Performance of The Binding Site Cerebrospinal Fluid Kappa Free Light Chains assay

Background Measurement of cerebrospinal fluid (CSF) kappa free light chains (KFLCs) for the detection of intrathecal immunoglobulin synthesis has generated interest as an alternative to CSF oligoclonal bands due to its rapid turnaround time and ability to automate the assay. A turbidimetric immunoassay - Freelite Mx, is available from The Binding Site, but published data on performance is scant. Therefore, we undertook a multi-centre sample comparison and investigated reagent lot-to-lot-variation (LTLV). Methods Intra-/inter-assay accuracy and imprecision of the Freelite Mx assay on The Binding Site Optilite analyser was assessed. Twenty paired CSF/serum samples were sent to 3 laboratories within Australia for the measurement of CSF/serum KFLC/albumin and concentrations compared using the Kruskal-Wallis test, Spearman's rank (rs), and Passing-Bablok analysis. Lot-to-lot-variation between 3 reagent lots was undertaken by analysis of 20 CSF samples. Results Intra- and inter-assay imprecision was ≤4.4 and ≤4.1%, respectively. There was a good correlation (rs = ≥0.98) between sites for the measured CSF KFLC concentration and no significant difference in the median concentration measured between sites (3.31, 2.78, and 3.48 mg/L, p = 0.98). The median bias between reagent lots was <4%, the intercept of the regression between lots was between -0.02 and 0.06 mg/L, and the slope ranged from 0.96 to 1.07 Conclusion Overall, there was a good agreement in CSF KFLC concentrations among laboratories, and LTLV was deemed acceptable. Ascertaining biological variability of CSF KFLCs and the participation of laboratories in quality assurance schemes would assist with harmonisation.