KFLC-index distinguishes multiple sclerosis from anti-myelin oligodendrocyte glycoprotein and aquaporin 4 diseases in a Chinese cohort

Background Kappa free light chain (KFLC) index has emerged as a diagnostic biomarker for multiple sclerosis (MS). This study aims to evaluate the diagnostic accuracy of the KFLC-index in Chinese patients with MS, and its capacity to discriminate MS from myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) and neuromyelitis optica spectrum disorders with aquaporin-4 antibody (AQP4+NMOSD). Methods 428 patients tested for KFLC-index were enrolled in the study, including 130 patients with MS, 41 with MOGAD, 25 with AQP4+NMOSD, 123 with other inflammatory or infectious neurological disorders (OIND) and 109 with non-inflammatory neurological disorders (NIND). Their oligoclonal band (OCB) results and clinical data were reviewed. Results KFLC-index was significantly higher in MS (20.1 (0.9–388.9)) compared with MOGAD (4.8 (0.8–56.1), p=0.003), AQP4+NMOSD (4.5 (1.5–46.4), p=0.011), OIND (2.9 (0.6–238.7), p<0.001) and NIND (1.8 (0.6–110.7), p<0.001). The optimal cut-off value for the KFLC-index to identify MS from the non-selective controls was 8.3, with an accuracy comparable to that of OCB (area under the curve 0.84 vs 0.81, p=0.249). The optimal cut-off values for differentiating MS from MOGAD and AQP4+NMOSD were 18.5 and 12.1, with performance similar to OCB (p=0.756 and 0.064). Combination of KFLC-index and OCB outperformed OCB alone in differentiating MS from non-selective controls and MOGAD (p<0.001 and p=0.044). Female (p=0.009) and higher cerebrospinal fluid leucocyte count (p<0.001) were associated with higher KFLC-index in MS. Conclusion KFLC-index is a valuable diagnostic tool for differentiating MS from other inflammatory demyelinating diseases.