Evaluation of the N Latex free light chain assay in the diagnosis and monitoring of AL amyloidosis

We compared a novel assay for free light chain (FLC) quantitation based on monoclonal antibodies (N-Latex, Siemens, Germany) to the established polyclonal antibody-based assay (Freelite , The Binding Site, UK) in AL amyloidosis. Sixty-two diagnostic samples were analysed on a BNII nephelometer, 32 of which also had a post-treatment sample. In the diagnostic samples: for AL of κ type, the median involved FLC (iFLC) was significantly lower by the N-Latex assay (289 vs. 667 mg/L, p=0.0002) whereas for λ AL the values were similar (148 vs. 161 mg/L, p=0.84). Measurable disease, defined as a difference between involved and uninvolved FLC (dFLC) >50 mg/L was present in 82% by the N-Latex assay compared to 89% by the Freelite assay. For diagnostic sensitivity, the FLC ratio was normal in 21% (95% CI 12%–33%) and 15% (95% CI 7%–26%) of patients by the N-Latex and Freelite assays, respectively. The combination of serum and urine immunofixation electrophoresis with either FLC assay allowed identification of the amyloidogenic clone in 98% producing comparable sensitivity. For the monitoring samples the median reduction in dFLC was 68% for the N-Latex assay and 77% for the Freelite assay (p=0.04). This led to some differences in assigning response categories. Partial response as assigned by both assays predicted overall survival (N-Latex p=0.0015, Freelite p=0.022). There are differences between FLC as measured by the N-Latex and Freelite assays, but overall the two assays have similar diagnostic sensitivity. Disease response calculated by both assays predicts survival but more clinical validation is required.