Serum j and k free light chain levels are markers of plasma cell proliferation, and their measurements have been included in recent guidelines by the International Myeloma Working Group for the management of patients with plasma cellular dyscrasias. Five in vitro diagnostic methods for the immunochemical quantification of serum free light chains (FLC) are available, three based on polyclonal antibodies (Freelite V R , The Binding Site; FLC ELISA j and k, Sebia; human j and k FLC, Diazyme Laboratories) and two on monoclonal antibodies (N Latex FLC, Siemens Healthineers; Seralite V R , Sebia). Several studies have shown that these methods cannot be used interchangeably for the follow-up of patients because measured j and k FLC concentrations may differ significantly, especially at high levels. Because no international reference material for the measurement of FLC is available, it is not possible to establish which method is the most accurate. For this reason, knowledge about the analytical and diagnostic performances of the assays used is important. The aim of this review is to describe the main analytical features of the j and k FLC assays and how they may influence the clinical use of these parameters.

Background The automated immunochemical serum free light chains (FLC) assays, Freelite (a polyclonal antiserum) and N Latex FLC (a mixture of monoclonal antibodies), are not interchangeable, as they may provide different results on a same sample. This study was aimed to establish if the calibrators contain FLC oligomers, and if different reactivity against monomers and dimers contributes to the discrepancy. Methods Gel filtration chromatography fractions of the calibrators were subjected to a Western blot (WB) and analyzed by each reagent. The procedure was repeated after pretreating the N Latex FLC calibrator with the reducing agent dithiothreitol (DTT). Results Both calibrators contain FLC dimers and monomers. Both reagents detect (with different sensitivity) FLC kappa monomers and dimers; instead, Freelite detects only FLC lambda dimers, while N Latex FLC detects only FLC monomers. After DTT treatment, only the N Latex lambda still detects FLC with reduced protein thiols, while the reactivity of all other reagents is abolished. Conclusions Due to their different reactivity against FLC monomers and oligomers, the Freelite and N Latex FLC are calibrated against different components of their own calibrators, making the two reagents not equivalent. The redox status of FLC determines the immunoreactivity not only of FLC dimers, but also of the monomers.

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