Kappa free light chains: Diagnostic performance in multiple sclerosis and utility in a clinical laboratory

Background: Automated, technically simple analytical methods offering objective results are highly valued in clinical laboratories. Kappa free light chains (KFLC) in cerebrospinal fluid (CSF) are promising multiple sclerosis (MS) biomarkers, particularly kappa (K) index. Methods: KFLC were determined in CSF and serum samples of patients diagnosed with MS, clinically/radiologically isolated syndrome (N, 39), and controls (N, 152; inflammatory and non-inflammatory neurological disorders). Diagnostic performance of several KFLC parameters, previously determined oligoclonal band (OCB) testing, and IgG index, was assessed. A K index decision threshold for sample screening was identified and reduction in performed OCB analyses estimated accordingly. Results: Higher KFLC parameters were detected in the MS group and K index performed best among them (AUC 0.92). At a 7.25 cut-off it showed better sensitivity (85% vs. 77%) though less specificity (88% vs. 91%) than OCBs. Comparatively, IgG index's performance was inferior (AUC 0.83). A decision K index threshold of 2.55 (97% sensitivity) would reduce OCB testing by 52% in the studied population. Conclusions: The proposed 7.25 cut-off could assist MS diagnostics and identify some false negative cases from OCB studies. Sequential algorithms using K index for the decision to perform OCB detection would improve laboratory efficiency and substantially reduce costs.