Commutability Assessment of Materials for Serum Copper Measurement Using the IFCC Difference-in-Bias and the CLSI EP30 Approaches

Background: Evidence on the commutability of material for serum copper when assessing multiple material categories and assay pairs remains limited. Methods: We evaluated 21 reference materials for serum copper across four routine colorimetric systems and an inductively coupled plasma mass spectrometry (ICP-MS)-based candidate reference method (System G). The materials included pooled frozen human serum, lyophilized external quality assessment (EQA) materials, aqueous materials, commercial control sera, a certified animal-serum reference material, and manufacturers' calibrators. Commutability was assessed pairwise using the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) difference-in-bias and the CLSI EP30 prediction-interval approaches. The homogeneity and stability of pooled frozen human-serum materials (RM1-RM3) were also evaluated. Results: Correlations were high across assay pairs, but proportional differences were generally larger in comparisons involving ICP-MS. With the IFCC approach, commutability varied by material type and assay pair; pooled frozen human serum and several serum-based EQA materials showed favorable performance, whereas aqueous materials and some calibrator-related materials more frequently showed assay pair-dependent non-commutability. The CLSI EP30 approach yielded more non-commutable classifications for pairs with ICP-MS than IFCC, but not uniformly across comparisons. Evaluated materials showed better commutability with routine-routine pairs than with pairs including ICP-MS. RM1-RM3 showed low between-vial variation, and bias after storage was lower at -20°C than at 4°C or 20-25°C. Conclusions: Material characteristics, assay combination, and assessment framework influenced serum copper commutability. Pooled frozen human-serum materials showed favorable commutability, particularly for routine-routine pairs, supporting the use of native serum-like materials in serum copper-commutability studies.