Let’s call a PT scheme a PT scheme!

Significance of the topic

Interlaboratory comparisons (ILCs) and proficiency testing (PT) schemes are foundational elements of laboratory quality assurance, method validation and accreditation. Clear, consistent terminology and harmonised practices are essential for interpreting results, meeting regulatory requirements and ensuring comparability between laboratories. Misuse or loose use of colloquial names for ILCs can lead to misunderstandings about objectives, design and expected outcomes; therefore, a concise conceptual framework and reference to international standards supports reliable assessment of measurement performance.

Objectives and overview of the leaflet

This leaflet clarifies basic terminology around ILCs and PT schemes, highlights potentially confusing terms, describes special types of comparisons, and advocates harmonisation through use of recognised standards and guidance documents. It aims to promote precise language so that organisers, participants and assessors share a common understanding of an exercise’s purpose and structure.

Methodology and key concepts

ILC is defined as the organisation, conduct and evaluation of measurements or tests on the same or similar items by two or more laboratories under predetermined conditions. The leaflet emphasises three common high-level objectives for running ILCs: assessing participant performance (typically called proficiency testing or external quality assessment), evaluating method performance (often called method performance or collaborative studies), and activities related to reference material characterisation (material certification studies).

Important distinctions and design options discussed include:

• Scheme formats: sequential schemes (a single unique item circulated between participants) versus simultaneous schemes (similar items from a batch distributed to all participants at once).
• Special comparison categories used in metrology: key comparisons, supplementary comparisons and pilot studies to demonstrate capabilities of National Metrology Institutes.
• Terminology pitfalls: colloquial labels such as ring test, round robin or circle analysis have been used inconsistently and may imply particular sample distribution methods or objectives that do not apply universally.

Main results and discussion

The leaflet’s principal argument is that imprecise terminology damages communication and can complicate interpretation of ILC results. Historical and sectoral variation in terms — and differing definitions across international standards — contribute to the problem. For example, terms like measurement audit, intercalibration or rapid performance evaluation scheme may not accurately reflect the laboratory category involved or the tasks participants undertake. IUPAC’s prior preference for the neutral term interlaboratory study over synonyms such as trial or exercise is highlighted as a useful precedent.

Harmonisation is recommended but acknowledged as slow because language usage evolves and local practices persist. The leaflet stresses that much confusion is preventable by explicitly stating the exercise’s objective and design, and by citing relevant standards and sectoral guidance.

Benefits and practical applications

Adopting standardised terminology and clearly documenting the scope and format of an ILC yields several practical benefits:

• Improved clarity for participating laboratories about their tasks and the form of required results.
• Easier comparison of performance outcomes across schemes and over time.
• Simplified interpretation by accreditation bodies and regulators, supporting compliance with ISO/IEC 17043 and ISO/IEC 17025 requirements.
• Better design choices (e.g., sequential vs simultaneous) matched to the exercise objective, reducing methodological bias and logistical errors.

Recommendations for organisers and participants

The leaflet advises explicit reference to relevant international standards and guides when describing an ILC or PT scheme. Principal references include ISO/IEC 17043 (general requirements for proficiency testing) and ISO 13528 (statistical methods for PT), with related concepts in ISO 5725 (repeatability and reproducibility), ISO Guide 35 (reference materials), and ISO/IEC 17025 (laboratory competence). The Eurachem PT Guide is noted as a freely available practical resource. Where local-language guidance is lacking, organisers should take extra care to define terms in communications.

Future trends and applications

Trends likely to influence ILC/ PT practice include increased emphasis on harmonisation across sectors, wider adoption of standardised statistical approaches, and greater transparency in reporting. Digital data exchange and interoperable reporting formats will facilitate meta-analyses of scheme performance and support more dynamic PT schemes. Expansion of PT to emerging analytical domains (e.g., complex matrices, non-targeted screening, bioanalytical and molecular methods) will require updated guidance and possibly new scheme formats. Finally, improved international coordination among accreditation bodies and PT providers will reduce ambiguity and help disseminate best practice.

Conclusion

Consistent use of terminology and explicit description of ILC objectives and design are small but high-impact measures that improve the utility of interlaboratory comparisons. Organisers should identify the exercise category (PT, method performance study, material certification study, key comparison, etc.), choose an appropriate distribution format (sequential or simultaneous) and reference the appropriate standards. Doing so enhances clarity, supports accreditation and increases confidence in the comparability of measurement results.

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 and 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, key and supplementary comparisons and pilot studies (relevant documents on measurement comparisons).
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).