Evaluation of free light chain measurement in EDTA and Lithium heparin plasma using the Diazyme assay

INTRODUCTION: Serum free light chain (FLC) measurement is widely used in the diagnosis and monitoring of monoclonal gammopathies. This study aimed to evaluate the analytical comparability of EDTA and lithium heparin plasma with serum for kappa and lambda FLC measurements using the Diazyme™ assay on the AU5800 analyzer.
METHODS: Twenty paired patient samples submitted for routine FLC testing were included. For each case, serum, EDTA-anticoagulated plasma, and lithium heparin plasma (Barricor tube) specimens were collected during the same phlebotomy session and analyzed in parallel. Paired comparisons were performed using the Wilcoxon signed-rank test. Agreement among serum, EDTA plasma, and lithium heparin plasma samples was assessed using Passing–Bablok regression analysis and Bland–Altman plots.
RESULTS: Kappa FLC concentrations measured in EDTA plasma did not differ from serum values (p>0.05). Lambda FLC concentrations in EDTA plasma showed a small but statistically significant difference compared with serum (p=0.037), while the kappa/lambda ratio was not significantly affected (p>0.05). In contrast, lithium heparin plasma demonstrated statistically significant differences from serum for kappa FLC (p=0.044), lambda FLC (p<0.0001), and the kappa/lambda ratio (p=0.0004). Lithium heparin plasma demonstrated broader limits of agreement and a substantially larger positive bias, particularly for lambda FLC.
DISCUSSION AND CONCLUSION: Compared with lithium heparin plasma, EDTA plasma demonstrated closer agreement with serum for kappa FLC, lambda FLC, and the kappa/lambda ratio. These findings suggest that EDTA plasma may represent a more suitable alternative specimen matrix for FLC measurement using the Diazyme assay on the AU5800 platform, although confirmation in larger studies is warranted.

Keywords: Free light chain, kappa, lambda, plasma, serum