B-094 Evaluating the Impact of Stabilized Calibrator on Free Light Chains Reference Intervals

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 < 0.05. Clinical significance was assessed per CLSI EP28-A3 guidelines. A RI was considered for partitioning by sex if the mean difference between males and females exceeded 25% of the RI width or if >4% of either sex had values beyond RI limits. Similar tests were applied to age groups (<60 and =60 years). Results 465 donors (48.2% male, 51.8% female) aged 30-89 years were used. Sixteen donors with incidental monoclonal proteins identified by MASS-FIX were excluded, for a final n=449. No clinically significant relationships of results were found with sex or age. The updated kappa calibrator resulted in a measurable shift down in FLC values with the new lot of reagents. The kappa FLC concentration and corresponding FLC ratio decreased by a median -12.1%, see table 1 for absolute values When reviewing Optilite results of the new lot, 98/449 samples were outside the kappa concentration original RI (21%). Lambda concentration RI was shifted up on Optilite when compared to original RI but passes verification (1.6% of samples, acceptance criteria < or =10%). 19 samples (4.2%) were outside the original RI for the FLC ratio of 0.26 to 1.65 (all >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.