- Anti-tissue Transglutaminase Antibody (tTG): Tissue transglutaminase is an enzyme responsible for crosslinking (joining) certain proteins, and is found in the gut as well as in other tissues. It has been identified as the target molecule for the anti-endomysial antibodies. Although “tissue” is in the name of this autoantibody, it nevertheless involves testing blood and not tissue.
- Anti-Endomysial Antibodies (EMA): Endomysium is the thin connective tissue layer that covers individual muscle fibres. Anti-endomysial antibodies develop as part of the ongoing damage to the intestinal lining.
- Anti-gliadin and anti-reticulin antibodies are autoantibodies that have been used to evaluate suspected coeliac disease in the past. However these tests have both been superceded by anti-EMA and anti-tTG antibodies, and are no longer used in the UK.
There are 5 isotypes of antibody ,, , and) in the body. Both IgG and IgA types of each autoantibody will often be present in the blood, and may be tested for. In many laboratories IgA antibodies are used as the first line test for coeliac disease, with other antibody tests being performed only when required (e.g. if the patient has IgA deficiency, which is more common in coeliac disease than the general population).
In the UK, the National Institute for Health and Clinical Excellence (NICE) recommends serological testing for IgA tTG as first choice, followed by IgA EMA testing if the result is equivocal. If both tests are negative, IgA deficiency should be tested for before moving on to IgG serological tests if this is the case.
In the past, the only way to diagnose coeliac disease was to take a biopsy (small piece of tissue) from the small intestine and to examine it under the microscope. This method is still used to confirm a diagnosis of coeliac disease in adults, but the availability of less invasive blood tests used to screen for coeliac disease has reduced the number of biopsies required for this purpose.
People should continue to follow a gluten containing diet during the diagnostic process; otherwise the accuracy of both serological tests and biopsies may be adversely affected.
Genetic markers that are strongly associated with coeliac disease exist, and include the Human Leukocyte Antigen (HLA) markers DQ2 and DQ8. The british and European guidance on testing was updated in the last few years. If a child has high levels of tTg antibodies (>10 x ULN) and is positive for EMA and on testing has the relevant genetic predisposition (specific DQ2 or DQ8 genes) then a biopsy is not required to make the diagnosis of coeliac. The combination of tests makes a false positive very unlikely. It is worth noting that a variety of DQ2 and DQ8 genetic forms exist and not all confer the same degree of risk for coeliac disease. It is recommended therefore that only specialist labs undertake testing and that results are clearly interpreted
Other tests used to evaluate the severity of the disease include:
- FBC (full blood count) to look for anaemia
- ESR (erythrocyte sedimentation rate) to evaluate inflammation
- CRP (C-Reactive protein), a more sensitive measure of inflammation
- Biochemistry testing to determine sodium, potassium, calcium, and protein concentrations, and to check kidney and liver function
- Iron, Vitamin B12 and folate, ferritin, Vitamins D, E to check for mineral and vitamin deficiencies