Kappa free light chain index in multiple sclerosis and other inflammatory CNS diseases: a pilot single-center study from Argentina

BackgroundIntrathecal immunoglobulin synthesis is a hallmark of multiple sclerosis (MS) and can be assessed using the kappa free light chain (κ-FLC) index. Although widely adopted, the optimal diagnostic cut-off of the κ-FLC index may vary according to the clinical context and comparison group.ObjectiveTo evaluate the diagnostic performance of the κ-FLC index for MS in a real-world series including inflammatory and non-inflammatory neurological disorders.MethodsWe conducted a retrospective cross-sectional study including consecutive patients aged ≥16 years undergoing diagnostic lumbar puncture at a tertiary public hospital in Argentina.

Patients were classified as MS/clinically isolated syndrome (MS/CIS), other inflammatory neurological diseases (OIND), or non-inflammatory neurological diseases (NIND). κ-FLC, IgG, and albumin were measured in paired cerebrospinal fluid and serum samples, and κ-FLC index was calculated using quotient-based normalization.

In addition, intrathecal κ-FLC synthesis was assessed using the hyperbolic Reiber diagram (Reiber’s diagram). Diagnostic performance was assessed using receiver operating characteristic (ROC) curves, with cut-off values derived from Youden’s index and compared with previously proposed thresholds.ResultsA total of 124 patients were included (49 MS/CIS, 48 OIND, 27 NIND).