The Gluten Challenge
Coeliac tests only work while gluten is still in your diet. If you've already cut it out, a short "gluten challenge" gets your body back to a state where the tests can give a reliable answer.
Don't start a gluten-free diet before testing
Going gluten-free before a coeliac blood test or biopsy can make the results falsely negative. If gluten has already been removed, you'll need a gluten challenge before testing to get a conclusive result.
How much gluten, and for how long?
If you've been gluten-free, resume a normal diet for six weeks before testing (blood test or biopsy).
What "4 slices of bread" looks like
Adults aim for the equivalent of four slices of wheat-based bread a day (children, two). One slice is about 2–2.5 g of gluten — roughly equal to any of these:
How much is enough?
There isn't full agreement on the exact dose and duration — so this should always be guided by your doctor.
Coeliac Australia notes there's limited consensus on how much gluten, and for how long, a challenge needs to be. Some evidence suggests 3–6 g of gluten a day for 4 weeks may be enough to change coeliac blood tests for most people (around 70%). To be as certain as possible, Coeliac Australia currently recommends more gluten over a longer period to give a reliable diagnosis for the majority. Be guided by your doctor throughout.
What about the coeliac gene test?
Over 99% of people with coeliac disease carry the HLA DQ2 or DQ8 genes (or parts of them). That makes the gene test useful in a specific way:
- A negative gene test effectively rules coeliac disease out.
- A positive gene test can't diagnose coeliac on its own — only about 1 in 40 people with these genes go on to develop it.
- Because it doesn't depend on gluten intake, the gene test can be done even if you're already gluten-free.
- If it's positive, a gluten challenge followed by a small-bowel biopsy is needed to confirm coeliac disease. A gluten-free diet should only be started after coeliac disease is confirmed by biopsy.
In summary
- Don't go gluten-free before testing — it can make coeliac tests falsely negative.
- If you're already gluten-free, do a gluten challenge (about 4 slices of bread a day for ~6 weeks) before a blood test or biopsy.
- The gene test can be done any time — even gluten-free — and a negative result rules coeliac out.
- Only start a gluten-free diet after coeliac disease is confirmed, and always be guided by your doctor. Source & disclaimer. Adapted from Coeliac Australia's Gluten Challenge fact sheet (coeliac.org.au). This is general educational information, not medical advice — always seek the guidance of your doctor before starting or stopping a gluten challenge. Reference: Leffler et al, Gut 2013;62:996–1004.