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The general position of inter-laboratory comparisons (ILCs), proficiency testing (PT) and measurement audit (MA) in accreditation

The objective for any organisation performing testing activities is to ensure that results are fit for their intended purpose. Quality assurance is fundamental in supporting robust analysis and there are a wide variety of processes and procedures that calibration, test and inspection bodies can implement to provide confidence in their data. 

Accuracy reflects how close a measurement is to a known or accepted value, while precision reflects how reproducible measurements are, even if they are not close to the recognised value. Measurements that are both precise and accurate are repeatable and very close to true values.

Internal quality control procedures provide important information on both the accuracy and precision of a test but can be limited by factors relating to the environment in which the test has been conducted. In an ideal world, results generated by one competent laboratory will be directly comparable to those produced by another for the same parameters on the same items. Many laboratories operate in isolation from other laboratories however and do not have ongoing opportunities to compare their data with others.

Inter-laboratory comparison is the organisation, performance and evaluation of tests on the same or similar test items by two or more laboratories in accordance with predetermined conditions. Participation serves the purpose of validating the capability of a laboratory to provide accurate and reliable results within permissible levels of uncertainty and therefore ensure that the adopted analytical method is suitable for the intended purpose. In summary, it provides independent verification of laboratory competence. 

So where does Proficiency Testing fit in?

Many people use the terms “inter-laboratory comparison” and “proficiency testing” interchangeably, but proficiency testing can be more accurately described as the evaluation of participant performance in inter-laboratory comparisons by an independent third party. 

The addition of independent oversight of the ILC process brings clear benefits in many ways. For example:

  • Laboratories can remain anonymous from each other so that participants can take part in the same scheme without fear that a competitor could use any performance issues to undermine them
  • When issues arise with laboratory performance, these can be quickly identified and investigated. Opportunity is then available for technical discussions with independent, competent scientists employed by (or under contract to) the PT provider
  • When issues arise with the comparison exercise itself, these too can be quickly identified and rectified by the PT provider
  • Trends in performance can be highlighted and used by participants (and often the technical sector in general) to improve the quality of testing or identify issues in analysis.

Proficiency testing providers (PTPs) are often accredited to confirm their competence to organise, conduct and evaluate ILCs. Accredited providers are recognised against the requirements of international standard ISO/IEC 17043:2010. This standard is currently under review for revision, probably within the next 12-18 months.

Measurement Audit

Historically, many areas of calibration were not well covered by appropriate PT schemes. As a result, UKAS created a “measurement audit” assessment tool which provided an independent means by which laboratories could demonstrate competence by calibrating UKAS-owned artefacts with known parameters. Whilst it does give an independent perspective on competence, measurement audit does not constitute an inter-laboratory comparison because the results are compared against known reference values rather than data produced by other laboratories. 

During the COVID pandemic, it was not possible to access the UKAS measurement artefact storage facility or distribute artefacts for calibration and the mechanism became dormant. Now that the restrictions imposed during the pandemic are being relaxed, UKAS has reviewed the ongoing viability of reinstating the measurement audit function and has reached a conclusion to stop offering the service because of the alternative mechanisms that have developed in the interim. As a result of that decision, UKAS policy document LAB 46 [] (UKAS Policy for Participation in Measurement Audits and Interlaboratory Comparisons) is currently under review and revision and will be reissued shortly.

The requirement for participation

The general requirements that testing and calibration laboratories must meet if they wish to demonstrate that they operate to a management system, are technically competent and are able to generate technically valid results are contained within ISO/IEC 17025:2017. ILAC document P9/06:2014 [] (currently under revision to reflect the requirements of ISO/IEC 17025:2017) also specifies ILAC policy requirements on laboratories and accreditation bodies in relation to proficiency testing activities.

Section 7.7 of ISO/IEC 17025:2017 requires that laboratories shall plan and undertake quality assurance procedures for monitoring the validity of tests and calibrations undertaken. This shall include participation in interlaboratory comparison or proficiency testing programmes where available and appropriate. As part of its routine assessment programmes, UKAS will review participation in external quality assurance to ensure any selected schemes are appropriate to the items being tested or calibrated and that the frequencies of participation are suitable. Additionally, in the absence of such participation, the laboratory can justify its approach or use of alternative mechanisms.

It is worth bearing in mind at the outset when considering an application for accreditation that UKAS policy will remain that before accreditation is granted for any measurement, there must be at least one instance of satisfactory participation in an appropriate ILC or equivalent activity. If no such audits or ILCs are available, the laboratory will need to put additional quality assurance activities in place or arrange their own inter-comparisons with another laboratory. UKAS experience is that this requirement for satisfactory participation in ILC can often lead to significant delays in obtaining accreditation while applicants wait for a suitable scheme to become available and then wait for the outcomes of the exercise. 

Going forwards, there is mounting interest in the development of “Rapid PT” which is where a PT provider sends out an unknown, well-characterised test sample (often a CRM) for which the assigned (certified) value is already known, so as soon as the laboratory send back their result a performance evaluation can be provided (in a similar manner to Measurement Audit). There are many pros and cons to this mechanism (not least the lack of comparability with other laboratories and whether this fits within the requirements of ISO/IEC 17043:2010 but on the plus side, the speed and ease with which fitness for purpose can be evaluated) and the laboratory committee (LC) of the European Co-operation for Accreditation (EA) is working in conjunction with the EA, Eurolab and Eurachem proficiency testing working group (EEE-PT WG) to determine the suitability and benefit of such Rapid PT (or Quick Response) events.

BMTA is evaluating the viability of creating a new, industry-led, inter-laboratory comparison service to assist you in meeting your accreditation obligations. 

If this would be of interest, our short survey, accessed here:  [], will take no more than two minutes to answer and will provide us with a better understanding of your ILC requirements.