Blood-Based Biomarkers for Identifying Disease Activity in AQP4-IgG–Positive Neuromyelitis Optica Spectrum Disorder

Importance: The temporal dynamics of serum glial fibrillary acidic protein (sGFAP) and serum neurofilament light chain (sNfL) as biomarkers of disease activity for neuromyelitis optica spectrum disorder (NMOSD) remain underexplored. Objective: To determine optimal timing for assessing sGFAP and sNfL, establish cutoff values differentiating between attacks and remissions in NMOSD, and evaluate these findings across independent cohorts. Design, Setting, and Participants: This retrospective, longitudinal, multicenter cohort study was conducted among patients with aquaporin-4 antibody (AQP4-IgG)-positive NMOSD. Patients with available stored serum samples were included, totaling 181 patients with 625 samples. Discovery cohort samples were collected from February 2008 to October 2023 and validation cohort samples were collected from January 2013 to October 2023. A combined analysis of both cohorts was conducted from November 2023 to March 2024. Exposures: sNfL and sGFAP concentrations, measured by a single-molecule array assay. Main Outcomes and Measures: The primary outcomes were the optimal timing of assessing sGFAP and sNfL and the adjusted cutoff values for evaluating disease activity in NMOSD. Results: The discovery cohort consisted of 366 samples from 78 Korean patients (median [IQR] age, 35 [30-42] years; 73 female patients [95%]), while the validation cohort included 190 samples from 34 German patients (median [IQR] age, 54 [39-61] years; 32 female patients [94%]) and 69 samples from 69 Brazilian patients (median [IQR] age, 46 [35-55] years; 62 female patients [90%]). Six-month postattack temporal biomarker dynamics were analyzed in 202 samples from 74 patients in the discovery cohort: sGFAP levels peaked within the first week and sNfL levels peaked at 5 weeks postattack. The optimal time frames for evaluating attacks were within 1 week for sGFAP and from 1 to 8 weeks for sNfL, with remission defined as at least 6 months postattack. z Score cutoffs of 3.0 for sGFAP and 2.1 for sNfL effectively distinguished between attack and remission phases, indicated by area under the curve values of 0.95 (95% CI, 0.88-1.02) and 0.87 (95% CI, 0.82-0.91), respectively. The discovery cohort time frames and cutoff values were applied to the validation cohort, achieving 71% sensitivity and 94% specificity for sNfL and 100% sensitivity and specificity for sGFAP in the German and Brazilian cohorts. Conclusions and Relevance: This longitudinal cohort study established optimal timing and thresholds for sGFAP and sNfL, which were consistent in independent cohorts, supporting these biomarkers' effectiveness in distinguishing NMOSD attacks from remission.