yes | — |
| Magnesium | Rarely | — | Yes |
The NHS coeliac screen answers one question: do you have coeliac antibodies? It doesn’t assess the nutritional damage already underway. Helvy’s Essential panel covers the coeliac screen plus the downstream markers, giving you the full absorption picture in a single test.
Evidence-based next steps by biomarker
| Biomarker | If abnormal | Evidence base |
|---|---|---|
| tTG-IgA elevated | GP referral for gastroenterology and duodenal biopsy; do NOT start GFD before biopsy | NICE NG20 1.2 |
| Total IgA low (<0.07 g/L) | Request IgG-class coeliac markers (IgG tTG or IgG DGP); tTG-IgA result is unreliable | NICE NG20 1.1 |
| Ferritin <30 µg/L | Iron supplementation (ferrous sulphate 200 mg, alternate days for absorption); retest at 3 months | NICE NG24, BSG 2021 |
| B12 <350 ng/L | GP review; if neurological symptoms, loading-dose injections before oral can be given | NICE CKS, BSH 2014 |
| Folate <7 µg/L | Folic acid 5 mg daily for 4 months; check B12 first (supplementing folate alone can mask B12 deficiency) | NICE CKS anaemia |
| Vitamin D <50 nmol/L | Loading dose (50,000 IU weekly for 6 weeks), then maintenance 1,000–2,000 IU daily; retest at 3 months | SACN 2016, NICE NG24 |
| hs-CRP >10 mg/L | Not typical of coeliac — consider IBD, infection, or other inflammatory process; faecal calprotectin recommended | NICE DG11 |
| Haemoglobin low + mixed MCV | Dual deficiency (iron + B12/folate) — common in coeliac; treat both, retest at 8 weeks | BSG 2021, NICE NG24 |
How and when to test
The gluten rule
You must be eating gluten in more than one meal every day for at least 6 weeks before testing. Two slices of wheat bread per day is the minimum. If you’ve been gluten-free, the test is not valid.
Medications to discuss with your GP
Immunosuppressants (e.g., azathioprine, methotrexate, systemic corticosteroids) can suppress antibody production and cause false negatives. If you’re on immunosuppressive therapy, discuss timing with your prescriber before testing.
Fasting
Fasting is not strictly required for tTG-IgA, but if you’re also testing ferritin, FBC, and other nutritional markers in the same draw, an overnight fast (10–12 hours, water only) gives the most accurate results.
When to retest
- Borderline tTG: retest with EMA confirmation at 3–6 months (while eating gluten)
- Post-diagnosis monitoring: tTG-IgA at 6 months and 12 months after starting GFD — should fall towards normal
- Nutritional markers: at diagnosis, 6 months, then annually
- Persistently elevated tTG on GFD: suggests ongoing gluten exposure (intentional or hidden) — dietitian review
Which Helvy panel to choose
If you’re investigating whether you have coeliac disease or monitoring downstream deficiencies after diagnosis, we recommend:
Essential Blood Test
Covers tTG-IgA + ferritin + FBC + B12 + folate + vitamin D + hs-CRP + TSH + FT4 + HbA1c. The most comprehensive single panel for coeliac investigation and deficiency screening.
View panelNutrition Blood Test
Adds magnesium, zinc, and omega-3 index to the core nutritional markers. Ideal for post-diagnosis monitoring when you need to track recovery across all malabsorption-vulnerable nutrients.
View panelFrequently asked questions
What blood test shows coeliac disease in the UK?
The first-line test is IgA tissue transglutaminase (tTG-IgA), recommended by NICE NG20 alongside a total IgA level. A positive tTG-IgA leads to referral for duodenal biopsy to confirm the diagnosis. If tTG is weakly positive, IgA endomysial antibody (EMA) is used for confirmation.
Can you have coeliac disease with a negative blood test?
Yes. False negatives occur in two main situations: IgA deficiency (where the standard tTG-IgA test cannot detect antibodies) and reduced gluten intake (where antibody levels have dropped below the detection threshold). If total IgA was not checked alongside tTG-IgA, the negative result may not be reliable.
How long do you need to eat gluten before a coeliac blood test?
At least 6 weeks, eating gluten in more than one meal per day. This is equivalent to roughly 2 slices of wheat bread daily. Shorter challenges or lower amounts increase the risk of a false-negative result.
What happens if you stop eating gluten before a coeliac test?
Your antibody levels drop, often to undetectable levels within weeks. The blood test will likely come back negative regardless of whether you have coeliac disease. You’ll need to resume eating gluten for 6 weeks and retest, or your GP may refer you for HLA-DQ2/DQ8 genetic testing as an alternative (a negative HLA result virtually rules out coeliac disease).
What is the difference between coeliac disease and gluten intolerance?
Coeliac disease is an autoimmune condition that causes measurable damage to the small intestine (villous atrophy) and long-term health risks including osteoporosis, anaemia, and lymphoma. Non-coeliac gluten sensitivity (NCGS) causes symptoms but no intestinal damage and no increased disease risk. Blood tests and biopsy distinguish the two — the management and long-term implications are completely different.
Is coeliac disease hereditary?
Coeliac disease has a strong genetic component. First-degree relatives have a 10–15% risk (compared to 1% in the general population). The HLA-DQ2 and HLA-DQ8 genes are necessary but not sufficient — about 40% of the UK population carries one or both, but only 1% develops coeliac disease. NICE NG20 recommends testing all first-degree relatives.
How do I know if my coeliac blood test result is accurate?
Three conditions must be met: you were eating gluten in more than one meal daily for at least 6 weeks before the test; your total IgA was checked alongside tTG-IgA (to exclude IgA deficiency); and you were not taking immunosuppressive medication. If any of these conditions was not met, the result may need repeating.
Related guides
Medical disclaimer
This guide is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Coeliac disease diagnosis requires clinical assessment, serological testing, and usually duodenal biopsy under the care of a gastroenterologist. Always consult a qualified healthcare professional before making decisions about your health. If you experience any red-flag symptoms listed above, seek medical attention without delay.
Reviewed by: PENDING — awaiting medical reviewer approval
Published: 2026-04-09 · Last updated: 2026-04-09