Coeliac Disease Blood Test: Getting Tested

Coeliac disease is one of the most underdiagnosed conditions in the United Kingdom. It affects approximately 1 in 100 people, yet an estimated 75% of those with the condition remain undiagnosed. The average time from symptom onset to diagnosis is a staggering 13 years. This diagnostic delay is not because coeliac disease is difficult to test for — the blood tests are well-established and highly accurate. It is because the symptoms are so varied and often so vague that coeliac disease simply is not considered.

If you have unexplained fatigue, iron deficiency anaemia, digestive symptoms, joint pain, recurrent mouth ulcers, skin rashes, or a family history of coeliac disease, a blood test could give you answers that have been eluding you for years.

What Is Coeliac Disease?

Coeliac disease is a systemic autoimmune condition triggered by gluten, a protein found in wheat, barley, and rye. When a person with coeliac disease eats gluten, their immune system mounts an inflammatory response that damages the villi — tiny finger-like projections lining the small intestine that are responsible for absorbing nutrients. Over time, this damage leads to villous atrophy, causing malabsorption of essential nutrients including iron, folate, B12, calcium, and vitamin D.

Unlike a food intolerance, coeliac disease causes measurable immune activation and structural damage to the gut lining. It is not a sensitivity or a lifestyle choice — it is a medical condition with serious long-term consequences if untreated, including osteoporosis, infertility, neurological complications, and a small but increased risk of intestinal lymphoma.

The Key Blood Test: Tissue Transglutaminase IgA (tTG IgA)

The first-line blood test for coeliac disease is tissue transglutaminase IgA (tTG IgA). This measures IgA-class antibodies against tissue transglutaminase, an enzyme that becomes a target of the immune response in coeliac disease. The test has a sensitivity of 93–98% and a specificity of over 95%, making it one of the most accurate serological tests for any autoimmune condition.

A positive tTG IgA result strongly suggests coeliac disease and, in most cases, leads to a referral for a duodenal biopsy (via gastroscopy) to confirm the diagnosis. NICE guidelines state that a tTG IgA level greater than 10 times the upper limit of normal, combined with positive endomysial antibodies (EMA), may be sufficient for diagnosis without biopsy in children, though adult guidelines still generally require biopsy confirmation.

The IgA Deficiency Trap

There is an important caveat that catches out many patients and, unfortunately, some clinicians. Approximately 2–3% of people with coeliac disease have selective IgA deficiency. This is a common immunodeficiency (affecting roughly 1 in 500 of the general population) that is even more prevalent among coeliacs.

If you have IgA deficiency, your tTG IgA result will be falsely negative — not because you do not have coeliac disease, but because your body does not produce enough IgA to generate a detectable antibody response. This is why total IgA should always be measured alongside tTG IgA. If total IgA is low, IgG-based coeliac tests (such as tTG IgG or deamidated gliadin peptide IgG) should be used instead.

This is not a rare academic concern. In a population of 1,000 people being tested for coeliac disease, approximately 20–30 may have IgA levels too low to produce a reliable tTG IgA result. If total IgA is not checked, these individuals receive a false negative and their coeliac disease remains undiagnosed.

Why Nutrient Markers Are Essential

Coeliac disease is fundamentally a malabsorption condition. Even before villous atrophy is severe enough to cause overt gastrointestinal symptoms, it can cause measurable nutrient deficiencies. In fact, iron deficiency anaemia is the single most common presentation of coeliac disease in adults — more common than diarrhoea or abdominal pain.

Iron and Ferritin

Iron is absorbed in the duodenum, which is precisely where coeliac disease causes the most damage. Iron deficiency anaemia that is unexplained (especially in men or post-menopausal women, where dietary deficiency is less likely) or resistant to oral iron supplementation should always raise suspicion of coeliac disease. Ferritin below 30 µg/L indicates depleted iron stores, and a persistent ferritin below 15 µg/L in the context of other symptoms is a strong indicator for coeliac testing.

Full Blood Count (FBC)

An FBC can reveal several patterns consistent with coeliac disease. Microcytic anaemia (low MCV) suggests iron deficiency. Macrocytic anaemia (high MCV) suggests folate or B12 deficiency. A mixed picture with a normal MCV but low haemoglobin can occur when both iron and folate/B12 are depleted simultaneously, which is characteristic of coeliac malabsorption. Mild thrombocytosis (elevated platelet count) is also seen, likely as a reactive response to chronic inflammation and iron deficiency.

