Let’s call a PT scheme a PT scheme!

Importance of the topic

Interlaboratory comparisons (ILCs) and proficiency testing (PT) schemes are foundational tools for assuring measurement quality, comparability and traceability across laboratories. They support accreditation, method validation, production of certified reference materials, and demonstration of national metrology capabilities. Clear, harmonised terminology and well‑defined designs are essential to avoid misinterpretation, ensure consistent application of standards and facilitate international mutual recognition of results.

Objectives and overview of the leaflet

This leaflet explains core terminology for ILCs and PT schemes, highlights common sources of confusion caused by colloquial or inconsistent terms, and advocates for harmonisation by referencing relevant international standards and guidance. It outlines typical objectives of ILCs, describes special types of comparisons, clarifies ambiguous labels and recommends using accepted terms that reflect the detailed aims and design of an exercise.

Key concepts and definitions

- Interlaboratory comparison (ILC): the organisation, conduct and evaluation of measurements or tests on the same or similar items by two or more laboratories under predetermined conditions.
- Proficiency testing (PT) / external quality assessment: evaluation of participant performance, often used as the practical label for performance assessment in accreditation and quality systems.
- Special ILC types: method performance studies (evaluating a standard method), collaborative studies, material certification studies (for reference material characterization), and metrology key/supplementary/pilot comparisons (for National Metrology Institutes).

Methodology and common designs

- Simultaneous scheme: distribution of similar items from the same batch to participants for near‑identical test conditions.
- Sequential scheme: a single unique item is transferred sequentially between participants. ISO/IEC 17043 uses these terms to distinguish distribution approaches.
- Different objectives (e.g., PT, method validation, material certification) require tailored statistical approaches and operational designs; ISO 13528 and ISO 5725 provide statistical and reproducibility frameworks.

Ambiguities in terminology and their consequences

- Colloquial labels such as ring test, round robin or circle analysis are widely used but can be ambiguous because they do not convey objective, design or distribution method unambiguously.
- Other labels—measurement audit, intercalibration, rapid performance evaluation—may misrepresent participant type or activities performed during the ILC.
- Variability in term usage across standards and local languages increases the risk of misunderstanding and inconsistent implementation in laboratories and among PT providers.

Recommendations for harmonisation and good practice

- Prefer precise, standardised terms: use ILC as the umbrella concept, and specify the subcategory (PT scheme, method performance study, material certification study, key comparison) based on objective and design.
- Reference relevant international standards and guidance when designing or describing an exercise: ISO/IEC 17043 and ISO 13528 are central for PT schemes; ISO/IEC 17025, ISO 5725 and ISO Guide 35 provide complementary requirements and methods.
- Provide clear procedural instructions specifying distribution mode (simultaneous vs sequential), statistical evaluation approach and intended use of results to avoid misinterpretation by participants.
- Use and disseminate sectoral guides and translated standards where needed to reduce confusion arising from language differences.

Main conclusions and practical benefits

- Consistent terminology and clear linkages to established standards improve the transparency, comparability and credibility of interlaboratory exercises.
- Well‑designed and properly labelled ILCs enable robust assessment of laboratory competence, method performance and material characterisation, supporting accreditation and regulatory decisions.
- Avoiding ambiguous terms reduces operational errors, incorrect expectations by participants, and variability in result interpretation.

Future trends and potential applications

- Continued harmonisation: wider adoption of ISO/IEC 17043 and ISO 13528 terminology across sectors and regions, coupled with clearer sectoral guidance, will reduce ambiguity.
- Digitalisation and traceability: electronic PT delivery, digital chain‑of‑custody and automated statistical evaluation will streamline schemes and improve data quality and timeliness.
- Capacity building and multilingual resources: expanded availability of translations, training materials and online guidance (e.g., Eurachem) will help laboratories and PT providers apply standards consistently.
- Tailored PT: development of more specialized PTs for complex matrices, non‑routine methods and emerging technologies (e.g., high‑resolution mass spectrometry, molecular assays) to meet evolving analytical needs.

Conclusion

ILCs and PT schemes are vital quality assurance tools in analytical chemistry. Precise use of terminology, clear description of objectives and adherence to internationally recognised standards substantially enhance the value and interpretability of interlaboratory work. Practitioners and PT providers should explicitly state the ILC category, distribution mode and intended application of results, and make consistent reference to ISO/IEC 17043, ISO 13528 and related guidance to promote harmonisation and confidence in comparative measurements.

Reference

1. ISO/IEC 17043:2010, Conformity assessment — General requirements for proficiency testing, ISO Geneva (2010).
2. W. Horwitz, Nomenclature of interlaboratory studies (IUPAC Recommendations 1994), Pure & Applied Chemistry, 66(9), 1903-1911.
3. ISO 5725–2:2019, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method, ISO Geneva (2019).
4. ISO Guide 35:2017, Reference materials — Guidance for characterization and assessment of homogeneity and stability, ISO Geneva (2017).
5. BIPM resources on key and supplementary comparisons (accessible via BIPM web resources).
6. ISO 13528:2015, Statistical methods for use in proficiency testing by interlaboratory comparison, ISO Geneva (2015).
7. ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories, ISO Geneva (2017).
8. B. Brookman and I. Mann (eds.) Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes (3rd ed. 2021).
9. ISO Online Browsing Platform (ISO OBP) for standard texts.