B-094 Evaluating the Impact of Stabilized Calibrator on Free Light Chains Reference Intervals
Brandon Copet, Ashley Thompson, Carin Smith, Grant Smith, Brandy L. Jaszewski, David L. Murray, Angela A. Dispenzieri, Maria Alice V. Willrich · Clinical Chemistry · 2025
Abstract Background The Freelite serum free light chain (FLC) assay has been widely used for detecting and monitoring monoclonal gammopathies. Reference intervals (RIs) established by Katzmann et al. in 2002 have served as the standard values for interpreting kappa/lambda FLC ratios. However, multiple reports suggested an assay drift and clinical laboratories started to move towards locally established RIs given the challenge in verifying original RIs. The assay drift over time increased kappa concentration measurements and was attributed to an interaction between the kappa FLC calibrator and serum proteins, including alpha-1-antitrypsin. A stabilized calibrator was developed to improve consistency of results with original RI. The objective of this study was to assess whether the new updated kappa calibrator would warrant a RI update. Methods Healthy adult donors’ serum samples were selected from the institution quality unit. Exclusion criteria included autoimmune diseases, cancers and renal disease. FLC measurements were obtained on the Optilite by turbidimetry using routine reagents for kappa (old lot); the new lot of reformulated kappa calibrator, and a single measurement for lambda FLC. FLC ratios were calculated with the old lot and new lot. All samples were tested by MASS-FIX to confirm absence of a monoclonal protein. Bootstrap resampling (10,000 replicates) estimated standard errors, with statistical significance defined as p 4% of either sex had values beyond RI limits. Similar tests were applied to age groups (1.65), which was originally established as a 100% diagnostic range. The calibrator reformulation, while decreasing test results by >10%, was not sufficient to verify the original RI for kappa and still left a few cases outside the original diagnostic range for the FLC ratio. Conclusion This study assessed the impact of the stabilized calibrator on RIs for the Optilite Freelite assay. These findings suggest revisiting existing RIs to ensure continued accuracy in clinical interpretation and should be further evaluated with additional lots of reagents for robustness.