Coeliac Disease Testing Explained

Coeliac disease is far more common than many people realise, affecting roughly one in a hundred people in the UK — yet a large proportion remain undiagnosed. It is not an allergy or a simple intolerance. It is an autoimmune condition in which eating gluten, a protein found in wheat, barley and rye, triggers the immune system to attack the lining of the small intestine. Over time this can affect how well nutrients are absorbed. A straightforward blood test is usually the first step in looking for it.

How the antibody blood test works

When someone with coeliac disease eats gluten, their body produces specific antibodies that can be measured in the blood. The main test looks at two things together:

• Tissue transglutaminase IgA (tTG-IgA) — the primary screening antibody, raised in most people with active coeliac disease.
• Total IgA — measured alongside because some people are naturally low in IgA, which could otherwise make the tTG result falsely reassuring.

Checking both together makes the screen more reliable. A raised tTG-IgA suggests coeliac disease is likely and should be investigated further.

Why you must keep eating gluten first

This is the single most important point about coeliac testing. The blood test detects your immune reaction to gluten, so if you have already removed gluten from your diet, the antibodies fall and the test can come back negative even if you genuinely have the condition. Before testing you should be eating gluten regularly — the equivalent of a few slices of bread daily — for at least several weeks. Never cut out gluten "to see if it helps" before being tested, as this can make an accurate diagnosis much harder.

Who should consider testing

Testing is worth discussing if you have ongoing digestive symptoms — bloating, diarrhoea, tummy pain, wind — or less obvious signs such as unexplained tiredness, iron-deficiency anaemia, mouth ulcers or weight loss. It is also more common in people who have a close relative with coeliac disease, or who have another autoimmune condition such as type 1 diabetes or an autoimmune thyroid problem.

What happens after a positive result

A positive antibody test is not the final word — it is a strong signal that needs confirmation. In the UK, diagnosis is usually confirmed by a specialist, often with a gut biopsy taken during a gastroscopy while you are still eating gluten. Only once coeliac disease is confirmed should a strict, lifelong gluten-free diet begin, ideally with the support of a dietitian. Starting the diet too early can leave you without a clear diagnosis.

When to seek prompt advice

See a doctor sooner rather than later if you have unexplained weight loss, persistent diarrhoea, blood in your stool, difficulty swallowing, or signs of anaemia such as marked tiredness and breathlessness. These may point to coeliac disease or to other conditions that need assessment in their own right.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic