For info from Coeliac UK website:
"Keep eating gluten throughout the diagnosis process
If you think you may have coeliac disease, it’s essential to continue eating gluten until your doctor makes a diagnosis.
The diagnostic tests for coeliac disease look at how the body responds to gluten. Some people may start to reduce or eliminate gluten from their diet because they feel ill. Unfortunately this is very likely to cause an inaccurate result for both the blood test and the gut biopsy. Therefore, it’s very important to keep eating gluten throughout the diagnosis process.
If you've already reduced or eliminated gluten from your diet, you will need to reintroduce it to make sure you get the most accurate test results. While it may be difficult and uncomfortable, it’s essential for your long term health.
As a general guideline, the recommendation is to eat some gluten in more than one meal every day for at least six weeks before testing.
For children, you can mix wheat flour into foods such as yoghurt or baked beans to add more gluten into their diet.
If you are reintroducing gluten into your diet, you should discuss how best to manage your symptoms with your GP. Your GP will be able to arrange for you to be tested for coeliac disease as soon as it is appropriate.
We understand how hard it is to get diagnosed if you have started a gluten-free diet. Getting a medical diagnosis, however, is important to ensure you get the medical follow up and support that is recommended for people with coeliac disease. Unfortunately, there are no other tests that can diagnose coeliac disease other than the antibody blood test and the gut biopsy. These tests depend on people eating gluten."
"Irritable bowel syndrome:
Almost a quarter of people with coeliac disease had previously been told they had IBS or were treated for it before they were diagnosed with coeliac disease, according to recent research1. It is important that the NICE Guidelines are followed and that coeliac disease is screened for before a diagnosis of IBS is given".