Background: High serum concentrations of monoclonal free light chain (FLC) kappa or lambda are markers of plasma cell dyscrasia. Methods: We developed new, latex-enhanced, specific nephelometric assays based on monoclonal antibodies for the determination of FLC kappa and lambda in serum, EDTA plasma and Li-heparin plasma for use on the Siemens BN systems. Results: Reference ranges were determined from 369 samples: FLC kappa 6.7-22.4 mg/L, FLC lambda 8.3-27.0 mg/L and kappa/lambda ratio 0.31-1.56. Protection from falsely low results due to antigen excess is obtained with a built-in pre-reaction in the assay protocols. Lot-to-lot consistency between three different lots of reagent, calibrators and supplementary reagent lots showed normalized differences -7.5%. The reproducibility of serum samples varied between 4% and 7%. The method comparison with Freelite assays showed normalized differences of 19.7%, 32.7% and 21.7%, respectively, for FLC kappa, lambda and ratio, correlations of 0.94, 0.77 and 0.73, and concordance rates of 99.2%, 94.2% and 95%. Conclusions: N Latex FLC demonstrates high precision, good lot-to-lot consistency and freedom from a high-dose hook effect. The method comparison between Freelite and the N Latex FLC assays showed good clinical concordance. Further studies need to reveal the clinical value of the new FLC assays.

Background: New monoclonal antibody-based assays for serum-free light chains (FLC) have become available. Methods: In a clinical study with 541 patients, the new N Latex FLC assays were compared with the Freelite ™ FLC assays and immunofi xation electrophoresis (IF). Results:Comparison of the different FLC kappa ( κ ) assays showed a slope of 0.99 with a deviation of 5.0 % , r s = 0.92, for FLC lambda ( λ ) a slope of 1.22, deviation 13.8 % , r s = 0.90 and for the κ / λ ratio a slope of 0.72, deviation -4.6 % , r s = 0.72. The concordance for the FLC κ assays was 91 % , for FLC λ 85 % and κ / λ ratio 95 % . The clinical sensitivity and specifi city of the κ / λ ratios in the study were comparable: 60 % and 99 % for the N Latex FLC assay and 61 % and 97 % for the Freelite ™ assay. In IF-FLC positive samples, the N Latex FLC κ / λ ratio scored 20/23 (87 % ) samples outside the reference range and Freelite ™ 21/23 (91 % ). For IF-FLC negative samples, N Latex FLC assay κ / λ ratio scored 338/350 (97 % ) within the reference range and Freelite ™ scored 332/350 (95 % ). Conclusions:The concordance scores and the clinical sensitivity and specifi city of the new N Latex FLC assays and Freelite ™ assays appeared comparable, but there are some differences in measurement of concentrations between the methods.