Evolution of Celiac Disease Testing The laboratory is challenged to provide guidance on test ordering and interpretation while ensuring accurate performance and appropriate test utilization

Vol. 23 • Issue 9 • Page 38 Celiac Disease Testing Celiac disease (CD) is an immune-mediated inflammatory process elicited by gluten that occurs in genetically susceptible individuals.1,2 The inflammatory response is triggered when partially digested gluten peptides (especially gliadin) reach the intestinal mucosa.2 These peptides are a substrate for tissue transglutaminase (TTG), which acts to deamidate gliadin, forming deamidated gliadin peptide (DGP).3 Both DGP and DGP-TTG complexes can be immunogenic, resulting in the production of antibodies against TTG and DGP, local inflammation, and damage to the intestinal mucosa causing malabsorption.3 This can lead to signs and symptoms such as anemia, failure to thrive, weight loss, diarrhea and bloating.1,3 HLA molecules encoded by the DQ2 and DQ8 haplotypes are associated with this immune process, forming part of the genetic basis of CD.4 Traditionally, CD diagnosis requires duodenal biopsy to characterize the degree of mucosal damage. Histological changes seen in CD include villous atrophy, crypt hyperplasia and increased intraepithelial lymphocytes. Classification of damage severity is outlined by the Marsh Criteria, which remains the gold standard for CD diagnosis.1,5-7 Histological analysis is not without its limitations, however (Table).8,9 Biopsy procedures are invasive, so other means of identifying CD patients have been sought. Serological tests, which detect antibodies produced during the mucosal immune reaction, provide a less invasive alternative. Serological Testing Early indirect immunofluorescence methods detected CD-specific antibodies in a semi-quantitative fashion. The anti-endomysial antibody test (EMA) remains the most sensitive and specific of these.10,11 A positive test is defined by the presence of staining restricted to smooth muscle fibers, and titers can be reported for semi-quantitative results. This assay can be challenging due to scarcity of tissue substrates, subjective interpretation and method heterogeneity between labs.11-13 Other immunofluorescence-based markers such as reticulin and gliadin antibodies have been shown to have inferior sensitivity and specificity to EMA, and should not be used for CD testing.1,14,15 The antigenic target of EMAs was later discovered to be TTG;16 DGP was also found to be a target of antibodies in CD patients.17,18 This led to the production of quantitative immunoassays for both anti-TTG and anti-DGP antibodies. Solid-phase immunoassays using immobilized TTG or DGP are commonly used. Following incubation with a patient's serum, anti-TTG or anti-DGP antibodies are detected using labeled anti-human, class-specific detection antibodies. Radiobinding assays have also been described.19 These assay formats allow for the use of a calibration curve and reporting in concentration units. A positive result is defined at a specific cut-off.1 However, because assay reagents vary by manufacturer, laboratories and clinicians need to be aware that cut-offs and results are not interchangeable.19-21 Other Testing Considerations Several other important considerations must be made when using serological testing in CD. First, mucosal immune responses typically involve IgA-class antibodies; therefore, serological testing commonly targets IgA class antibodies. However, IgA deficiency occurs more frequently in CD patients compared to population estimates (2.5% versus 0.25%),22 and IgA-based serological tests may not provide informative results in these patients. IgA quantitation can help interpret CD serology ADVANCE for Laboratory Administrators | Lab Admin Magazine | Print... http://laboratory-manager.advanceweb.com/Editorial/Content/PrintFriend...