Assessing Free Light Chain Assays: A Comparative Study of Freelite and Diazyme Assays in an Indian Setting

This study aims to compare the performance of two serum free light chain (FLC) assays, Freelite by The Binding Site and Diazyme, on the Cobas 6000 analyser in a North Indian tertiary care centre, focusing on patients with chronic kidney disease (CKD) and plasma cell dyscrasias (PCD). A total of 65 patient samples, including those with CKD, PCD, and both conditions coexisting, were analysed. The correlation between the two assays was assessed using Spearman correlation analysis, and bias was evaluated using the Bland-Altman method. Concordance rates for kappa FLC, lambda FLC, and the kappa/lambda ratio were also calculated. The study found strong correlations between the Freelite and Diazyme assays for kappa and lambda FLCs (Spearman coefficients: 0.96 and 0.98, respectively). The Bland-Altman analysis revealed minimal bias, and the concordance rates for kappa FLC, lambda FLC, and the kappa/lambda ratio were 95%, 93.4%, and 82.5%, respectively. Variable concordance rates were found in subgroups of PCD, CKD and both. Discrepancies were observed in patients with extremely high FLC levels. Freelite and Diazyme FLC assays show strong correlation and concordance in PCD and/or CKD patients. However, differences at high FLC levels highlight the need for assay-specific reporting and consistent follow-up with the same assay. This study highlights the importance of context-specific evaluation of FLC assays, particularly in resource-limited settings like India, where assay availability and compatibility with existing analysers may influence diagnostic outcomes.