Clinical comparison of new monoclonal antibody-based nephelometric assays for free light chain kappa and lambda to polyclonal antibody-based assays and immunofixation electrophoresis.

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 immunofixation electrophoresis (IF).

Results: Comparison of the different FLC kappa (κ) assays showed a slope of 0.99 with a deviation of 5.0%, rs=0.92, for FLC lambda (λ) a slope of 1.22, deviation 13.8%, rs=0.90 and for the κ/λ ratio a slope of 0.72, deviation -4.6%, rs=0.72. The concordance for the FLC κ assays was 91%, for FLC λ 85% and κ/λ ratio 95%. The clinical sensitivity and specificity 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 specificity of the new N Latex FLC assays and Freelite assays appeared comparable, but there are some differences in measurement of concentrations between the methods.