Bloating after bread, a tired feeling that never quite lifts, low iron that keeps coming back. Many people wonder about gluten, and some cut it out on their own to see what happens. The coeliac blood test is the usual first step to look into coeliac disease, but there is one rule that matters more than any other: it only works if you are still eating gluten. This guide explains what the test measures, why timing is so important and who it is usually suggested for.
What coeliac disease is
A 2022 Lancet seminar describes coeliac disease as an autoimmune condition that mainly affects the small intestine and is triggered by gluten in people with a genetic susceptibility (Catassi et al., 2022). The same seminar estimates prevalence at between 0.5% and 2% of the general population, about 1% on average, and notes that the way it shows up is highly variable: from classic gut symptoms to problems outside the gut and cases with almost no symptoms.
Coeliac disease is different from a wheat allergy, which is an immediate allergic reaction, and from simply feeling better with less gluten. The blood test looks specifically at the immune response linked to coeliac disease.
What the coeliac blood test measures
Tissue transglutaminase IgA (tTG-IgA)
Tissue transglutaminase is an enzyme in the gut lining. In coeliac disease, the immune system makes IgA antibodies against it. LML describes the tTG-IgA test as primarily used to aid the assessment of coeliac disease. NICE recommends it as the first-choice test for young people and adults.
Total IgA
Some people naturally make very little IgA. If that is the case, the tTG-IgA test can come back falsely low. That is why NICE recommends testing total IgA alongside tTG-IgA. LML's Tissue Transglutaminase IgA test includes both.
IgG-based tests
When IgA is low, NICE suggests using IgG-based tests instead, such as IgG tTG or IgG deamidated gliadin peptide (DGP). LML notes its Tissue Transglutaminase IgG test is particularly useful in cases where IgA antibodies may be deficient.
Why you should keep eating gluten
This is the single most important point. The antibodies only appear while gluten is in the diet. If you cut gluten out before testing, the result can be falsely reassuring.
NICE advises people on a normal diet to eat some gluten in more than one meal every day for at least 6 weeks before testing. NICE and the NHS both advise not starting a gluten-free diet until a specialist has confirmed the result, even if the blood test is positive. If you have already cut gluten out and find it hard to reintroduce, NICE suggests talking to a gut specialist rather than testing on a gluten-free diet.
What happens after a positive result
The blood test is a first step. The Lancet seminar explains that confirming coeliac disease involves positive antibodies together with changes seen on a small-intestine biopsy. The NHS describes the same route: if antibodies are found, the GP refers you to a gut specialist (gastroenterologist), who may arrange a biopsy. British Society of Gastroenterology guidance on adult coeliac disease sets out how this process and the follow-up are organised (Ludvigsson et al., 2014).
A negative result, taken while eating gluten and with normal total IgA, makes coeliac disease less likely. NICE advises people who test negative, especially close relatives and people with type 1 diabetes, to speak to a professional again if relevant symptoms appear or persist.
Who the coeliac blood test is usually suggested for
NICE recommends offering coeliac testing to people with any of the following:
- Persistent unexplained abdominal or digestive symptoms.
- Faltering growth in children.
- Prolonged tiredness or unexpected weight loss.
- Severe or persistent mouth ulcers.
- Unexplained low iron, vitamin B12 or folate.
- Type 1 diabetes or autoimmune thyroid disease, at the time these are first identified.
- Irritable bowel syndrome, in adults.
- A first-degree relative (parent, brother, sister or child) with coeliac disease.
NICE also suggests considering it for reduced bone density and some unexplained neurological symptoms, among others.
How often? As a general guide, the test is done once, when there are symptoms or a reason such as a close relative with coeliac disease, and never after removing gluten. It may be repeated if new symptoms appear. Talk to a professional about whether and when testing suits you.
