Interlaboratory comparisons other than proficiency testing

Significance of the topic

Interlaboratory comparisons (ILCs) are fundamental tools in analytical chemistry for demonstrating measurement reliability, harmonising methods and supporting laboratory accreditation. While proficiency testing (PT) schemes represent a formally structured subset of ILCs intended to assess participant competence against defined criteria, a range of other ILC formats exist that serve complementary purposes such as method validation or material characterisation. Understanding the differences between these types of ILCs, their intended uses and their limitations relative to PT is essential for laboratories that must comply with ISO/IEC 17025 and wish to ensure the ongoing validity of their results.

Objectives and overview of the document

The document clarifies the nature and scope of ILCs other than PT and highlights where they can or cannot substitute for PT in the context of ISO/IEC 17025 requirements. It categorises ILCs into three principal types by purpose, summarises common limitations when such ILCs are used to monitor laboratory performance, and indicates relevant international guidance and accreditation considerations.

Methodology and study types

The leaflet organises non-PT ILCs into three major categories, each using identical portions of a homogeneous stable test sample but differing by intent:

• Evaluation of measurement procedure performance (method performance studies / collaborative studies)
• Characterisation of candidate reference materials (material certification studies)
• Other ILC formats (small ILCs, split-sample comparisons and similar exercises)

For each type, typical study designs and their objectives are described. The document contrasts these designs with PT schemes that are specifically configured to evaluate participant performance and are required to conform to ISO/IEC 17043 when used for that purpose.

Main results and discussion

Key conclusions highlight that although many ILCs involve multiple laboratories analysing the same samples, only PT is primarily intended and structured to assess and monitor participant competence. Important distinctions and limitations noted include:

• Method performance studies typically require all participants to use the same procedure under evaluation. Consequently, such studies do not assess a laboratory’s routine performance with its own methods or enable comparison between different routine methods.
• New or non-routine procedures used in collaborative studies may lack established quality-control practices at participating sites, so observed variability may not reflect typical routine operation.
• Statistical approaches in some collaborative studies may assume homogenous variability among labs, preventing extraction of individual laboratory performance metrics from the study report.
• Designs that demand multiple samples or extensive replication (common in method validation or material certification) are often not representative of routine workflows and therefore do not substitute for routine PT-based monitoring.
• Material certification studies focus on assigning property values (often with stated uncertainty) to candidate reference materials; they emphasize assignment accuracy rather than routine laboratory comparability, and their reporting formats and replication requirements can differ from routine reporting.
• Small-scale ILCs and split-sample exercises can be used to evaluate participant performance but may lack the statistical robustness, scope and provider accreditation that full PT schemes offer. Laboratories must therefore assess the competence of the organising provider and the suitability of the service (ISO/IEC 17025 clause 6.6).

Benefits and practical applications of the methods

While limited as PT replacements, non-PT ILCs provide valuable functions:

• Method performance studies support validation, interlaboratory reproducibility assessment and identification of method weaknesses prior to broad adoption.
• Material certification studies enable establishment of reference materials with assigned values and uncertainties, essential for calibration, method validation and traceability.
• Small ILCs and split-sample checks are pragmatic tools for rapid, local comparison of results between a few laboratories, useful for troubleshooting or short-term checks.

When used appropriately and interpreted with awareness of their constraints, these ILCs strengthen measurement science, method development and material characterisation activities.

Future trends and applications

Trends likely to influence ILC practice include increased harmonisation of statistical approaches, wider adoption of ISO 17043-conformant PT where participant evaluation is intended, and growth in tailored small-scale comparisons for rapid response needs. Developments in digital data exchange, shared quality-management platforms and greater transparency of provider competence are expected to improve comparability and traceability across ILC types. There is also potential for hybrid schemes that combine method validation, material certification and targeted participant assessment while maintaining clarity about their primary objectives.

Conclusion

Non-PT interlaboratory comparisons play important and distinct roles in analytical chemistry, particularly for method validation and material characterisation. However, they are generally not equivalent to proficiency testing when the goal is to evaluate and monitor routine laboratory performance. Laboratories seeking to meet ISO/IEC 17025 obligations should prioritise PT (ISO/IEC 17043-compliant) for participant assessment and use other ILC forms with an understanding of their scope, design limitations and reporting implications. Where other ILCs are employed to support accreditation or competence arguments, careful appraisal of the study design, statistical treatment and the organiser’s competence is essential.

References

1. ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories, ISO, Geneva, 2017.
2. Horwitz W., 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, 2023.
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, 2018.