To accurately diagnose coeliac disease there are two steps:

1. Blood test – In the first instance you will need to go to your GP who will arrange for a blood test.
2. Referral to a specialist – If that blood test indicates that it's likely you do have coeliac disease then you will be referred to a gut specialist (gastroenterologist) who may conduct a confirmation test called a biopsy, or may carry out further blood tests to confirm the diagnosis as a biopsy may not be needed in all cases.

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 a specialist

If you have a positive blood test for coeliac disease, 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. Read more about diagnosis of children.

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.

In some cases, an endoscopy might not be needed to diagnose coeliac disease. Guidelines published in June 2020 in response to the Covid-19 pandemic recommend that some adults can be diagnosed without the need for a biopsy if they:

• are 55 years or younger
• don't need an endoscopy to rule out another condition
• have symptoms of coeliac disease
• have very high antibody levels (if IgA tTG is at least 10 times the upper limit of normal)
• and have a second positive antibody blood test (EMA or IgA tTG if EMA is not available)

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.