AQP4 autoantibody assay performance in clinical laboratory service

OBJECTIVE:To compare performance of contemporary aquaporin-4–immunoglobulin (Ig) G assays in clinical service. METHODS:Sera from neurologic patients (4 groups) and controls were tested initially by service ELISA (recombinant human aquaporin-4, M1 isoform) and then by cell-based fluorescence assaysfixed (CBA, M1-aquaporin-4, observer-scored) and live (fluorescence-activated cell sorting [FACS], M1 and M23 aquaporin-4 isoforms). Group 1all Mayo Clinic patients tested from January to May 2012; group 2consecutive aquaporin-4-IgG–positive patients from September 2011 (Mayo and non-Mayo); group 3suspected ELISA false-negatives from 2011 to 2013 (physician-reported, high likelihood of neuromyelitis optica spectrum disorders [NMOSDs] clinically); group 4suspected ELISA false-positives (physician-reported, not NMOSD clinically). RESULTS:Group 1 (n = 388)M1-FACS assay performed optimally (areas under the curvesM1 = 0.64; M23 = 0.57 [p = 0.02]). Group 2 (n = 30)NMOSD clinical diagnosis was confirmed byM23-FACS, 24; M1-FACS, 23; M1-CBA, 20; and M1-ELISA, 18. Six results were suspected false-positiveM23-FACS, 2; M1-ELISA, 2; and M23-FACS, M1-FACS, and M1-CBA, 2. Group 3 (n = 31, suspected M1-ELISA false-negatives)results were positive for 5 seraM1-FACS, 5; M23-FACS, 3; and M1-CBA, 2. Group 4 (n = 41, suspected M1-ELISA false-positives)all negative except 1 (positive only by M1-CBA). M1/M23-cotransfected cells expressing smaller membrane arrays of aquaporin-4 yielded fewer false- positive FACS results than M23-transfected cells. CONCLUSION:Aquaporin-4-transfected CBAs, particularly M1-FACS, perform optimally in aiding NMOSD serologic diagnosis. High-order arrays of M23-aquaporin-4 may yield false-positive results by binding IgG nonspecifically.