Use of Fluid Biomarkers in NMOSD and MOGAD: Clinical and Research Applications.

Neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) are inflammatory disorders of the CNS with distinct immunopathologic mechanisms and treatment responses and partially overlapping clinical phenotypes. The identification of aquaporin-4 (AQP4)-IgG and MOG-IgG has transformed disease classification and diagnosis, enabled a classification of antibody-defined subgroups, and facilitated the development of targeted therapies. However, optimal use of these biomarkers in clinical practice requires careful interpretation within the appropriate clinical and radiologic context. This review synthesizes current evidence on established and emerging fluid biomarkers in NMOSD and MOGAD, with emphasis on analytical performance, biological relevance, and clinical utility. We review antibody detection using cell-based assays, highlighting differences between live and fixed platforms and the impact of antigen conformation on sensitivity and specificity, particularly for MOG-IgG. Common causes of false-positive and false-negative results are discussed, including low-titer reactivity, testing in low pretest probability populations, treatment-related antibody titer reduction, and assay-specific limitations. The diagnostic challenges posed by indiscriminate testing in adult cohorts with multiple sclerosis, in whom disease prevalence markedly exceeds that of MOGAD, are emphasized. We also discuss the role of repeat testing during acute attacks and paired serum-CSF analysis in improving diagnostic confidence when results are equivocal or discordant. Beyond disease-defining antibodies, we examine biomarkers of tissue injury and immune activation. Serum and CSF neurofilament light chain and glial fibrillary acidic protein provide complementary measures of neuroaxonal and astrocytic damage and show associations with attack severity, disease activity, relapse risk, and long-term disability. Cytokines, chemokines, and complement components reflect inflammatory pathways, including IL-6-driven immune activation in NMOSD and MOGAD and complement-mediated astrocytopathy in NMOSD, and may support mechanistic stratification and treatment monitoring in both conditions. We further review the contribution of CSF analysis, neuropathology, genetics, and antigen discovery platforms to refine disease classification, particularly in seronegative or atypical presentations. Finally, we outline priorities for future research, including assay harmonization, standardized sampling protocols, longitudinal biomarker profiling, and integrative multiomic approaches. Collectively, advances in biomarker science have the potential to improve diagnostic precision, guide individualized therapeutic strategies, and support de-escalation decisions in NMOSD and MOGAD.