Regarding the Overestimation of Serum κ Free Light Chains

To the Editor:I read with interest 2 recent Letters to the Editor regarding the accuracy of serum free light chain (FLC)1 measurement (1, 2). According to de Kat Angelino et al., the nephelometric serum FLC assay is inaccurate, and an ELISA method may be more accurate. Inaccuracy of the nephelometric method has been demonstrated by showing nonparallelism between individual patient samples, with the slopes for different patient samples varying between 0.54 and 0.76, compared with a slope of 1.0 for the FLC calibrator (3). We noticed that 3 of the 10 diluted serum samples showed a concentration that was increased 2- to 4-fold above that of the undiluted baseline value (4). In another study, 10 of 10 serum samples showed recoveries of 175% to 381% after multiple dilutions, compared with the lowest dilution (5). The reagent manufacturer (Binding Site) has acknowledged variation in values at different dilutions and has indicated that the concentration …