Sources and performance criteria of uncertainty of reference measurement procedures
Introduction
According to the International Vocabulary of Metrology (VIM) a reference measurement procedure (RMP) is defined as “measurement procedure accepted as providing measurement results fit for their intended use in assessing measurement trueness of measured quantity values obtained from other measurement procedures for quantities of the same kind, in calibration, or in characterizing reference materials” [1]. With regard to the measurement procedure, “a detailed description of a measurement according to one or more measurement principles and to a given measurement method, based on a measurement model and including any calculation to obtain a measurement result” is reported. Among the notes, it is written that “a measurement procedure can include a statement concerning a target measurement uncertainty”. Within this work we try to make a comprehensive list of the currently available RMPs mostly useful in the field of laboratory medicine, to present the reference measurement services having implemented some of these RMPs and to review how their performance may be regularly monitored.
Section snippets
Reference measurement procedures
The first point to address when looking for such a topic is where to find the relevant information. The Joint Committee for Traceability in Laboratory Medicine (JCTLM) database Working Group was charged for identifying, reviewing and publishing lists of available higher order certified reference materials, higher order reference measurement procedures, and reference measurement services of laboratories. The information is available through a dedicated website [2], which is regularly updated.
Reference measurement services
It is important to remember that a published RMP is only the basis and that has to be transformed into a standard operating procedure (SOP) in every calibration laboratory. As an example, in the case of HbA1c, the original description of the RMP was published as a research paper back in 1997 [15], and that only a few years later the SOP was fully described and published in another paper [11]. Indeed, the measurement design in the individual calibration laboratory has strong influence on the
Reference measurement services and evaluation of their performance
Most of the readers are familiar with EQAS procedures, which are regularly performed in order to monitor the long-term accuracy of routine methods. Various EQAS are present worldwide, but none of them is suited to evaluate the long-term performance of RMPs. In order to overcome this issue, ten years ago an external quality assessment scheme for reference laboratories in laboratory medicine was designed, and it is now regularly available through the JCTLM facility [16]. Essentially, specific
Conclusions
From the brief survey we have conducted, we have seen that for some measurands there are several RMPs listed in the JCTLM database. This is not a limit, on the contrary it is a richness, because there is absolutely no need to limit the number of RMPs as long as they meet the criteria of an RMP. For primary RMPs, the decisive factor is that the analytical measurement principle used allows the development of a RMP of higher/highest metrological order. If so, then it is quite possible that there
Acknowledgments
We acknowledge Dr. Anna Carobene and Dr. Elena Guerra (San Raffaele Hospital, Milano, Italy) for having shared with us their reports from the cholesterol network, and Dr. Cas Weykamp and Dr. Carla Siebelder (Queen Beatrix Hospital, Winterswijk, The Netherlands) for the permission of reproducing part of the report of the 2016 Shanghai study related to the IFCC network for HbA1c.
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