Method comparison of four clinically available assays for serum free light chain analysis

Background: Serum free light chain (sFLC) measurements are increasingly important in the context of screening for monoclonal gammopathies, prognostic stratification and monitoring of therapy responses. In this study we have performed a method comparison of four sFLC assays that are currently available for routine clinical use. Methods: In a retrospective study, sFLC analyses were performed on a cohort that included 139 patients with various monoclonal gammopathies and 54 control sera without an M-protein. Method comparisons of the following four FLC assays were performed: Freelite (Binding Site), N-Latex FLC (Siemens), Seralite (Abingdon Health) and Sebia FLC (Sebia). Results: Bland-Altman agreement analysis showed biases varying between -0.1 and 16.2 mg/L for kappa FLC, -6.0 and 6.8 mg/L for lambda FLC and -0.04 and 0.38 for the ratio of the involved to uninvolved FLC. Strong agreements were observed for FLC-concentrations below 100 mg/L. The clinical concordance of the kappa/lambda FLC-ratio of the four methods varied between 86% and 92%. Significant quantitative differences were observed between the different methods, mainly in sera with high FLC concentrations. Most assays consistently overestimated FLC concentrations compared to SPE. Conclusions: Good overall clinical concordances were observed between the four sFLC assays that were compared in this study. Although good agreements were observed between the FLC assays, significant absolute differences in FLC concentrations in individual patients can be seen, particularly at higher FLC concentrations. Because of inequivalent absolute sFLC values between the methods in individual patients, none of the four sFLC assays can be used interchangeably.