Evaluating Commutability of Control Materials in Three Nordic External Quality Assessment Schemes for Lipoproteins.

Abstract

The quality of control materials is crucial for evaluating external quality assessment (EQA) results. To detect method differences, the EQA material should behave the same as a patient sample, meaning the material must be commutable. Noncommutable materials may cause misinterpretations of EQA results. Here, we examined the commutability of EQA materials used in 3 Nordic EQA schemes for lipids. The study was designed according to the procedures recommended for assessing commutability by the International Federation of Clinical Chemistry and Laboratory Medicine. Commutability was assessed based on the difference in bias between a control material (CM) and clinical samples (CS) consisting of human plasma using 2 different measurement procedures (MPs). Measurands: LDL-cholesterol (LDL-C), total cholesterol (TC), HDL-cholesterol (HDL-C), and triglycerides (TG). Four CMs (CM1-4) were assessed for commutability by using 40 CS and 3 MPs (Abbott Architect, Roche Cobas, and Siemens Atellica). Unmodified native CMs (CM1 and CM3), stored at -80 °C, were commutable for all included measurands, except for LDL-C that was indeterminate, when comparing MPs pairwise. Modified CM2 was noncommutable for HDL-C, LDL-C, non-HDL-C, and LDL-C calculations. Unmodified native CM4, stored at -20°C, was noncommutable for LDL-C. Unmodified serum samples stored at -80 °C were commutable for lipids on the evaluated MPs, and therefore suitable as CMs in EQA schemes. Moreover, the study demonstrated that minor modifications of samples may lead to noncommutability.