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The K free light chains index (K-FLC index) has been proposed as an alternative test for intrathecal immunoglobulin synthesis in MS diagnosis. Aim of the study was to assess the accuracy of the K-FLC index in differentiating MS from other immune-mediated CNS disorders and NMOSD. Data were available from a cohort of 371 patients. K-FLC index was significantly higher in MS: MS mean K-FLC index 90.897 ± 134.198; NMOSD 17.992 ± 15.103; other immune-mediated CNS disorders 12.568 ± 24.440. The overall diagnostic accuracy of the K-FLC index was similar to intrathecal oligoclonal bands detection. However, as a quantitative variable, K-FLC index allowed easier discrimination of MS from other immune-mediated CNS disorders: highest K-FLC index values (> 100) were observed almost only in MS and are therefore strongly predictive of MS, in patients with the appropriate clinical presentation. Unlabelled Image • K-FLC index can diagnose MS with sensibility and specificity comparable to CSF OBs • K-FLC index is higher in MS compared to NMOSD or other immune-mediated CNS disorders. • While some overlap exists, highest K-FLC index values are characteristic only of MS • K-FLC index provides added diagnostic value in OBs negative MS patients [ABSTRACT FROM AUTHOR]