tTG-IgA

Coeliac disease is an autoimmune disease in which the immune system reacts to gluten, the protein in wheat, rye and barley. In that reaction antibodies arise against tissue transglutaminase, an enzyme in your own gut wall, and the gut wall becomes inflamed and flattens, so nutrients are absorbed less well. tTG-IgA measures those antibodies, of the IgA kind, the antibody of the mucous membranes. The test is good: in anyone who eats gluten and has a normal IgA, it finds the great majority of people with coeliac disease, and a clearly positive result is almost always coeliac disease. It has two conditions. The antibodies are only made as long as gluten comes in, so anyone already eating gluten-free tests negative without that meaning anything. And because it is an IgA antibody, anyone who makes little IgA themselves always tests negative; that is why total IgA belongs with it as standard. Your report shows tTG-IgA as a number with a limit: below 7 kU/L normal, above 10 kU/L positive, and in between equivocal. The unit differs per lab; the limit is the same everywhere. The higher above the limit, the more certain the result.

Coeliac disease occurs in about one in a hundred people, and most of them do not know it. The complaints are often vague: tiredness, a bloated belly, changeable stools, an iron deficiency that will not go away, osteoporosis at a young age, or no complaints at all. Untreated, the disease damages the gut wall for years, with deficiencies of iron, vitamin B12, folate and vitamin D as the result, and an increased chance of fractures and other autoimmune diseases. The treatment is not a medicine but a diet: lifelong gluten-free, after which the gut wall recovers and the deficiencies disappear. That makes coeliac disease a disease worth knowing about, and tTG-IgA the test that makes that possible. Anyone with an iron deficiency that does not respond to iron, anyone with a parent or sibling with coeliac disease, anyone with type 1 diabetes or an autoimmune thyroid disease has good reason to have it measured. The pitfall is the order. Many people who feel better without gluten have already gone gluten-free before being tested. Then the antibodies are gone and the test measures nothing, and the question of whether it was coeliac disease, or a sensitivity without autoimmune disease, stays open. Anyone who wants to know eats normal gluten first for at least six weeks, daily, and tests after that.

A positive tTG-IgA, above 10 kU/L, fits coeliac disease strongly; the higher, the more certain, and a value many times the limit is enough for the diagnosis in children. In adults the diagnosis is usually confirmed with an endoscopy of the small intestine, and that is done while you are still eating gluten. A slightly raised value sometimes occurs without coeliac disease, with type 1 diabetes, an autoimmune thyroid disease or a liver disease, and is then repeated and supplemented with a second antibody. A negative tTG-IgA is reliable if you ate gluten in the weeks before and your total IgA is normal. If you were already gluten-free, the result says nothing; if your IgA is low, an IgG variant of the test is used. In a small share of people with coeliac disease the test still stays negative, and with strong suspicion an endoscopy follows anyway. TTG-IgA belongs beside your total IgA, and with complaints beside ferritin, vitamin B12, folate and vitamin D, the deficiencies coeliac disease causes.