Getting Tested: Why You Must Keep Eating Gluten
The six-week rule, what the blood test actually measures, why total IgA matters, and what to do if you have already cut gluten out.
Part of the Complete Coeliac Disease Guide.
Key fact: Eat gluten in more than one meal every day, for at least six weeks, before a coeliac test. Going gluten-free first makes both the blood test and the biopsy unreliable — and getting tested afterwards means eating gluten again for six weeks.
Why the tests need gluten
Every coeliac test measures the body's reaction to gluten rather than the condition itself.
- The blood test looks for antibodies produced in response to gluten. Remove gluten and the antibodies fall, often into the normal range within weeks to months
- The biopsy looks for damage to the lining of the small intestine. Remove gluten and the lining heals, so the biopsy looks normal
So a gluten-free diet does not hide a mild case; it erases the evidence of any case. A person with well-established coeliac disease who has been gluten-free for six months will often test entirely negative on both.
What the six weeks actually means
NICE advises eating gluten in more than one meal every day for at least six weeks before testing. Two practical points people get wrong:
| Requirement | What that looks like |
|---|---|
| More than one meal a day | Not a slice of toast on Sundays. Gluten at two or more meals daily — ordinary bread, pasta, cereal, chapatti |
| At least six weeks | Continuously, up to and including the day of the test. Stopping a week before undoes it |
| Right through the process | Keep eating gluten until a specialist has confirmed the diagnosis — that includes the gap between blood test and biopsy |
The gap between the blood test and the biopsy is where this most often goes wrong. A positive antibody result feels like confirmation, and the natural response is to stop eating gluten immediately. Doing so lets the gut heal before the biopsy that is meant to confirm the diagnosis, and the result can come back normal.
If eating gluten makes you genuinely unwell, say so rather than abandoning the process — the amount and duration can be discussed, and the alternative is an unconfirmed diagnosis you will be managing for life.
The blood test
For young people and adults, the first-choice tests are total IgA and IgA tissue transglutaminase (tTG). If the tTG result is weakly positive, IgA endomysial antibodies (EMA) are used as well.
- It is an ordinary blood sample, with no preparation beyond continuing to eat gluten
- A strongly positive result makes coeliac disease very likely, and leads to referral
- A weakly positive result is followed up rather than dismissed
- A negative result in someone still eating gluten makes coeliac disease unlikely, but does not entirely exclude it
Why total IgA is measured too
The main test is an IgA antibody. Some people do not make normal amounts of IgA at all — selective IgA deficiency is the commonest primary immune deficiency and is more frequent in people with coeliac disease than in the general population.
In someone IgA deficient, an IgA-based test will read negative regardless of whether they have coeliac disease. Measuring total IgA alongside catches that, and where IgA is deficient, IgG-based tests are used instead — IgG EMA, IgG deamidated gliadin peptide (DGP) or IgG tTG.
Worth checking on your result. If you were told your coeliac blood test was negative, it is reasonable to ask whether total IgA was measured. A tTG result reported without it is not fully interpretable.
The biopsy
A positive blood test leads to referral to a gastrointestinal specialist for an endoscopic biopsy of the small intestine — the test that confirms the diagnosis.
- A thin flexible tube with a camera is passed through the mouth down to the small intestine
- The throat is numbed, and medication to help you relax is usually offered
- Several tiny tissue samples are taken and examined under a microscope
- Taking the samples is not painful, and the procedure itself takes only a few minutes
- Keep eating gluten until it is done
The reason a biopsy is still used, rather than relying on blood tests, is that the diagnosis commits you to a lifelong diet, family screening and long-term monitoring. It is worth confirming properly.
If the blood test is negative
A negative antibody test in someone eating a normal diet makes coeliac disease unlikely, but it is possible to have coeliac disease without detectable antibodies. If symptoms continue and there is no other explanation, referral to a specialist may still be appropriate — and worth asking about rather than accepting the result as final.
Two things to check before accepting a negative: were you eating enough gluten, for long enough; and was total IgA measured.
If you have already gone gluten-free
This is a common and genuinely awkward position. The options:
| Option | What it involves |
|---|---|
| A gluten challenge | Reintroducing gluten in more than one meal a day for at least six weeks, then testing. The only route to a definite answer |
| Testing anyway | A positive result is still meaningful. A negative one tells you very little |
| Staying gluten-free without a diagnosis | You keep the dietary restriction but lose the annual review, the family screening, the bone health monitoring, and gluten-free prescriptions where available |
The third option is the one most people drift into, and it is worth naming the trade-off rather than leaving it implicit. A challenge is unpleasant; going through life on a restricted diet for an unconfirmed condition has costs of its own.
Children
The first-line blood tests are the same. Children with positive results are referred to a paediatric gastroenterology specialist for further investigation rather than being diagnosed in primary care, and the same rule about continuing gluten applies — which is harder to manage in a child who feels unwell, and worth raising with the paediatric team rather than handling alone.
Home tests and genetic tests
- Home finger-prick coeliac tests are available, and a positive result should be followed up properly through a GP. They do not replace the diagnostic pathway, and a negative one carries the same caveats as any antibody test
- HLA DQ2/DQ8 genetic testing is not used for initial diagnosis outside specialist settings. Around a third of the population carries these genes and the vast majority never develop coeliac disease, so a positive result tells you almost nothing. Its value is the reverse: a negative result makes coeliac disease very unlikely, which is occasionally useful in specialist hands
- IgG food intolerance tests sold commercially do not diagnose coeliac disease or anything else reliable
Frequently Asked Questions
How much gluten do I need to eat before a coeliac test?
NICE advises eating gluten in more than one meal every day for at least six weeks before testing, and continuing until a specialist has confirmed the diagnosis. That means ordinary gluten-containing food at two or more meals daily, right up to and including the day of the test and the biopsy.
Can I stop eating gluten after a positive blood test?
No, not until the biopsy is done. The biopsy looks for damage to the gut lining, and that heals once gluten is removed, so stopping between the blood test and the biopsy can produce a normal biopsy result and an unconfirmed diagnosis.
Why is total IgA tested as well as tTG?
Because the main test is an IgA antibody, and some people do not produce normal amounts of IgA. In someone IgA deficient, an IgA-based test reads negative regardless of whether they have coeliac disease. If IgA is deficient, IgG-based tests are used instead.
What if my coeliac blood test is negative but I still have symptoms?
It is possible to have coeliac disease without detectable antibodies, so referral to a specialist may still be appropriate. First check two things: whether you were eating enough gluten for long enough before the test, and whether total IgA was measured alongside the tTG.
Is a home coeliac test enough?
No. A positive home test should be followed up through a GP, because diagnosis requires the standard blood tests and usually an endoscopic biopsy. Genetic HLA DQ2/DQ8 testing is not used for initial diagnosis outside specialist settings, as around a third of the population carries these genes without ever developing the condition.
References
- National Institute for Health and Care Excellence. Coeliac disease: recognition, assessment and management (NG20). nice.org.uk
- NHS. Coeliac disease. nhs.uk
- NHS. Coeliac disease: diagnosis. nhs.uk
- Coeliac UK. Non-coeliac gluten sensitivity. coeliac.org.uk
- National Institute for Health and Care Excellence. Irritable bowel syndrome in adults: diagnosis and management (CG61). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Do not start a gluten-free diet before coeliac testing is complete, as doing so makes the tests unreliable. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.