Background: Commutability is essential to harmonize different measuring systems and to evaluate the performance of participating laboratories in external quality assessment (EQA). This study aims to assess the commutability of EQA materials in the tumor marker program of the Korean Association of External Quality Assessment Service.
Methods: We analyzed commercial quality control (QC) materials, individual patient samples, and frozen human serum pools (FSPs) based on the Clinical and Laboratory Standards Institute guidelines. Alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA), and total prostate-specific antigen (PSA) were assayed in triplicate in all samples using four analytical systems at the three laboratories. The results obtained from pairs of assays were plotted and assessed using Deming regression analysis. The criterion for commutability was a 95% prediction interval, and bias for noncommutability was calculated.
Results: FSPs were commutable in all AFP, CEA, and total PSA assay methods. Bias for AFP, CEA, and total PSA ranged from –203% to 27%, –67% to 45%, and –9% to 12%, respectively. Commercial QC materials for AFP and PSA were commutable in four assays, whereas for CEA, noncommutability was observed.
Conclusions: Our results validated that the frozen serum pools were commutable across different platforms for tumor marker assays. Therefore, validation findings from materials like FSPs and information about their commutability needs to be reported, for the applicability of EQA programs.

Keywords: Commutability, External quality assessment, Frozen human serum pools, Immunoassay, Tumor biomarkers