Evaluation of the N Latex FLC free light chain assay on the Siemens BN analyser: precision, agreement, linearity and variation between reagent lots

Background: The clinical utility of serum kappa and lambda free light chains (FLC) for the diagnosis and prognosis of plasma cell proliferative disorders is well established. We assessed the analytical performance of the N Latex FLC assays and compared it with the Freelite (TM) assays. Methods: Analytical precision was assessed according to the CLSI EP5-A2 protocol. Method comparison was performed with 116 clinical samples and linearity was assessed on samples with monoclonal and polyclonal elevations of FLC. Analytical bias and variance was measured with two lots of N Latex FLC reagent. Results: The N Latex FLC kappa and lambda assays had a total coefficient of variation of 5-7% across the analytical range. The slopes were 1.36 and 1.37 and the between-method variances were 40.0% and 45.4%, respectively, compared with the Freelite (TM). Good agreement in classification was observed for kappa, lambda and the kappa/lambda ratio (Cohen's kappa 0.84, 0.76 and 0.89). Statistical non-linearity occurred commonly, but clinically significant non-linearity was observed in only one instance. Between-reagent lot variation was 7.3% and 10% for kappa and lambda, respectively. Conclusions: The N Latex FLC assay has good precision, did not exhibit gross antigen excess and can be used in clinical practice based on the analytical performance characteristics.