Comparison of Free Light Chain Assays:  Freelite and N Latex in Diagnosis, Monitoring, and Predicting Survival in Light Chain Amyloidosis.

Objectives: Measurement of serum free light chains (FLCs) is critical in diagnosis, prognosis, and monitoring treatment responses in light chain (AL) amyloidosis. We compare the Freelite assay (polyclonal antibodies to hidden light chain epitopes), which is the current gold standard, with a new assay: a mixture of monoclonal antibodies to light chain epitopes (N Latex). Methods: We collected 240 serum samples from 94 consecutive patients with newly diagnosed AL amyloidosis (at least three serial serum samples during the first 6 months) analyzed at the National Amyloidosis Centre, London, from January 2011 to April 2012. Concordance in detecting abnormal light chain components and hematologic response was assessed at 2, 4, and 6 months. Results: The kappa and lambda clonal light chain involvement was 21% and 79%, respectively, with an abnormal kappa/lambda ratio or detectable protein in 78.7%. Median kappa, lambda, and difference in involved and uninvolved FLCs by Freelite and N Latex assays were 17.3 vs 16 mg/L (R-2 = 0.91), 48.8 vs 52.6 mg/L (R-2 = 0.52), and 43.2 vs 39.1 mg/L, respectively. Discordant kappa/lambda ratios at presentation were as follows: 10 of 90 abnormal by Freelite/normal by N Latex and 11 of 90 abnormal by N Latex/normal by Freelite. Conclusions: Both FLC assays show good correlation in detecting the abnormal light chain subtype with discordance in absolute values and thus are not interchangeable.