Improving the antibody-based evaluation of autoimmune encephalitis.

Objective: We tested whether antibody screening samples of patients with suspected autoimmune encephalitis with additional research assays would improve the detection of autoimmune encephalitis compared with standard clinical testing alone. Methods: We examined 731 samples (333 CSF, 182 sera, and 108 pairs) from a cohort of 623 patients who were tested for CNS autoantibodies by the University of Pennsylvania clinical laboratory over a 24-month period with cell-based assays (CBAs) on commercially obtained slides of fixed cells for antibodies to NMDA receptor (NMDAR), -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), -aminobutyric acid-B receptor (GABA(B)R), leucine-rich glioma-inactivated 1 (LGI1), contactin-associated protein-like 2 (Caspr2), and glutamic acid decarboxylase (GAD65). In parallel, our research laboratory screened all samples for reactivity to brain sections and for anti-NMDAR using in-house CBAs. Samples with brain reactivity or positive clinical studies were examined with CBAs for a larger panel of antibodies. Results: The clinical laboratory reported positive findings for NMDAR (80 samples), GAD65 (8), LGI1 (5), Caspr2 (2), and GABA(B)R (4). Sixty-five serum samples and 32 CSF samples were indeterminate for one or more antibodies. In our research laboratory, all but 4 positive results were confirmed, 88 of 97 indeterminate results were resolved, and 15 additional samples were found positive (10 NMDAR, 1 AMPAR, 3 LGI1, and 1 Caspr2). Clinical information supported these diagnoses. Overall, informative autoantibodies were detected in 15.5% of cases. Conclusions: Standard clinical laboratory kits were specific, but some tests were insensitive and prone to indeterminate results. Screening with immunohistochemistry for reactivity to brain sections, followed by additional CBAs for cases with brain reactivity, improves the diagnostic accuracy of testing for autoimmune encephalitis.