How can proficiency testing help my laboratory?

Importance of the topic

Proficiency testing (PT) is a cornerstone of quality assurance in analytical laboratories. Regular participation in PT schemes verifies laboratory performance for quantitative, qualitative and interpretative analyses, increases confidence in reported results, supports accreditation and regulatory compliance, and helps identify method- and personnel-related weaknesses before they affect routine work.

Objectives and overview of the guidance

This guidance summarises how PT schemes assess laboratory performance, explains common scoring approaches (notably z scores), outlines how PT complements internal quality control, and describes practical uses of PT data for bias estimation, measurement uncertainty checks, method comparison and demonstration of competence. It aims to help laboratories understand expected actions following unsatisfactory results and how to exploit PT outcomes to improve analytical reliability.

Methodology and assessment

PT providers assign an assigned value (xpt) and a standard deviation for proficiency assessment (spt) that are used to evaluate each participant result (xi). A commonly used performance metric is the z score calculated as z = (xi - xpt) / spt. Assessment criteria normally applied are:

• |z| ≤ 2.0 — satisfactory
• 2.0 < |z| < 3.0 — questionable (warning)
• |z| ≥ 3.0 — unsatisfactory (action required)

The spt may be derived from target analytical performance, expert judgement, or from the observed distribution of participant data. Because a single spt value may not suit all laboratories or applications, participants may justify and use an alternative spt to calculate their own z scores where appropriate.

For evaluating a laboratory’s uncertainty claims, the zeta (ζ) score can be used: ζ = (xi - xpt) / sqrt(u(xi)^2 + u(xpt)^2), where u(xi) and u(xpt) are the standard uncertainties of the laboratory result and the assigned value. The same interpretation bands as for z apply; values outside the acceptable range suggest underestimated measurement uncertainty or an inability to meet claimed performance.

Main results and discussion

Key practical messages from PT practice include:

• PT quantifies laboratory bias and precision relative to an assigned value and consensus or reference-based expectations.
• Unsatisfactory scores signal the need for prompt investigation into transcription, calculation, calibration, method bias or precision issues; failure to act negates the value of PT participation.
• Repeated PT rounds enable trend analysis and early detection of performance drift when PT scores are plotted over time (control-chart approach).
• When participants report method details, PT results permit comparison across methods, revealing systematic differences between technologies or procedures.
• Although certified reference materials (CRMs) or reference-method comparisons are preferred for bias estimation, PT can provide practical bias checks across real sample matrices and concentration ranges when reliable assigned values are available.

Benefits and practical applications

Laboratories gain multiple practical advantages from systematic PT participation:

• Independent verification of analytical performance for clients, accreditation bodies and regulators.
• Objective evidence for accreditation and quality-management records where satisfactory PT performance demonstrates competence.
• Educational feedback on staff training effectiveness and identification of training needs.
• Empirical data to support or revise measurement uncertainty budgets, especially by comparing reported uncertainties with observed PT reproducibility.
• Information to support method selection, validation and continuous improvement by benchmarking against peer performance.

Future trends and potential applications

Developments likely to influence the role and utility of PT include:

• Increased use of robust statistical procedures and consensus-based approaches for assignment of xpt and spt to improve relevance across diverse laboratories.
• Enhanced digital reporting and automated trend-analysis tools that integrate PT scores with internal QC data for near-real-time performance monitoring.
• Expansion of PT schemes into complex matrices, lower concentration levels and broader compound classes to match evolving analytical demands in areas such as environmental monitoring, food safety and clinical testing.
• Greater use of PT data to support measurement uncertainty estimation, method harmonisation and inter-method equivalence studies.

Conclusion

Proficiency testing is an essential component of a laboratory’s quality system. When PT outcomes are actively reviewed and acted upon, they provide robust evidence of competence, a mechanism for continual improvement, and a valuable complement to internal quality control. Laboratories should treat PT as a diagnostic tool: investigate unsatisfactory results, adopt corrective actions, monitor trends across rounds, and use PT-derived information to refine uncertainty estimates and method choices.

References

1. Brookman B., Mann I., editors. Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes. 3rd ed. 2021.
2. Eurachem Proficiency Testing Working Group. Proficiency testing leaflet, Second English edition, July 2022.
3. ISO 13528. Statistical methods for use in proficiency testing by interlaboratory comparison.