Tissue transglutaminase antibody (IgA)

The upper limit depends on the method: 7, 10, 15 and 20 U/mL are all in use in UK laboratories, and some report Negative, Weak positive or Positive. The test relies on IgA, so in people who make very little IgA (about 1 in 600 in the general population) it reads falsely low; laboratories measure total IgA alongside it for this reason.

Tissue transglutaminase is an enzyme in the lining of the gut. In coeliac disease, eating gluten leads the immune system to make antibodies against it, and this test measures those antibodies, of the IgA class. It is the standard first blood test for coeliac disease in the UK, and it only works if the person has been eating gluten regularly in the weeks before the sample.

Doctors request it when looking at coeliac disease in someone with unexplained tiredness, anaemia, low iron, bloating, diarrhoea, weight loss or bone thinning, and in close relatives of people with coeliac disease and in people with type 1 diabetes or thyroid conditions.

• Coeliac disease, in someone eating gluten
• Dermatitis herpetiformis, the skin form of coeliac disease
• Type 1 diabetes, autoimmune thyroid and liver conditions, at low levels
• Some long-term infections and inflammatory bowel conditions, at low levels
• Reduced kidney function, a little

• A gluten-free diet, which brings the level down over months and can make the test negative even in coeliac disease
• Negative is the usual finding in healthy adults; there is no lower limit
• Immune-suppressing medicines
• Low IgA, which makes the test read falsely low

• A low or negative result means little if gluten has been cut out of the diet before the test
• Selective IgA deficiency, a common inherited trait, makes the test falsely negative, which is why total IgA is checked with it
• Methods and cut-offs differ between laboratories, so a number from one lab does not map onto another's range
• A low positive is found in some people with other autoimmune conditions

No fasting needed, but gluten must have been eaten regularly, in more than one meal a day, for at least six weeks before the sample. Always read alongside the total IgA from the same sample. In people with known coeliac disease it is followed over time as a guide to how strict the diet has been.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.