Polyclonal versus monoclonal immunoglobulin free light chains quantification

Background: The clinical usefulness of the serum-free light chain assays has expanded since their first description, and further applications other than plasma cell dyscrasia are emerging. Currently, we have the ability to perform the measurements with two certified methods: the FreeliteTM assay (The Binding Site Ltd, Birmingham, UK) and the new N Latex free-light chain assay (Siemens, Germany). In the present study, we investigated the impact of free light chain concentrations and structures on their quantification, performed with both tests. Methods: A total of 524 serum samples from 497 patients from our routine laboratory were analysed with the FreeliteTM and the N Latex free light chain assay. The results were compared in two subgroups: with or without monoclonal component. Twenty-four samples were subsequently investigated for the presence of dimeric and monomeric free light chain with sodium dodecyl sulphate polyacrylamide gel electrophoresis and densitometric quantification. Results: Methods comparison showed that the Pearson rank correlation coefficients were 0.90 for polyclonal k and 0.91 for polyclonal free light chain. Conversely for monoclonal immunoglobulins, the Pearson rank correlation coefficient was lower with 0.82 for kM >500 mg/L and 0.56 for M >500 mg/L. Furthermore, densitometric quantification of the involved monoclonal free light chains showed that both assays do not reflect the Coomassie-stained protein mass. Conclusion: Samples containing high amounts of a single pathologic free light chain may not be considered like a sample containing a sum of different polyclonal free light chains. Indeed, free light chain dimerization leads to different scatter efficiency of macromolecular complexes.