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 provides an independent, external check of laboratory performance for quantitative measurements, supports accreditation and regulatory compliance, and helps verify the effectiveness of methods, personnel training and internal quality control. PT also creates documented evidence of competence and a data source for bias and uncertainty assessment under realistic sample matrices and concentration ranges.

Objectives and overview of the study/article

This leaflet explains how PT works for quantitative analyses and summarizes the primary benefits for participating laboratories. It outlines common performance metrics (notably z and related scores), assessment criteria, options for deriving performance standards, the role of corrective actions, and how PT results can be used to monitor long-term performance, compare methods, estimate bias and check measurement uncertainty estimates. Practical guidance for acting on PT outcomes is emphasized to ensure that participation produces real quality gains.

Methodology and performance evaluation

• Assigned value and dispersion: PT schemes define an assigned value (xpt) and a standard deviation for proficiency assessment (spt). Individual laboratory results (xi) are evaluated relative to these parameters.
• Common score: The z score is widely used and calculated as z = (xi − xpt) / spt. Equivalent scoring systems and decision limits are often applied.
• Assessment criteria: Typical thresholds are
  • |z| ≤ 2.0 — satisfactory
  • 2.0 < |z| < 3.0 — questionable (warning signal)
  • |z| ≥ 3.0 — unsatisfactory (action signal)
• Derivation of spt: Providers may set spt from performance-based goals, desirable analytical performance, or the observed data spread. Because a provider’s spt may not suit every laboratory or application, participants can justify and calculate alternative spt values when appropriate.

Corrective actions and follow-up

• Unsatisfactory (action) results trigger an obligation to investigate root causes. Typical steps include checking transcription and calculation errors, re-evaluating method trueness and precision, and searching for systematic or random error sources.
• Corrective measures must be documented and implemented; without corrective action, PT participation provides little benefit.
• Questionable (warning) results should prompt targeted reviews to prevent drift into unsatisfactory performance.

Evaluation of results over time

Regular PT rounds allow plotting performance scores on control charts to detect trends, shifts or increased variability before they affect routine reporting. This longitudinal view complements internal QC and supports decisions such as recalibration, retraining or method revision.

Method comparisons

When PT returns include method details from participants, laboratories can benchmark their methods against alternatives used by peers. This comparison helps identify method-dependent biases, relative robustness across matrices and concentration ranges, and opportunities for method improvement or harmonization.

Use of PT data to estimate bias

• PT can provide practical bias checks when certified reference materials (CRMs) or reference methods are unavailable or non‑commutable for the tested matrix. A reliable assigned value in PT is required to make meaningful bias estimates.
• Repeated PT participation reveals bias variability over time, which can be incorporated into a laboratory’s estimation of measurement uncertainty and method performance review.

Use of PT to check measurement uncertainties

PT supports plausibility checks of a laboratory’s reported measurement uncertainty via the zeta (ζ) score, which accounts for uncertainties of both the laboratory result and the assigned PT value. If zeta or z scores fall outside acceptable limits, this indicates that the laboratory’s uncertainty estimate may be underestimated or that performance does not meet claimed accuracy. Additionally, reproducibility observed across PT participants often exceeds an individual laboratory’s internal uncertainty estimate; large discrepancies should trigger a review of the uncertainty budget.

Demonstration of competence

Consistent satisfactory PT results (|z| ≤ 2.0) serve as documented evidence of analytical competence for customers, accreditation bodies and regulators. PT also delivers educational value by highlighting training needs, method weaknesses and successful interventions in staff competency or process control.

Benefits and practical uses of PT

• Independent verification of method trueness and precision under realistic sample conditions.
• Support for accreditation and regulatory reporting through documented external assessment.
• Tool for continuous improvement — early detection of trends, method comparison and benchmarking.
• Input for measurement uncertainty estimates and risk‑based decision making.
• Evidence for customer confidence and procurement decisions.

Future trends and potential applications

• Improved assigned-value determination using commutable materials and consensus algorithms to better reflect true values across diverse matrices.
• Digitalization of PT workflows and automated data exchange between laboratories and providers to accelerate feedback and analysis.
• Advanced data analytics and machine learning to identify subtle trends, method-dependent biases and systemic issues across scheme participants.
• Expanded use of virtual or inter-laboratory comparisons where physical samples are impractical, complemented by simulated uncertainty and bias assessments.
• Greater harmonization of performance specifications across sectors and increased focus on clinical/consumer impact-oriented criteria rather than purely statistical thresholds.

Conclusion

Proficiency testing is an indispensable element of a robust quality management system in analytical laboratories. Beyond a single performance score, PT provides longitudinal performance monitoring, practical bias and uncertainty insights, method benchmarking and demonstrable competence for stakeholders. To maximize the value of PT, laboratories must participate regularly, investigate and act on unsatisfactory findings, and use PT data proactively to refine methods, training and uncertainty estimates.

References

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