Monoclonal vs Polyclonal Free Light Chain Assays

We do not agree with many comments made by Davern and colleagues1 regarding the performance comparisons of their monoclonal serum free light chain (FLC) assay with the polyclonal Freelite assay (The Binding Site, Birmingham, England) and welcome the opportunity to respond. The authors stated that the Freelite assay missed some λ molecule subgroups. During the 8-year commercial availability of Freelite, there have been millions of these tests used in patients and hundreds of peer-reviewed publications from independent groups. During this period, many studies have found 100% diagnostic sensitivity for Freelite in patients with both κ and λ light chain only multiple myeloma.2 In contrast, only 1 study has shown that Freelite failed to recognize monoclonal FLCs, and that was in a single patient.3 The urine monoclonal light chain (Bence Jones protein [BJP]) from the patient was subsequently included in the immunization protocols 3 years ago, and it was recognized by subsequent antiserum batches. Freelite specificity is, therefore, validated against thousands of cases vs the 71 reported by Davern et al.1 Indeed, the higher specificity of the Freelite assay is actually demonstrated in their own results Figure 1 . Figure 1 demonstrates that there is a nonlinear relationship between the results using the different assays, indicating these are not …