B-122 Discrepant transglutaminase IgA results between Bioplex 2200 and Phadia 250 assays

Abstract

Background Transglutaminase (tTg) IgA plus total IgA is the recommended screening test for patients over two years old for Celiac Disease. Other tests, such as Endomysial IgA (EMA), may be useful in diagnosing patients when the tTg IgA is equivocal due to the higher specificity of EMA compared to tTg IgA. For patients with Celiac Disease, tTg IgA is a useful marker to monitor disease activity when subscribing to a gluten-free diet. To consolidate antibody testing to a single vendor, tTg IgA was moved from a Phadia 250 assay (EliA Celikey IgA) to a Bioplex 2200 assay (Celiac IgA). Methods To verify the accuracy of tTg IgA on the Bioplex 2200 assay, 65 residual patient sera samples were analyzed on both the Phadia 250 and Bioplex 2200 assay. Results 90.7% were qualitatively concordant between instruments (table 1). Conclusion In our laboratory we offer a Celiac Cascade consisting of tTg IgA plus total IgA. In the cascade when tTg IgA is equivocal it is reflexed to EMA. With the implementation of the Bioplex 2200 assay there is no longer an equivocal range. In our accuracy study we identified a small cohort of patients that tested equivocal on the Phadia 250 assay (7-10 U/mL) and as low positives on the Bioplex 2200 assay (reference range <15 U/mL). The five samples that were equivocal on the Phadia 250 assay resulted in a median (min, max) Bioplex 2200 result of 34.4 U/mL (29.6-73.3). As a result, we modified our cascade so any weakly positive tTg IgA (15-30 U/mL) will reflex to EMA. Post go-live there have been three clinical reports of Celiac Disease patients adhering to a gluten-free diet who tested either as high negative or equivocal on the Phadia 250 assay before the assay transition and are now weakly positive (<30 U/mL) on the Bioplex 2200 assay with a negative EMA. In conclusion, it is recommended that if the tTg IgA is a low positive (15-30 U/mL) by the Bioplex 2200 assay, reflex to EMA. If the EMA is negative, it can be concluded that the patient does not have active disease and is most likely compliant with a gluten-free diet.