Abstract
Dear Editor,
We read with interest the article by Pretorius et al. 1 comparing the new N Latex FLC (Siemens) and Freelite assays (The Binding Site) for the measurement of serum free light chains (FLC). The authors carefully pointed out the deficiencies in Freelite, but were less critical of N Latex.
International guidelines recommend the use of serum FLC analysis for monitoring monoclonal gammopathies (MG) and, alongside serum protein electrophoresis, for the detection of MG. 2 This was based upon retrospective analysis of 428 MG samples with Freelite 3 and subsequently validated in prospective studies. 4 Although Pretorius et al. reported significant quantitative differences between the two assays and reported clinical information on only 16 samples, they conclude that the N Latex assays can be used in clinical practice on the basis of the assay's analytical performance; a safer conclusion would be that separate clinical validation studies are required.
The study design concerns us. The expression of serum free light chains is highly variable in MG. In diseases such as AL amyloidosis, serum concentrations may be close to normal. 5 Why then have the authors excluded 58% of their samples (those <50 mg/L by Freelite), risking biasing their analysis?
Clinical data were presented only for samples giving grossly discrepant results, mainly limited to the initial diagnoses. Importantly, the N Latex assays gave normal FLC ratios for three patients with known B-lymphoproliferative disorders. In this highly selected population ∼3% of the samples would not have been identified by the N Latex.
Antigen excess is a common problem in nephelometric assays. The incidence of antigen excess for Freelite is well documented 6 while data for N Latex, which has antigen excess protection, are sparse. Pretorius et al. evaluated the antigen excess performance of the N Latex assay in samples where excess had been identified by Freelite, leading inherently to a bias when comparing antigen excess susceptibility of the two assays. We recommend the authors repeat their analysis on all samples identified as discordant between the two assays. The experiment comparing the precision of the two assays does not provide a fair comparison. The FLC value of the five serum pools used was determined by the N Latex assay. There was a large discrepancy between the FLC levels measured by the N Latex assays and Freelite with the latter giving 25–40% lower values for kappa and lambda. In consequence, for the lower level samples the precision for Freelite was tested at the lowest point of the dynamic ranges, while with the N Latex assays, precision was tested at least 50% above the bottom of the dynamic range.
Nevertheless, we agree with the authors that non-linearity of FLC assays is a property of the individual sample 7 and not method specific. Similarly, aggregation will cause an over estimation of the light chain concentration in both assays.
We question the logic for suggesting a change in the reference range of the N Latex assay. The N Latex FLC was calibrated against Freelite; 8 therefore, a recalibration of the N latex assay based on the differences in monoclonal samples cannot be justified. Furthermore, changing the reference range for serum FLC would prevent the use of Freelite as a predicate device.
In summary, the data presented highlight that the two assays are different and does not provide support for the clinical utility of N Latex FLC. Prospective, clinically relevant studies are required to assess the utility of the N Latex assay and until these are reported its use should be limited to academic comparison studies.