OBJECTIVES: Kappa free light chains (κ-FLC) have emerged as a reliable biomarker for diagnosing multiple sclerosis (MS). Compared to oligoclonal band (OCB) measurement, κ-FLC presents distinct advantages, including enhanced accessibility in clinical practice. This study evaluates κ-FLC index values in MS patients and explores its potential as a practical alternative to the OCB test. METHODS: Cerebrospinal fluid (CSF) and serum κ-FLC concentrations were quantified using an immunonephelometry analyzer, while OCB analysis was performed via agarose isoelectric focusing combined with immunoblotting. The cut-off values were set at ≥0.7 for the CSF IgG index and ≥6.6 for the κ-FLC index, with values exceeding these thresholds considered positive. κ -FLC index values were compared between OCB-negative and OCB-positive patients, between patients with negative and positive CSF IgG index, and across different OCB types. RESULTS: OCB positivity was detected in 82.7 % of patients, whereas a positive κ-FLC index was observed in 91.7 %. The IgG index was positive in 51.9 % and negative in 48.1 %. Among patients with a negative IgG index, 84.3 % exhibited a positive κ-FLC index. Additionally, CSF κ-FLC values were significantly higher in patients with a positive IgG index compared to those with a negative IgG index. On the other hand, patients with type 2 (+) OCBs had higher κ-FLC index values than those with negative OCBs. CONCLUSIONS: The κ-FLC index may serve as a valuable tool for identifying OCB-negative patients with a high likelihood of MS, offering a practical and accessible alternative for diagnostic evaluation.