Tissue transglutaminase IgA with a total IgA, on a gluten containing diet
Expansion
First line: IgA tissue transglutaminase antibody (anti-tTG) with a total IgA.
The two pitfalls
- The patient must be eating gluten, in more than one meal a day, for at least 6 weeks before testing. A gluten free diet produces a false negative, and the commonest practical problem is the patient who has already excluded gluten
- Selective IgA deficiency affects about 1 in 500 people and is ten times commoner in coeliac disease. An IgA based test in an IgA deficient patient is falsely negative, which is why the total IgA is mandatory. If deficient, use IgG based tests: IgG tTG, IgG endomysial or IgG deamidated gliadin peptide
Second line: endomysial antibody (EMA), which is more specific but operator dependent and more expensive.
Diagnosis
- Adults: serology, then duodenal biopsy for confirmation, taking at least four samples from the second part and one or two from the bulb, since the changes are patchy. Histology shows villous atrophy, crypt hyperplasia and intraepithelial lymphocytosis, graded by the Marsh classification
- Children: biopsy may be avoided where the tTG is more than 10 times the upper limit with a positive endomysial antibody on a second sample
Who to test: chronic diarrhoea, unexplained iron deficiency, weight loss, mouth ulcers, dermatitis herpetiformis, unexplained deranged liver enzymes, osteoporosis, infertility and unexplained neurological symptoms, plus type 1 diabetes, autoimmune thyroid disease, Down syndrome, Turner syndrome and first degree relatives.
HLA-DQ2 and DQ8 typing has a high negative predictive value, so a negative result essentially excludes coeliac disease; a positive is unhelpful, since a third of the population carries it.