Evaluation of Serum Kappa Free Light Chain Reference Interval Using a New Stabilized Calibrator

Measurement of serum free light chains (FLC) including kappa FLC, lambda FLC, and calculated kappa to lambda FLC ratio play a vital role in the diagnosis, prognosis, and monitoring of plasma cell disorders. Recent concerns regarding upward drift in The Binding Site's kappa FLC measurements have prompted a reformulation of the kappa FLC calibrator by the manufacturer. This study aims to assess the impact of the reformulated/new kappa FLC calibrator on both the kappa FLC reference interval (RI) and the FLC ratio diagnostic range.

Healthy volunteers (n = 124) from 3 tertiary care medical centers had FLCs measured using both the new and the prior lot of kappa FLC calibrators on an Optilite analyzer (The Binding Site). All samples were screened for monoclonal proteins using capillary electrophoresis. Creatinine was measured to confirm renal function.

Comparison of kappa FLC values from the new vs prior calibrator demonstrated minimal differences. Neither calibrator was able to verify manufacturer-suggested RI in our study population. The proportion of samples that had FLC ratios within the manufacturer's claimed diagnostic range was equivalent between calibrators, with roughly 6% to 7% of study participants falling above the diagnostic range of 0.26 to 1.65.

The new reformulated kappa FLC calibrator from The Binding Site does not support verification of the manufacturer's claimed kappa FLC RI or improve the number of healthy patients falling within the FLC ratio diagnostic range. Both the kappa FLC RI and diagnostic range for FLC ratio should be reevaluated and updated in consensus guidelines.