Limitations of Free Light Chain Assays caused by the Matrix Effect.

Background: Serum free light chain (FLC) assays are used clinically to measure the concentration of kappa and lambda FLC in patients with suspected or diagnosed plasma cell proliferative disorders. Previous studies have demonstrated a loss of linearity in low concentration ranges of these assays. We hypothesized that this result could be caused by a matrix effect. Methods: Recovery studies were performed for kappa and lambda FLC in both serum and saline using the Freelite assay (Binding Site) on a Cobas c502 system (Roche). Samples were analyzed either at the recommended dilution or undiluted. Follow-up studies were performed in varying matrices ranging from 0% to 100% saline. Retrospective patient data were analyzed to assess the impact on reported kappa FLC, lambda FLC, and kappa/lambda ratio. Results: FLC in a serum matrix demonstrated underrecovery relative to samples diluted in saline for both kappa and lambda FLC. Of 255 patient samples with lambda FLC measured undiluted (lambda FLC <6.0 mg/L), an unexpected gap was observed in patient results between 2.0 and 6.0 mg/L. In addition, 23 patients measured serially with lambda FLC between 2.0 and 6.0 mg/L demonstrated dramatic changes in kappa/lambda ratio, with no changes in kappa FLC, likely because of the matrix effect. Conclusions: The kappa and lambda Freelite assays exhibit a matrix effect when samples are tested undiluted, which has the potential to affect the kappa/lambda ratio. Consequently, our laboratory has stopped reporting lambda FLC <6.0 mg/L.