Interlaboratory comparisons other than proficiency testing

Significance of the topic

Participation in interlaboratory comparisons (ILCs) and proficiency testing (PT) is a core requirement of ISO/IEC 17025 for laboratories that wish to demonstrate the validity of their results. Understanding the differences between PT and other forms of ILCs is essential for laboratories to select the appropriate external comparison for validation, method development, accreditation evidence, or routine quality assurance. Misinterpreting the purpose or limitations of a particular ILC can lead to incorrect conclusions about a laboratory’s routine performance and competence.

Objectives and overview

The source leaflet clarifies the range of ILCs that fall outside formal PT schemes and compares their aims, strengths and limitations relative to PT. It categorizes ILCs into three principal types: studies aimed at evaluating measurement procedure performance (collaborative/method performance studies), material certification studies (characterization of candidate reference materials), and smaller or split-sample comparisons that may nevertheless be used to assess participant performance. The core objective is to make laboratories aware that only PTs are specifically designed to evaluate participant performance under the rules of ISO/IEC 17043, while other ILCs serve different main purposes and therefore have specific limitations when used instead of PT for monitoring laboratory competency.

Methodology and instrumentation

The leaflet describes the designs and typical practices of three classes of ILCs rather than particular instruments. Key methodological features are:

• Method performance (collaborative) studies: multiple laboratories use the same measurement procedure (often a new or revised method) to assess its performance characteristics (accuracy, precision, robustness). Designs frequently include multiple samples and replicate measurements under tightly controlled conditions.
• Material certification studies: participating laboratories analyse candidate reference materials to assign property values and estimate uncertainties; these studies typically require more careful replication and specific reporting formats than routine analysis.
• Other ILCs (small comparisons, split-sample tests): these can be organized to evaluate participant performance but often lack the procedural rigor or scale of a formal PT provider operating under ISO/IEC 17043.

Because the leaflet addresses study design and standards, it highlights relevant normative documents (ISO/IEC 17025 and ISO/IEC 17043) and guidance (EA-4/21, Eurachem), rather than listing analytical instruments. Laboratories are advised to consider the competence of the organization managing an ILC (ISO/IEC 17025 clause 6.6) when the scheme is not a formally accredited PT.

Main results and discussion

The principal conclusions and practical observations are:

• PT is a subset of ILCs specifically designed to evaluate participant performance: it provides infrastructure, assigned values, scoring, and compliance with ISO/IEC 17043. Other ILCs generally are not structured primarily for participant assessment.
• Method performance studies assess the method under uniform conditions; because all participants use the same procedure, these ILCs do not measure how laboratories perform with their routine methods or with alternative procedures.
• Studies validating a method or certifying materials often require non-routine replication and reporting formats, so results do not necessarily reflect typical laboratory operations.
• Statistical models used in some collaborative studies may assume equal variability across laboratories; if so, the study report will not provide reliable, laboratory-specific performance indicators.
• Small ILCs and split-sample comparisons can offer useful information, but they may lack the scope, statistical robustness or provider competence checks inherent in accredited PT schemes; consequently they should be used with caution when the objective is assessment of routine performance for accreditation purposes.

Benefits and practical applications

Other ILCs remain valuable in many contexts:

• Method validation: collaborative studies are appropriate when the primary goal is to establish fitness-for-purpose, repeatability and reproducibility of a single measurement procedure across laboratories.
• Reference material development: material certification studies are the correct mechanism to assign property values and uncertainties to candidate reference materials.
• Troubleshooting and targeted comparisons: small or split-sample comparisons are pragmatic tools for rapid checks, method harmonization, or focused investigations between a few laboratories.
• Supplementary quality evidence: results from these ILCs can complement PT and internal QC, especially during method implementation or when PT schemes are not available for a given measurand.

However, when the objective is to demonstrate routine laboratory performance for accreditation, participation in accredited PT schemes (ISO/IEC 17043) remains the preferred and expected route.

Future trends and opportunities

Expected developments and opportunities include:

• Greater harmonization between ILC types and PT schemes, including clearer guidance on when non-PT ILCs may substitute for PT evidence in accreditation.
• Increased use of digital data exchange, standardized reporting formats and advanced statistical tools to extract laboratory-level performance indicators from collaborative datasets.
• Hybrid approaches that combine elements of PT (assigned values, scoring) with collaborative method studies to provide both method validation and participant performance feedback.
• Expanded application of small comparisons and split-sample testing as rapid response tools in emergent fields, provided their design and provider competence are verified.

Conclusion

Non-PT ILCs serve important and distinct roles—method validation, material characterization and focused interlaboratory checks—but they are not direct substitutes for accredited proficiency testing when the objective is to evaluate routine laboratory performance for accreditation or external demonstration of competence. Laboratories should select the form of comparison that aligns with their objective, understand the methodological limitations of the chosen ILC, and verify the competence of the organizing body when the scheme does not conform fully to ISO/IEC 17043.

Reference

1. ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories, ISO, Geneva.
2. W. Horwitz, Nomenclature of interlaboratory studies (IUPAC Recommendations 1994), Pure & Applied Chemistry 66(9), 1903–1911.
3. Eurachem leaflet “Let’s call a PT scheme a PT scheme!”. Eurachem.
4. ISO/IEC 17043:2023, Conformity assessment — General requirements for the competence of proficiency testing providers, ISO, Geneva.
5. EA-4/21 INF:2018, Guidelines for the assessment of the appropriateness of small interlaboratory comparisons within the process of laboratory accreditation, European Accreditation.