Gluten intolerance or coeliac disease? How the blood test works

Gluten intolerance and coeliac disease are not the same thing, and that difference decides whether a blood test tells you anything. Coeliac disease is an autoimmune condition with gut damage and a strong serological test. Gluten sensitivity without coeliac disease causes similar symptoms, but no blood value demonstrates it.

The mistake I see most often: going gluten free for months first, then getting tested. That makes the one good test in this story unreliable.

What is the difference between gluten intolerance and coeliac disease?

In coeliac disease your immune system responds to gluten with autoantibodies that damage your small intestinal lining. That damage is visible and measurable. In gluten sensitivity without coeliac disease you get symptoms after gluten, but without an autoimmune reaction and without demonstrable gut damage.

The symptoms overlap heavily: bloating, abdominal pain, diarrhoea or constipation, sometimes fatigue. You cannot tell the two apart from symptoms alone.

There is a third category: wheat allergy, a genuine IgE allergy to wheat protein. That runs faster and can cause symptoms outside the gut as well.

Thuisarts, the Dutch GP information service, describes coeliac disease as a lasting condition in which gluten damages the gut lining. That is exactly why a blood test exists for this one and not for the other form.

Which blood values are measured for coeliac disease?

The first step is tTG-IgA, antibodies against tissue transglutaminase of the IgA type, together with your total IgA. That second value is not decoration: if you have an IgA deficiency, the tTG-IgA can come back falsely negative. In that case IgG-based tests are used instead.

That last row is often misread. Close to a third of the population carries DQ2 or DQ8 and never develops coeliac disease, so a positive says little. A negative is informative, because without those genes coeliac disease is very unlikely.

The international guideline for diagnosing coeliac disease builds on this combination of serology, genetics and, where needed, tissue examination (PMID 31568151).

Why do you have to keep eating gluten before the test?

Because the test measures antibodies your body only makes while gluten keeps arriving. Drop gluten for weeks or months and those antibodies fall, so the result can turn negative while you do have the condition. The test then measures your diet, not your gut.

That is why a self-started gluten free diet makes the diagnosis so difficult.

If you still want to know whether gluten drives your symptoms, do it in the right order: test first, remove afterwards. The other way round costs you months, because to test at all you have to reintroduce gluten.

Take a runner who goes gluten free in March and feels better. In September he wants certainty and gets tested. His tTG-IgA is perfectly normal, and nobody knows whether that is because he is healthy or because he has not touched bread in six months.

The test is negative but my symptoms continue, now what?

Coeliac disease has become less likely, but your symptoms have not gone away and that is a real fact. What remains is gluten sensitivity without coeliac disease, a reaction to fructans in wheat, or an irritable bowel. None of those three has a blood value that confirms it.

Researchers use a double-blind challenge of around 8 grams of gluten a day against placebo, where at least a 30 percent symptom difference counts as positive (PMID 26096570). That is strict for good reason: the placebo effect is large with gut symptoms.

An interesting detail: with wheat it is not always the gluten in the way. Wheat also contains fructans, and some people who feel better gluten free are probably reacting to those.

How to approach such a suspicion methodically is covered in food intolerance: which tests work. If your pattern centres on dairy, look at the lactose intolerance test instead.

What going gluten free costs an athlete

Without a demonstrated reason, gluten free eating is rarely a win for athletes. Wholegrain products supply carbohydrates, fibre and B vitamins, and many gluten free substitutes carry less of all three. Under a heavy training load that is an awkward trade.

There is another reason to be careful about cutting things yourself. Long-running symptoms and a narrower diet can together affect your absorption and your stores.

Values often reviewed in that light are ferritin, vitamin B12 and folate. A panel such as 360 Health covers those in one draw, with a doctor's assessment.

If you are unsure whether your gut or your training load is the culprit, also read leaky gut.

What I would do

Keep eating gluten until your blood has been drawn, however tempting it is to stop early. Then discuss the result with your GP alongside your symptom history, because serology is one piece of the puzzle and not the whole picture.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Husby S, Koletzko S, Korponay-Szabo I, et al. European Society Paediatric Gastroenterology, Hepatology and Nutrition Guidelines for Diagnosing Coeliac Disease 2020. J Pediatr Gastroenterol Nutr. 2020;70(1):141-156. PMID 31568151.
• Catassi C, Elli L, Bonaz B, et al. Diagnosis of Non-Celiac Gluten Sensitivity (NCGS): The Salerno Experts' Criteria. Nutrients. 2015. PMID 26096570.
• Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793-796. PMID 18489614.