Folate

Folate is absorbed in the jejunum, adjacent to the duodenum, and is frequently depleted in coeliac disease. Low folate levels are found in up to 20% of newly diagnosed coeliacs. Because folate is important for DNA synthesis and red blood cell production, deficiency contributes to anaemia and, in women of childbearing age, carries the additional risk of neural tube defects in pregnancy.

Vitamin B12

B12 deficiency is less common than iron or folate deficiency in coeliac disease because B12 is absorbed further down the gut (in the terminal ileum), which is less frequently affected. However, it is still found in approximately 10–15% of newly diagnosed patients, and its symptoms — fatigue, neuropathy, cognitive impairment — overlap with coeliac disease itself, making it important to identify and treat.

Vitamin D and Calcium

Vitamin D and calcium absorption are both impaired in coeliac disease. Chronic malabsorption of these nutrients leads to reduced bone mineral density, and osteoporosis is a well-documented complication of untreated coeliac disease. NICE recommends a DEXA scan (bone density assessment) for adults diagnosed with coeliac disease. Measuring vitamin D levels at the time of initial testing helps establish a baseline. In the UK, where vitamin D deficiency is already endemic, coeliacs are at particularly high risk — levels below 25 nmol/L (deficiency) are common at diagnosis.

The Critical Rule: You Must Be Eating Gluten

This is arguably the most important practical point about coeliac testing, and it is frequently misunderstood. The blood test for coeliac disease measures antibodies produced in response to gluten. If you have already removed gluten from your diet before being tested, your antibody levels will fall, and your result may be falsely negative.

NICE guidelines recommend eating gluten in more than one meal per day for at least six weeks before being tested. Some guidelines suggest a minimum of two slices of wheat-based bread per day as a practical guide. If you have already gone gluten-free and want to be tested, you need to undertake a "gluten challenge" — reintroducing gluten for the required period before the blood test.

This can be difficult for people who feel significantly worse when eating gluten, but it is necessary for accurate diagnosis. A confirmed diagnosis of coeliac disease has lifelong implications for dietary management, follow-up care, and screening of first-degree relatives (who have a 1 in 10 risk of also having the condition).

Who Should Be Tested?

NICE guidelines recommend testing for coeliac disease in anyone with:

• Unexplained iron deficiency anaemia
• Chronic diarrhoea or steatorrhoea (fatty stools)
• Unexplained weight loss
• Recurrent abdominal pain, bloating, or distension
• Unexplained fatigue
• Dermatitis herpetiformis (an intensely itchy blistering skin rash)
• Unexplained recurrent mouth ulcers
• Unexplained abnormal liver function tests
• Type 1 diabetes (coeliac is more common in people with other autoimmune conditions)
• Autoimmune thyroid disease
• Irritable bowel syndrome (IBS) — NICE specifically states that coeliac disease should be excluded before diagnosing IBS
• First-degree relatives of someone with coeliac disease

Testing Options at Lola Health

Our Core Health 45 test (£120) includes a full blood count, iron studies, ferritin, B12, folate, vitamin D, and liver function — all the markers that reveal the nutrient deficiency patterns associated with coeliac disease. Tissue Transglutaminase IgA (tTG IgA) is available as an add-on biomarker, allowing you to combine a coeliac screen with a comprehensive health check in a single blood draw.

Key Takeaways

• Coeliac disease affects 1 in 100 people in the UK, but 75% remain undiagnosed, with an average 13-year diagnostic delay.
• The tTG IgA blood test has 93–98% sensitivity — it is highly accurate when performed correctly.
• Always test total IgA alongside tTG IgA — 2–3% of coeliacs have IgA deficiency, which causes false negative results.
• Iron deficiency anaemia is the most common presentation of coeliac disease in adults.
• You must be eating gluten for at least 6 weeks before testing, or your result may be falsely negative.
• First-degree relatives of coeliacs have a 1 in 10 chance of also having the condition and should be screened.

Coeliac disease is treatable with a strict gluten-free diet, and early diagnosis prevents the long-term complications of malabsorption. If you have symptoms that could be consistent with coeliac disease, or if you have a family history, getting tested is straightforward and could change your health trajectory significantly.