B-367 Correlation of kappa and lambda free light chain results between Siemens N Latex assays on BN II analyzer and Binding Site Freelite assays on Optilite platform

Background Free Kappa, free lambda light chain levels and their calculated ratio are incorporated into diagnostic and monitoring criteria for multiple myeloma and related lymphoproliferative disorders. The cut-offs in the International Myeloma Working Group consensus are based on the Binding Site Freelite assays. It is imperative for a healthcare facility that implemented a different assay to establish site-specific cut-offs by a comparison study with the local patient population. Methods Correlation data of free kappa, free lambda light chain measurements were collected by parallel testing between Siemens N Latex assays on BN II analyzer and Binding Site Freelite assays on Optilite platform from 12/27/2023 to 2/29/2024. Kappa to lambda ratios were calculated from results of the two methods, respectively. 2094 data pairs were obtained for free kappa, free lambda light chains and their ratio. 1772 pairs from discrete patients remained after duplicate removal. The data ranges based on the Binding Site assays were <0.29 to 1820.76 mg/dL, <0.52 to 1136.49 mg/dL and 0.00 to 2193.69 for free kappa, free lambda light chains and the ratio, respectively. Linear regression was performed by correlating the Siemens with the Binging site results in Excel. Two outlier identification mechanisms were employed: visual inspection of high values and the interquartile range method. Results Linear relationships were established with numerical results for free kappa light chains <300 mg/dL, free lambda light chains <100 mg/dL and the ratio <100, after removing 6, 5 and 10 high outliers by empirical visual inspection, respectively. The equations were derived as Siemens = 0.65 × Binding Site + 0.44 with a correlation coefficient of 0.93 for free kappa light chains (N = 1719), Siemens = 0.94 × Binding Site + 0.59 with a correlation coefficient of 0.75 for free lambda light chains (N = 1694), and Siemens = 0.49 × Binding Site + 0.34 with a correlation coefficient of 0.86 for the kappa to lambda ratio (N = 1669). The kappa and ratio equations were further verified at lower ranges by interquartile range exclusion, as Siemens = 0.71 × Binding Site + 0.13 with a correlation coefficient of 0.92 for kappa <= 10.30 mg/dL (N = 1531) and as Siemens = 0.50 × Binding Site + 0.28 with a correlation coefficient of 0.71 for kappa to lambda ratio from 0.12 to 2.88 (N = 1463). P-values for the above slopes and intercepts are all <0.001. The lambda equation was not verified at a lower range of lambda < 5.53 mg/dL statistically (p = 0.869 for intercept, N = 1522), which reflects high variation of the free lambda light chain measurements. Conclusion Based on the locally derived equations above, the cut-off for free kappa to free lambda light chain ratio of 100 resulted from the Binding Site method correlates with 50 from the Siemens method, and the involved free light chain level at 10 mg/dL corresponds to 7 mg/dL for free kappa light chains and 10 mg/dL for free lambda light chains.