To accurately diagnose coeliac disease there are two steps: 

Blood test 

In the first instance you will need to go to your GP who will arrange for a blood test. There are specific blood tests used to diagnose coeliac disease. They look for antibodies that the body makes in response to eating gluten. Your GP will carry out a simple blood test to check for these antibodies. 

In adults and children, with suspected coeliac disease, the following blood tests are recommended: 

• Total immunoglobulin A (IgA). 

• IgA Tissue transglutaminase antibody (shortened to tTG). 

It’s important to continue eating gluten until you have had all the required tests – blood tests and biopsy where necessary. If you remove or reduce the amount of gluten in your diet before testing this will affect your results. 

Guidelines recommend that you eat some gluten in more than one meal every day for at least six weeks before testing. 

Negative blood test results 

It’s possible to have a negative blood test and still have coeliac disease. If you weren’t eating gluten at the time of your blood test, you may have received an inaccurate result. 

If you have ongoing symptoms that suggest coeliac disease but have had a negative blood test, ask your GP to check to see if you have been tested for IgA deficiency. 

IgA deficiency 

Some people with coeliac disease do not make the usual coeliac disease antibodies. This is called IgA deficiency. When the laboratory is measuring your antibody level they should also check your total serum IgA to detect IgA deficiency. If you are IgA deficient the following tests should be considered: 

• Immunoglobulin G (IgG) EMA. 

• IgG deamidated gliadin peptide (DGP). 

• IgG tTG. 

If you have tested negative for coeliac disease, particularly if you have Type 1 diabetes or you are a close relative of someone with coeliac disease, it is important to note that coeliac disease may present with a wide range of symptoms and you should consult with your GP if any symptoms arise or persist. 

Referral to secondary care 

If you have a positive blood test for coeliac disease or there is clinical suspicion, your GP will refer you to a gut specialist – a gastroenterologist. Usually, a biopsy of the gut is carried out to confirm whether or not you have coeliac disease but in some cases, both adults and children can be diagnosed based on blood tests without the need for a biopsy.  

Around 20-30% of adults with symptoms associated with undiagnosed coeliac disease, will have a blood test with antibody levels that are very high, ten times the upper limit of normal, that they can be diagnosed with coeliac disease without the need for having an endoscopy and biopsy. The decision not to have an endoscopy and biopsy should be made in shared decision making between the gastroenterologist and the patient.  

For children, a biopsy may not be necessary if antibody levels are ten time the upper limit of normal followed by a second positive blood test. A no biopsy diagnosis should be made in discussion with a paediatric gastroenterologist or paediatrician with a special interest in gastroenterology and the parents/carers and child, where appropriate.  

Biopsy  

A biopsy involves a small camera called an endoscope being passed through your mouth and stomach into the gut. It is done using an anaesthetic spray to numb your throat, or with a sedative given by injection. 

Biopsies are collected and examined under a microscope to check for damage to the gut lining, which is typical of coeliac disease. 

If you have any concerns about testing, the best thing to do is to talk with your healthcare team. 

Classification of biopsy results 

In people with coeliac disease there is damage to the lining of the intestine. Your healthcare professional will classify the damage according to a scale called the Marsh classification: 

Marsh 0: the lining of the intestine is normal and it’s unlikely that the person has coeliac disease. 

Marsh 1: increased number of lymphocytes (small white blood cells that are involved in the body’s immune system response to the disease) is seen, but there are normal villi. 

Marsh 2: increased number of lymphocytes, the depressions in the lining of the intestine are deeper than normal but normal villi length. 

Marsh 3: the villi are becoming flattened. 

Marsh 4: the villi are completely flattened. 

Potential coeliac disease  

Potential coeliac disease is used to describe someone who has repeatedly high antibody blood tests, has the HLA genetics associated with coeliac disease but has normal or minimal changes to the lining of their gut, as described in Marsh 0 and 1, and other conditions have been ruled out.  

Potential coeliac disease is often considered as an early manifestation of coeliac disease.  

For people with symptoms, the guidelines suggest that a gluten free diet could be trialled with the support of a Specialist Dietitian. This approach should be reconsidered if there is no improvement after around 3 – 6 months and an alternative cause for symptoms should be explored.  

Follow up for individuals with potential coeliac disease who have adopted a gluten free diet should be the same as for those with a diagnosis of coeliac disease. 

If someone with potential coeliac disease remains on a gluten containing diet, they should be made aware of their risk of developing coeliac disease. A discussion between patient and gastroenterologist should take place, regarding a repeat blood test and endoscopy and biopsy if new symptoms develop or repeat testing after 1 – 2 years if stable.   

Seronegative coeliac disease  

Seronegative coeliac disease can be diagnosed when someone has extensive gut damage, the HLA genetics associated with risk of coeliac disease, blood tests using IgA- and IgG-based coeliac antibodies are negative and there are improvements to the lining of the gut after adopting a gluten free diet. 

Seronegative coeliac disease accounts for around 2 – 3% of coeliac disease cases and around a third of people who have extensive gut damage but don’t have a positive coeliac disease antibody test, so it’s important to rule out other causes of the gut damage.