Background: Measurement of serum free light chains (FLC) is useful for the diagnosis and monitoring of monoclonal light-chain diseases. It has been suggested that there will be widespread replacement of urine Bence Jones protein measurement by serum FLC assay. We report on our experience with the assay during monitoring of light-chain myeloma (LCMM) and AL amyloidosis (AL).
Methods: Serum FLC immunoassay, serum and/or urine protein electrophoresis and immunofixation were performed on serial samples during monitoring of LCMM. Recovery and immunoreactivity of FLC were tested by sample dilution. Assay precision was determined by repeat assay of samples over several reagent lots.
Results: In one of 23 patients with LCMM there was non-reaction of a monoclonal kappa FLC with some reagent lots and the assay did not indicate disease relapse. Samples showed non-linear, non-parallel immunoreactivity on dilution. Several tested monoclonal FLC gave lower values at the assay starting dilution compared with higher sample dilution and non-parallel dose-response curves. The median between-reagent lot variation for FLC measurement was 19-20% CV.
Conclusions: Laboratory staff and clinicians need to be aware of the potential for non-reactivity of individual monoclonal FLC, and the effects of dilution and precision on FLC values and their interpretation.