As explained in detail elsewhere (7), (8) the concept of harmonization applies for “mixture” analysis. In that case, the components in the measurand are mostly not unequivocally defined, hence, SI-units for expression of measurements do not apply, nor is there a certified primary calibrator available. Instead, a conventional calibration material is used, such as the World Health Organization (WHO) international standard (IS) or international reference preparation (IRP), which define an arbitrary unit for expression of measurement results – in case of the WHO, the International Unit (IU). IVD manufacturers use that WHO IS/IRP to make their assays traceable to, as described in the ISO 17511. However, this IU-traceability does not (necessarily) warrant equivalence or interchangeability of measurement results. This is due to the fact that the WHO IS/IRP is (potentially) not commutable, i.e., does not behave like patient samples with the assays. This can be due to, among others, the fact that the WHO IS/IRP is from human cadaver organs (e.g., the pituitary, or placenta, etc.) and, therefore, comprises forms that differ from those typically present in the mixture components circulating in the blood stream. Therefore, as long as the components in the measurand are not clearly defined, this issue is difficult to solve. It prevents the expression of measurement results in SI-units, the development of a certified primary calibrator, and a reference method/RMP. Therefore, the concept of harmonization has been proposed as (temporary) substitute of standardization (9).