Proficiency testing schemes and other interlaboratory comparisons

Significance of the topic

Proficiency testing (PT) and other interlaboratory comparisons are cornerstone activities for assuring the quality and comparability of analytical measurements across laboratories, sectors and countries. They provide independent external evaluation of routine testing, help identify systematic and random errors, and support accreditation, regulatory compliance and trade. In regulated fields such as environmental monitoring, food safety and clinical analysis, PT results influence confidence in decisions that affect public health, the environment and commerce.

Objectives and overview of the document

This document summarizes the role, types and practical considerations for PT schemes and interlaboratory comparisons. It differentiates common comparison types (certification trials, collaborative method validation, key comparisons and PT/EQA schemes), outlines advantages and limitations of PT participation, and highlights how PT fits into a broader measurement quality framework (validation, internal quality control, accreditation). The aim is to guide laboratories and stakeholders in selecting appropriate PT schemes and interpreting their outcomes.

Methodology and typical design features of PT schemes

Interlaboratory comparisons involve multiple laboratories measuring the same or similar samples under predefined conditions. Important design elements include sample selection and preparation (matrix match to routine samples, homogeneity and stability), participant instructions, schedules and frequency, analytical scope (analytes and concentration ranges), and statistical evaluation methods used to judge performance.

Key distinctions in purpose drive design choices:

• Certification trials: measurements are used to assign values to reference materials and require highly characterized samples and strict protocols.
• Collaborative method validation: multiple labs evaluate method performance (trueness, precision) to support method acceptance.
• Key comparisons: high-accuracy laboratories compare top-tier measurement capabilities internationally.
• Proficiency testing/external quality assessment (PT/EQA): routine laboratories receive periodic challenges to assess ongoing measurement quality.

Practical compromises often appear in sample preparation (e.g., stabilization or freeze-drying) to achieve acceptable transport stability and homogeneity; participants should be informed of such treatments because they may affect comparability with routine sample handling.

Main results and discussion (practical implications)

Participation in PT provides objective benchmarking against peer laboratories and reveals method- or instrument-specific biases and variability. Regular PT data complement internal quality control (IQC) and method validation by demonstrating whether routine procedures perform acceptably in real operational conditions. However, PT alone cannot guarantee all aspects of measurement quality: sample processing differences, limited frequency of challenges, and variability in provider evaluation protocols can complicate interpretation. Stakeholders should therefore consider PT results alongside validation data, control charts and accreditation audits.

Advantages of proficiency testing

• Independent, external assessment of routine measurement quality.
• Benchmarked comparison to peer laboratories and methods.
• Actionable feedback that supports technical improvements and corrective actions.
• Sector-level insight into analytical capabilities across regions or industries.

Limitations and cautions

• PT samples may not perfectly mimic routine matrices; stabilization or other processing can alter analyte behavior.
• Not all schemes cover the full range of analytes, concentration levels or methods used by a laboratory; careful selection is required.
• Different PT providers may use distinct evaluation protocols, so the same numerical result can be judged differently by different schemes.
• Infrequent participation limits the ability to detect transient problems; PT should complement, not replace, continuous IQC.

Benefits and practical use of PT results

PT outcomes are increasingly used by laboratories, accreditation bodies and regulators as evidence of competence and method fitness-for-purpose. Practical uses include:

• Demonstrating measurement capability for accreditation or contractual requirements.
• Identifying and quantifying method biases and precision issues to target corrective actions.
• Supporting method selection and instrument procurement decisions through comparative performance data.
• Providing sector-wide metrics that inform policy, trade and public-health decisions.

Educational aspects and provider accreditation

PT providers often offer user meetings, result interpretation workshops and discussion forums that enhance laboratory competence. Virtual PT schemes using digital representations of samples are emerging, enabling unlimited participation, immediate feedback and repeated training assessments. Some PT providers pursue formal accreditation of their schemes to demonstrate that their organisation and statistical evaluations meet international standards, increasing stakeholders' confidence in scheme outputs.

Future trends and applications

• Growth of virtual and digital PT platforms to extend reach, speed feedback and allow frequent training-oriented assessments.
• Increased accreditation of PT providers and harmonization of evaluation protocols to reduce inter-provider variability in performance judgments.
• Expanded use of PT data by regulators and purchasers as part of risk-based surveillance, supplier evaluation and conformity assessment frameworks.
• Integration of PT results with laboratory information management systems (LIMS) and statistical process control tools for near real-time quality oversight.

Conclusion

Proficiency testing is a flexible and essential external quality tool that complements validation and internal quality controls. Appropriate selection of PT schemes — considering matrix, analytes, levels and frequency — together with regular participation and proper interpretation, strengthens laboratory credibility and measurement reliability. PT should be viewed as one element of an integrated quality strategy that supports accreditation, regulatory compliance and continuous improvement.

Reference

Eurachem Proficiency Testing Working Group. Proficiency testing schemes and other interlaboratory comparisons. Second English edition, July 2022. Produced by Eurachem.