Background: Kappa free light chains (KFLCs) are emerging as promising biomarkers for intrathecal B cell activity for diagnosing multiple sclerosis (MS) through cerebrospinal fluid (CSF) analysis. In this study, we evaluated the ability of KFLC formulas to identify the presence of MS and their agreement with the ‘gold standard’ of CSF IgG oligoclonal bands (OCBs). Methods: A total of 233 patients were included in this study: 149, comprising 43 males and 106 females, had MS, and the remainder, 40 males and 44 females, had other neurological diseases (ONDs). We evaluated the potential of KFLCs in terms of sensitivity, specificity, and accordance. All analyses were conducted using a sex-disaggregated approach. Results: KFLCs showed a high sensitivity for both sexes with respect to the diagnosis of MS, with values between 74.42% and 93.03%. The specificity of the various formulas was much lower for females when compared to males, with values between 45.45% and 59.09%, with a significant difference between the two sexes for the K Index > 5.9 (p = 0.0451). Cohen’s kappa showed substantial agreement for men and moderate agreement for women between the KFLC indices and OCB. Conclusions: This study highlights the potential of KFLCs as a biomarker for MS but emphasises the need for sex-specific thresholds to improve diagnostic accuracy.