Anti-tTG IgA test explained (anti-tTG IgA)

Anti-tTG IgA is an antibody the immune system produces against the enzyme tissue transglutaminase when the gut is going through an autoimmune reaction to gluten — the reaction typical of coeliac disease. The test belongs to the wider group of coeliac serology tests, alongside total IgA, antibodies against deamidated gliadin peptide (anti-DGP) and endomysial antibodies (EMA). According to MedlinePlus, the test is used for three purposes: to help diagnose coeliac disease, to monitor its course, and to assess whether a gluten-free diet is helping to reduce symptoms. Because the test detects IgA class antibodies, and some people have a congenital total IgA deficiency, total immunoglobulin A is often tested alongside it — where it is deficient, the anti-tTG IgA result can be falsely negative even in someone who has coeliac disease. Blood is taken from a vein in the usual way.

• When a child has nausea, bloating, constipation, chronic diarrhoea, weight loss or delayed puberty
• When an adult has nausea and vomiting, chronic diarrhoea, unexplained weight loss or abdominal pain
• When there are signs unrelated to digestion — iron-deficiency anaemia, a skin rash, mouth ulcers, thinning of the bones, tiredness, depression or headaches
• When coeliac disease is suspected because of family history or other autoimmune conditions
• When monitoring already diagnosed coeliac disease and assessing whether the gluten-free diet is working

A positive (raised) anti-tTG IgA indicates that coeliac disease is likely — according to MedlinePlus, such a result is taken to mean that the person most probably has coeliac disease. A clear-cut result is often confirmed with a biopsy of the small intestine. Unclear (borderline) results may call for further tests.

What can raise anti-tTG IgA

• Coeliac disease — an autoimmune reaction to gluten in the small intestine
• More rarely, other autoimmune or bowel conditions, for which a doctor may order further tests

Symptoms that can accompany a positive result

• Nausea and vomiting
• Chronic diarrhoea
• Unexplained weight loss
• Abdominal pain and bloating
• Iron-deficiency anaemia
• A skin rash, mouth ulcers
• Thinning of the bones, tiredness, depression or headaches

• Discussing the result with a doctor together with the total IgA level — a deficiency changes the interpretation
• Whether a biopsy of the small intestine is needed to confirm the diagnosis
• Whether gluten-containing food was being eaten before the test — this can affect the result
• Family history and other autoimmune conditions

A negative (low) anti-tTG IgA

A negative (low) anti-tTG IgA indicates that coeliac disease is unlikely — according to MedlinePlus, such a result means the person most probably does not have coeliac disease. Even so, if gluten was avoided before the test or there is a total IgA deficiency, the result can be falsely negative even in someone who does have coeliac disease.

What can lie behind a negative result

• The absence of coeliac disease — the most common reason for a negative result
• Not eating gluten before the test, which can make the result falsely negative even in someone who does have coeliac disease
• A total immunoglobulin A deficiency, because of which an IgA class antibody test can wrongly show a negative result

What is usually assessed alongside a negative result

• If symptoms persist, discussing with a doctor a total IgA check and the other coeliac tests (anti-DGP, EMA)
• Whether gluten-containing food was eaten for long enough before the test
• Whether something else could be causing the symptoms, if coeliac disease has been ruled out