We sought to assess the false positivity rate and positive predictive value of aquaporin-4 antibody (AQP4-IgG) using live cell-based flow cytometry assay for a diagnosis of neuromyelitis optica spectrum disorder (NMOSD) at a tertiary referral center.

AQP4-IgG is a serum diagnostic biomarker of neuromyelitis optica spectrum disorder (NMOSD). Antibody detection methods have improved but false positive rates of up to 5% are reported with some assays (e.g., enzyme-linked immunosorbent assay).

We searched our neuroimmunology laboratory database (1/1/2018-12/31/2019) and identified consecutive Mayo Clinic patients that tested positive for AQP4 IgG over the two year study period. We used a live cell-based fluorescent activated cell sorting (FACS) using the M1 isoform of aquaporin-4.  IgG-binding-index ≥2 was the cut-off during screening and an end titration ≥5 confirmed a positive result. Clinical records were reviewed independently by two neurologists to determine if positives met diagnostic criteria for NMOSD with AQP4-IgG.

Of 1704 Mayo Clinic patients consecutively tested for AQP4-IgG, 37 (2%) were positive. All 37 fulfilled 2015 NMOSD diagnostic criteria for NMOSD with AQP4-IgG with 100% agreement between two neurologists evaluating the medical records. Thus, no false positives occurred. The positive predictive value and specificity of AQP4-IgG for NMOSD were 100%. Among the AQP4-IgG positive samples, the median IgG-binding-index was 11.7 (range, 2.1-71.7) and median end titer was 1000 (range, 5-10000). The median age of AQP4-IgG seropositive patients (81% female) was 56 years (range, 4-90). AQP4-IgG NMOSD included one or more attacks of: transverse myelitis, 28; optic neuritis, 14; or area postrema syndrome, 8.

AQP4-IgG, assessed by live cell-based FACS assay, is a highly specific diagnostic biomarker of NMOSD and did not yield false positive results in this study despite a large testing volume.