Interlaboratory comparisons other than proficiency testing

Significance of the topic

Interlaboratory comparisons (ILCs) and proficiency testing (PT) are essential quality assurance tools for testing and calibration laboratories. ISO/IEC 17025 requires laboratories to participate in PT and/or other ILCs to ensure the validity of their results. Understanding the distinctions between PT and other types of ILCs is critical for laboratories to select appropriate external checks, correctly interpret outcomes, and demonstrate competence to customers and accreditation bodies.

Objectives and overview of the article

The source document clarifies what interlaboratory comparisons other than proficiency testing are, classifies the principal types of such ILCs, and highlights their intended purposes and limitations relative to PT. The leaflet aims to help laboratories choose the appropriate comparison format for method validation, reference material characterization, or targeted inter-laboratory checks while indicating when PT remains the preferred route for assessing laboratory performance.

Methodology and scope

The document is a guidance leaflet summarizing international standards and community practice (not an experimental study). It synthesizes definitions and recommendations from ISO/IEC 17025, ISO/IEC 17043, and relevant Eurachem and EA guidance documents, and it categorizes ILCs by their principal objectives: method performance evaluation, candidate reference material characterization, and other targeted comparisons (e.g., small-scope or split-sample exercises).

Main findings and discussion

The leaflet identifies three principal types of ILCs in which laboratories analyse identical portions of a homogeneous, stable test sample. Key distinctions and limitations compared with PT are summarized below.

• Proficiency testing (PT): Designed specifically to evaluate participant performance and provide infrastructure to monitor and assess result validity. PT schemes that assess participant competence should comply with ISO/IEC 17043.
• Method performance / collaborative studies: These are intended to validate or evaluate a specific measurement procedure across laboratories. Typical limitations when used as a substitute for PT include:
  
  • All participants often use the same, possibly new, measurement procedure, so results do not reflect routine performance using each laboratory’s standard methods.
  • New methods may lack established quality control and stable routine practice, producing performance estimates that differ from normal operations.
  • Statistical models for the study may assume uniform variability across laboratories, preventing direct assessment of individual laboratory performance.
  • Study designs frequently require multiple samples or replicates that depart from routine workflows, limiting representativeness.
  • Outcomes can conclude that the method itself is not fit for purpose, which is a different aim than assessing laboratory competence.
• Characterization of candidate reference materials (material certification studies): These ILCs assign property values (often with uncertainty) to a material. Limitations for using them as PT replacements include:
  
  • Analyses may demand atypical care, extensive replication, or alternative reporting (e.g., reporting all valid replicates) that are inconsistent with routine practice.
  • Results reporting is typically aggregated to support certification and may not provide evaluative feedback on individual laboratory performance.
  • Repetitive or non-routine analytical requirements reduce representativeness of regular laboratory workflows.
• Other ILC forms (small ILCs, split-sample comparisons): Some exercises are explicitly designed to evaluate participant performance (for example small schemes as defined in EA-4/21 or split samples sent to two or three labs). While targeted at participant evaluation, such formats often lack the comprehensiveness and procedural rigor of full PT schemes and may not comply fully with ISO/IEC 17043.

The leaflet emphasizes that only PT schemes conducted in full conformity with ISO/IEC 17043 should be considered a complete and accredited route for objectively evaluating participant competence. For other ILCs, laboratories must assess the competence of the scheme organiser and the appropriateness of the external service (per ISO/IEC 17025 clause 6.6) before relying on results for accreditation evidence.

Benefits and practical applications

When selected and used with awareness of their aims and limitations, non-PT ILCs offer practical benefits:

• Method performance studies are appropriate for multi-laboratory method validation, estimating method precision, bias, robustness, and transferability prior to broad deployment.
• Material certification studies are essential to assign property values and uncertainties to candidate reference materials, supporting traceability and quality control across industries.
• Small ILCs or split-sample checks provide rapid, low-cost checks to identify gross inter-laboratory discrepancies or to support ad hoc investigations.

However, for routine external quality assessment of laboratory competence and accreditation evidence, accredited PT schemes remain the gold standard because they are specifically designed and governed to evaluate participant performance impartially.

Future trends and potential uses

Expected developments and opportunities include:

• Greater clarity and harmonization between ISO standards and guidance documents on when non-PT ILCs are acceptable alternatives for accreditation evidence.
• Increased use of hybrid approaches combining method validation ILCs and targeted PT elements to both validate methods and monitor ongoing laboratory performance.
• Broader adoption of small, rapid-turnaround intercomparisons and digital data-exchange platforms to support real-time troubleshooting and method transfer, while maintaining traceability and statistical rigour.
• Improved guidance and accreditation frameworks for split-sample and small-scope ILCs to ensure transparent assessment of provider competence and data interpretation.

Conclusion

Interlaboratory comparisons other than proficiency testing serve distinct and valuable purposes—principally method validation and reference material certification—but they have important limitations if used instead of PT for evaluating routine laboratory performance. Laboratories must select ILC formats aligned with their objectives, critically evaluate the competence of the organising body, and prefer ISO/IEC 17043-conformant PT schemes when the goal is to demonstrate participant competence for accreditation.

Reference

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 and 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.