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 κ and λ 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 κ and λ 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 κ FLC, λ FLC, and κ/λ ratio. Results: FLC in a serum matrix demonstrated underrecovery relative to samples diluted in saline for both κ and λ FLC. Of 255 patient samples with λ FLC measured undiluted (λ 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 λ FLC between 2.0 and 6.0 mg/L demonstrated dramatic changes in κ/λ ratio, with no changes in κ FLC, likely because of the matrix effect. Conclusions: The κ and λ Freelite assays exhibit a matrix effect when samples are tested undiluted, which has the potential to affect the κ/λ ratio. Consequently, our laboratory has stopped reporting λ FLC <6.0 mg/L.