P.028 Is there utility in duplicating antibody testing for autoimmune encephalitis? A comparison of results obtained from Mayo and Mitogen Dx

Abstract

Background: Autoantibody testing for suspected autoimmune encephalitis (AIE) in Alberta is commonly performed by Mitogen Dx (MDx) using cell-based assays (CBAs) for cell surface antibodies and line immunoassay (LA) for intracellular antibodies without confirmatory tissue immunofluorescence/immunohistochemistry (TIFF/IHC). Duplicate testing is often sent to Mayo Clinic (MC) verify, resulting in increased costs. Methods: Antibody panel results were obtained for all patients who had testing sent to both MC and MDx from adult hospitals in Calgary between 2018 and 2020. Positive antibodies were evaluated to be pathogenic/non-pathogenic by chart review and expert consensus. Results: Thirty-four individuals had antibody panels completed at both labs. Overall agreement (positive/negative panel) was fair (κ = 0.24, p =.08), even after excluding low-titre GAD65 antibodies through MC (n=9, 26.5%). MDx reported more non-pathogenic serum results, including: anti-SOX1 (n=3), anti-NMDAR (n=2) and anti-GABA(B)R (n=1). All pathogenic antibodies (n=3) were positive in both laboratories. Conclusions: No new pathogenic antibodies were identified by sending duplicate testing to MC; however, a larger number of non-pathogenic antibodies were reported by MDx, likely due to lack of confirmatory TIFF/IHC. Antibody testing for AIE should be done in labs performing confirmatory TIFF/IHC on all CBA/LA results to avoid unnecessary investigations and/or treatments.