Hierarchical scheme of measurement traceability
Measurement traceability (defined as “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty” according to VIM 2008)  forms the basis for standardization in laboratory medicine which is described in detail in ISO 17511 . The strategy of traceability in laboratory medicine is described in detail in ISO 17511 . As demonstrated in Figure 1, a result of measurement in a patient sample should be traceable along a cascade of measurement procedures and calibrators/reference materials of increasing metrological order up to the highest level represented by the definition of the measurand in SI units.
This hierarchical scheme represents a sequence of measurement procedures and materials of increasing metrological order – from the routine sample obtained from a patient applying the end-user’s routine measurement procedure (test kit) – to the highest level: the definition of the measurand in SI units. The increasing uncertainty µc(y) from the top level down to the result in a patient sample is symbolized by the horizontal bars. The responsibilities for the procedures and materials at different levels lie, on the top, with the International Bureau for Weights and Measures (BIPM), which serves as a functional unit for the General Conference on Weights and Measures (CGPM); this is followed by the National Metrology Institutes (NMI), the Accredited Reference (Calibration) Laboratories (ARML), the Manufacturer’s Laboratories (ML) down to the basic level of the end-user’s routine (testing) laboratories.
In many situations, it is acceptable to skip pairs of materials and procedures. However, it is important to maintain the highest levels in the scheme, e.g., the primary reference measurement procedure of outstanding specificity and trueness to assign values to secondary matrix reference materials.
The hierarchical scheme for the measurement of catalytic concentrations of enzymes shows some differences as, in this case, the top level cannot be described by a primary calibrator and its certification. Instead, the enzyme measurand has to be defined by an exactly described conventional reference measurement procedure. This concept is described in ISO 18153  and the respective primary reference measurement procedures have been published by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) for a series of enzymes .
Requirements for establishing the concept of traceability
The implementation of the traceability concept obviously requires the establishment of internationally accepted:
reference measurement procedures;
reference materials; and
reference measurement laboratory services.
With respect to these requirements, the Joint Committee for Traceability in Laboratory Medicine (JCTLM) , established by the BIPM, the IFCC and the International Laboratory Accreditation Cooperation (ILAC), has launched two projects in its working groups (WG).
WG-1 lists tables of reference materials and reference procedures on the JCTLM website . The materials and procedures are first inspected by review teams dedicated to different groups of measurands on the basis of criteria defined in the International Standards ISO 15193  and 15194 . The criteria for listing reference measurement materials and measurement procedures are described in detail in the respective procedure manuals on the JCTLM website .
The list of materials and methods is updated annually according to the decisions made by the JCTLM Executive Committee.
The identification of reference measurement laboratories is the objective of JCTLM WG-2. Establishing networks of reference laboratories is – in addition to reference measurement procedures and reference materials – one of the biggest challenges in implementing the concept of measurement traceability. Each individual proposal of a reference laboratory for a particular measurement service (measurand) is first inspected by a review team dedicated to the appropriate group of measurands and then approved by the JCTLM Executive Committee according to the rules of the corresponding WG-2 procedure manual .
There is general agreement now that reference laboratories should be identified:
according to the metrological level of the procedures applied;
on the basis of accreditation as calibration laboratory according to ISO 15195  and ISO 17025 ; and
on the basis of their ability to demonstrate performance in regular inter-laboratory comparisons (ring trials).
With respect to the metrological level the measurement principle [e.g., isotope dilution-mass spectrometry (IDMS)] is probably one of the most important criteria to accept a method as reference measurement procedure in view of its outstanding specificity and trueness. In addition, the use of a primary reference material as calibrator is essential. Furthermore, all gravimetric and volumetric steps must be carefully calibrated gravimetrically using certified test weights. Each certified measurement result achieved after a campaign of at least three independent measurement occasions must be accompanied by an individual uncertainty statement. Also, extensive quality assessment of each analytical campaign is obligatory. Matrix control materials with certified target values must be included in each series of measurements.
Laboratories must show their accreditation as a reference (calibration) laboratory to demonstrate adherence to the requirements of ISO standards 15195 and 17025.
In order to support the JCTLM activities regarding inter-laboratory comparisons, the IFCC has launched a ring trial program for reference laboratories (RELA) – particularly dedicated to reference measurement laboratories – for some 30 different measurands . Ring trial results not only demonstrate the competence of individual laboratories, they also reveal the equivalence of individual laboratory results or, in some cases, the bias of different reference procedures. In this special issue of Clinical Chemistry and Laboratory Medicine, the article “External Quality Assessment Scheme for reference laboratories – review of eight years experience”  describes the details.
Hierarchical scheme of laboratories
In summary, a hierarchical scheme of laboratories has been established as demonstrated in Figure 2.
At the top, we find the NMIs; they must demonstrate their competence in key comparisons and are listed in the “CIPM-MRA-Key-Comparison-Data-Base (International Committee on Weights and Measures – Mutual Recognition Arrangement – Key Comparison – Data Base)”. Their services include certification of primary and secondary reference materials and provision of calibration and measurement capabilities on the highest metrological level, e.g., by participating in Ring Trials for Reference (Calibration) Laboratories. Thereby, they provide the link to the second level of laboratories in the hierarchical order.
The Reference (Calibration) Laboratories have to demonstrate their competence:
by accreditation as calibration laboratories according to ISO 15195 ;
by using JCTLM accepted reference measurement procedures; and
by regular participation in ring trials dedicated to reference/calibration laboratories.
Once the requirements for particular services (measurands) are fulfilled, these laboratories will be listed by the JCTLM on the respective website.
The service of the reference/calibration laboratories is to support diagnostic kit manufacturers and regulatory bodies in the establishment and control of traceability of diagnostic test kits. Furthermore, they provide certified reference method target values for External Quality Assessment Schemes (EQAS), thus linking to the basic group of routine/testing laboratories.
These may demonstrate their competence by accreditation according to ISO 15189  and by participation in ring trials where target values are certified by reference/calibration laboratories. Obviously, their service is the analysis of samples from patients.
Implementation of the concept of measurement traceability
The implementation of the concept of traceability improves standardization in laboratory medicine as evident from the results of EQAS for routine laboratories using reference method target values.
The concept of traceability as described here has to date become available for only a limited number of measurands for which certified primary reference materials and/or reference measurement procedures are available.
Up to now, the JCTLM has listed 241 reference materials, 156 reference measurement procedures and 63 reference measurement services from eight reference laboratories. Extension of the concept of traceability to include further measurands by developing new reference materials and reference measurement procedures and establishing reference measurement services is scheduled for the future. Nevertheless, it can be estimated that in the current situation, about 65% of all measurements in a typical clinical chemical laboratory can be covered by the concept of traceability.
For measurements where the concept is not applicable another approach has been proposed based on “harmonization”. This has been discussed extensively in recent years . However, it must be emphasized that harmonization should not be applied when the metrological concept of traceability can be implemented.
Conflict of interest statement
Author’s conflict of interest disclosure: The author stated that there is no conflict of interest regarding the publication of this article.
Research funding: None declared.
Employment or leadership: None declared.
Honorarium: None declared.
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