Overestimation of Serum κ Free Light Chain Concentration by Immunonephelometry

To the Editor: We were interested to read the recent letter from de Kat Angelino et al. (1) and agree with many but not all of their conclusions. We are, of course, aware that our assays report higher concentrations of free light chains (FLCs) than those measured by gel techniques, and these observations have been published on several occasions [e.g., (2–4)]. Likewise, our own studies have indicated that this observation is more prevalent with monoclonal κ and is probably due to the presence of high molecular weight polymers. Fractionation and analysis of sera from 2 patients with “nonsecretory” myeloma (3) suggested that the presence of variably sized polymers prevented the formation of a single clear band in a gel—leading to underestimation of the …