You have now learned about metrological traceability and the metrological traceability chain. You may then ask who is responsible for handling instrument accuracy checking. The purpose of checking is to make sure that an instrument has the same or similar accuracy as other similar instruments. In this section, we will introduce the calibration hierarchy.
ISO/IEC 17025 requires testing organizations/laboratories to calibrate their instruments before putting them into service. Every country has a national metrology institute (NMI) to maintain measuring standards. All measurement standards trace back to SI units. An NMI will use the measuring standard to calibrate equipment from its country's other laboratories.
For example: all reference laboratories send their instruments to an NMI for calibration before they provide the calibration service to the other testing laboratories. In this section we discuss this infrastructure and how it works.
The National Centre for Biotechnology Information at the US National Library of Medicine defines traceability as being realized by the establishment of a measurement infrastructure that consists of three levels. Figure 1.5 illustrates this hierarchy.
Source: http://www.bipm.org/en/convention/wmd/2004/figure.html
You should note how the level of precision decreases along the chain of responsibility.
We'll now take a closer look at each level as follows:
- Level One: National Metrology Institutes (NMI);
- Level Two: Reference (Calibration) Laboratories; and
- Level Three: Testing Laboratories.
Level 1: National Metrology Institute (NMls)
Every NMI will conduct inter-laboratory correlation and make sure that its own primary standards are consistent when compared with other NMIs. In simple terms, this means that the NMI can
offer its calibration and measurement capabilities (CMC) for certifying specific reference materials. The competent NMI will be listed in the BIPM. See Table 1.3 for examples of NMIs in some different countries.
| Country/District | Example of NMI |
|---|---|
| China | National Institute of Metrology P.R.China |
| United Kingdom | National Physical Laboratory |
| United States | National Institure of Standards and Technology |
| Australia | National Measurement Institute |
You can search for the NMIs of different countries through the BIPM website.
Level 2: Reference (Calibration) Laboratories
Reference laboratories operate at a higher metrological level than routine laboratories. The level of the results from Reference Laboratories should be appropriate for medical requirements. These laboratories are also known as expert institutions because they perform measurements with the greatest competence. A laboratory will qualify as a Reference Laboratory if it satisfies all the following requirements:
- accreditation as a calibration laboratory according to ISO 17025 and 15195
- use of a reference method that has been approved and listed by JCTLM WG2
- participation in reference laboratory ring trials.
Laboratories that satisfy these conditions will be listed in the JCTLM list of Reference Laboratories. The laboratories that fall into this category offer their calibration and measurement capabilities to Diagnostic Kit Manufacturers, Regulatory Organizations and External Quality Assessment (EQA) Organizations. NMI and Calibration Laboratories listed in JCTLM provide the required link between routine laboratories and the reference materials and measurement procedures of a higher metrological order.
Level 3: Routine (Testing) Laboratories
These laboratories provide the routine measurement services to the community and are accredited by the Hong Kong Accreditation Service (HKAS) in Hong Kong. Because the HKAS has a mutual recognition agreement with other accreditation bodies, its certificates are recognized worldwide.
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