Comparison of polyclonal and monoclonal antibody based free light chain assays taking account of renal function

Introduction Serum free light chain (FLC) assay is an increasingly important method in diagnostic and follow-up approaches of monoclonal gammopathies. Since 2011, two immunonephelometric assays are available: the FreeliteTM assay (The Binding Site, UK) using polyclonal antibodies, and the N Latex FLC assay (Siemens, Germany) using monoclonal antibodies. While Hutchison highlighted extended reference range for the/ratio obtained with the FreeliteTM assay for patients with renal impairment (CKD-EPI 0.390). The analysis of qualitative results showed independence between proportion of discrepancies and renal function (p=0.639 in case of use of the extended reference ranges for the FreeliteTM assay), which is not improved by consideration of the technical assay variability (p=0.270). In all, the results showed that the lack of correlation and the discrepancies for 7.6% of patients are observed for extremely high and low values, but without relationship to the renal function. Conclusion Our results confirm good agreement between the 2 tests which are both reliable and robust. Renal failure has no impact on the correlation between the two tests. However we observed few discordant results which emphasize the need to analyse the results of FLC tests in conjunction with the results of traditional tests and clinical evolution.