Tissue transglutaminase IgA antibody (anti-tTG)

Anti-tTG IgA measures an antibody the immune system produces against tissue transglutaminase, an enzyme in the lining of the small intestine, in people with coeliac disease. Coeliac disease is an autoimmune reaction to gluten — a protein in wheat, barley and rye — in which the gut lining is damaged, impairing the absorption of nutrients. When someone with the condition eats gluten, they make this antibody, so measuring it in the blood is the first-line screening test for coeliac disease. It is both sensitive and specific, which is why it is used to decide who needs a confirming biopsy of the small intestine. A crucial caveat is that the test measures the IgA class of antibody, and some people are deficient in IgA overall; in them the test can be falsely negative, so a total IgA level is checked at the same time. The test is only valid while the person is still eating gluten — going gluten-free before testing can turn a true positive negative. It is also used to monitor how well someone with known coeliac disease is sticking to a gluten-free diet, as the level falls when gluten is removed.

• Coeliac disease — the main reason for a positive result, especially with symptoms; needs confirming with a small-bowel biopsy.
• A high level (several times the upper limit) — makes coeliac disease very likely and, in some pathways, may allow diagnosis without biopsy.
• Ongoing gluten exposure in known coeliac disease — a persistently positive level suggests the gluten-free diet is not being fully followed.
• Occasionally other autoimmune conditions or liver disease — a weakly positive result can be non-specific.

• Negative — coeliac disease is unlikely, provided the person was eating gluten and is not IgA-deficient.
• A falling level on a gluten-free diet — in known coeliac disease, shows good adherence and gut healing.
• A false negative — possible if IgA-deficient (check total IgA) or already gluten-free before testing.