BACKGROUND AND OBJECTIVES: The differential diagnosis between aquaporin-4-immunoglobulin G-positive neuromyelitis optica spectrum disorder (AQP4-NMOSD), myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease (MOGAD), and multiple sclerosis (MS) can be complex. Kappa free light chain index (KFLC-Index) emerged as an effective biomarker for distinguishing patients with MS from patients with other conditions. The main aim of this study was to assess the diagnostic performance of KFLC...