Proficiency testing schemes and other interlaboratory comparisons

Significance of the topic

Proficiency testing (PT) and other interlaboratory comparisons are central tools for assuring the quality and comparability of analytical measurements. They provide independent, external assessment beyond internal quality control and method validation, and are crucial for accreditation, regulatory compliance, trade, environmental monitoring and public health decisions. PT schemes inform laboratories about method performance in routine conditions, reveal systematic and random errors, and stimulate continuous improvement across sectors and regions.

Objectives and overview of the study/article

The document outlines types of interlaboratory comparisons (certification trials, collaborative method validation trials, key comparisons) and focuses on proficiency testing schemes as the most common instrument for evaluating routine laboratory performance. It describes the advantages, limitations, roles of PT within a quality framework, educational aspects, and how PT results are used by laboratories, customers, accreditation and regulatory bodies. The guidance also highlights practical considerations when selecting and interpreting PT schemes.

Methodology and key principles

• Types of comparisons: interlaboratory comparisons vary by objective—assigning certified values, validating methods, or comparing highest-accuracy measurements internationally.
• PT scheme design: schemes should use samples resembling routine materials, be homogeneous and stable, and be distributed under predetermined conditions with clear evaluation protocols.
• Statistical evaluation: providers apply statistical models to evaluate participants (z-scores, robust statistics, equivalence tests); however, different providers may use different protocols leading to divergent judgments.
• Integration with quality systems: PT complements method validation, internal quality control (IQC), control charts and accreditation activities, acting as an external check that procedures remain fit for purpose.

Main results and discussion

• Advantages: PT enables benchmarking against peers; delivers regular, objective assessments; provides feedback to improve laboratory practice; offers comparative data on methods and instruments; and gives an overview of analytical quality in a sector or region.
• Limitations: PT samples are sometimes processed (stabilised, freeze-dried) which can differ from routine matrices and affect comparability. Perfect PT protocols do not exist; heterogeneity in provider methods can lead to inconsistent evaluations.
• Practical considerations: laboratories must verify that scheme materials, analytes and concentration ranges match routine measurements, that frequency of rounds is appropriate, and that provider reports supply sufficient information for corrective actions and accreditation evidence.
• Educational value: many providers run user meetings and interpretative workshops. Emerging virtual PT (digital images or data) offers scalability, immediate feedback and repeated assessments for training purposes.
• Accreditation of providers: some PT providers seek accreditation for their schemes, which increases confidence in scheme design and evaluation processes and aligns PT operations with international standards applicable to testing and calibration laboratories.

Benefits and practical applications of the method

• Support for accreditation and regulatory compliance by providing objective, external evidence of measurement capability.
• Identification of systematic biases and poor precision that can affect client decisions, trade outcomes, environmental reporting and health assessments.
• Benchmarking of methods and instruments across laboratories to inform method selection, training needs and method improvement.
• Use in supplier surveillance, inter-company comparisons and sector-wide quality surveillance to raise overall analytical standards.

Future trends and potential applications

• Virtual and digital PT schemes will expand, enabling immediate and repeatable assessments and wider participation without physical sample distribution.
• Harmonisation and standardisation of evaluation protocols across PT providers to reduce inconsistencies and improve comparability of PT outcomes.
• Increased accreditation of PT providers and tighter integration of PT results into regulatory decision-making and customer procurement criteria.
• Advanced analytics and automation (including AI-assisted evaluation) to detect patterns in interlaboratory data, improve uncertainty estimation, and provide targeted corrective actions.
• Broader use of commutable reference materials and improved sample design to better mimic routine matrices and reduce matrix effects in PT assessments.

Conclusion

Proficiency testing is an indispensable component of a robust laboratory quality system. While not a substitute for method validation or internal QC, regular and well‑designed PT participation provides independent evidence of measurement performance, drives improvement, and supports accreditation and regulatory needs. Laboratories should carefully select schemes that match their routine scope, interpret results in the context of other quality tools, and engage with providers to maximise the educational and corrective value of PT participation.

References

• Eurachem Proficiency Testing Working Group. Proficiency testing schemes and other interlaboratory comparisons. Second English edition, July 2022. Eurachem.
• EPTIS (European Proficiency Testing Information System) and national accreditation bodies for information on providers and schemes.