Dr Anthony Kelleher
Department of Immunopathology

1. Introduction

Coeliac  disease, also known as gluten sensitive enteropathy, results from immune mediated damage to small bowel mucosa triggered by an immune response to the gliadin fraction of gluten, a component of wheat, barely and oats. The pathogenesis of the disease is dependent upon the de-amidation of proteins within the gliadin fraction by the intestinal enzyme, tissue transglutaminase (tTG). This modification induces antibodies to both gliadin and tTG. The presence of auto-antibodies has lead to the development of a number of serological tests which assist with the diagnosis of coeliac disease. Final diagnosis however, is still dependent on characteristic histological changes found on small bowel biopsy and improvement on a gluten free diet.

2. Diagnosis

Autoantibodies in Coeliac disease.

A number of antibodies have been described as having an association with Coeliac disease. These include anti-gliadin and anti-transglutaminase antibodies.  The later can be measured by either immunofluorescence and reported as anti-endomysial antibodies or by ELISA and reported directly as anti-transglutaminase antibodies.  In each case both IgG and IgA isotype antibodies have been described.  In general the IgA antibodies have greater sensitivity and specificity and are the preferred diagnostic tool. However, a negative IgA autoantibody result must be interpreted  carefully.  IgA deficiency occurs more frequently in patients with Coeliac disease (approx 2%) than in the normal population (approx 0.15%).  Furthermore, individuals with IgA deficiency have at least a 10 fold increased risk of coeliac disease than the general population.  Therefore the potential for a false negative IgA based serological test is real and a concurrent IgA estimation is recommended when ordering coeliac serology. Currently the tests offered by SYDPATH include, IgA anti-endomysial antibody and IgA and IgG anti gliadin antibodies, but the IgA based tests are only interpretable if one knows that IgA deficiency is absent.

Anti-transglutaminase antibodies

Detected as anti-endomysial antibodies by indirect immuno-fluorescence on tissue sections: These are measured by indirect immuno-fluorescence. IgA anti-endomysial antibodies have consistently been shown to have high sensitivity and specificity for coeliac disease.   This is the most widely used test in Australian laboratories.  The major protein stained in this assay is tissue transglutaminase (tTG).  The main technical problem with this assay is that the presence of concomitant anti-smooth muscle antibody making the interpretation of the slides more difficult. The presence of a high titre anti-smooth muscle antibody may make the test uninterpretable and an alternative serological test should be performed.

Detected by ELISA as anti-transglutaminase antibodies: This assay is an alternative to the anti-endomysial antibody and has similar levels of sensitivity and specificity.  However it is incompletely validated and this has limited its widespread adoption. Recent data indicate that low level reactivity occurs in a range of autoimmune disorders complicating the interpretation of low titre results.

Given the parity of the anti-endomysial and anti-tissue  transglutaminase assays most laboratories do not offer both. SydPath currently offers the anti-endomysial antibody assay.

Anti-gliadin antibodies

This ELISA based assay has a lower sensitivity and specificity than the IgA anti-endomysial and IgA anti-transglutaminase antibody tests.  However, there may be some patients with coeliac disease who have detectable anti-gliadin antibodies but no antibodies to transglutaminase.  The IgG form of this test is most useful in children less than 2 years of age and in guiding diagnosis in individuals with IgA deficiency.  It is also used to monitor dietary compliance.

3. Indications

Autoantibodies should be used as a guide to identify those who are most likely to have a positive small bowel biopsy which is required to confirm the diagnosis of coeliac disease. In those older than 2 years of age, an IgA anti-endomysial antibody is the test of choice. This ideally should be ordered in conjunction with a total serum IgA level to exclude IgA deficiency. In those with IgA deficiency then IgG isotype antibodies against gliadin may be of utility. Diagnostic suspicion must be confirmed by small bowel biopsy. Adherence to diet can be monitored by anti-gliadin antibody. However, normalisation of antibody levels does not guarantee resolution of tissue histopathology.

4. Supplemental Tests

Immunoglobulin levels should be ordered in those in whom these are not known, as IgA deficiency is more common in those with coeliac disease and will result in a false negative test.

Antibody testing in IgA deficient individuals.

There is no diagnostic utility in measuring  IgG isotype autoantibodies  in individuals with normal IgA levels.  In IgA deficient  individuals  measurement of IgG isotype anti-endomysial,  tissue transglutaminase or gliadin  antibodies may  be useful.