How can proficiency testing help my laboratory?

Importance of the topic

Proficiency testing (PT) is a cornerstone of analytical quality assurance for laboratories performing quantitative measurements. Regular participation in PT schemes helps laboratories demonstrate analytical competence, verify method performance across matrices and concentration levels, support accreditation and regulatory compliance, and provide objective evidence for customers and auditors. PT also serves as an external check on internal quality control, highlighting systematic errors, method bias and training needs before they affect routine results.

Objectives and overview of the document

This leaflet focuses on the role and practical benefits of PT for quantitative tests. Its aims are to describe how PT performance is evaluated, how results should be interpreted and acted upon, and how PT data can be used to support estimates of bias and measurement uncertainty. The leaflet summarizes scoring approaches, recommended corrective actions, use of PT for method comparison and long-term performance monitoring, and the value of PT as evidence of competence.

Methodology and performance evaluation

PT providers assign an assigned value (x_pt) for each test item and define a standard deviation for proficiency assessment (s_pt) that forms the basis of performance scoring. The most common performance metric is the z score, calculated as z = (x_i - x_pt) / s_pt, where x_i is the laboratory result. Assessment criteria commonly used are:

• |z| ≤ 2.0: satisfactory
• 2.0 < |z| < 3.0: questionable (warning signal)
• |z| ≥ 3.0: unsatisfactory (action signal)

These thresholds rely on the assumption of approximate normal distribution of results (≈95% within ±2 s_pt, ≈99.7% within ±3 s_pt). PT providers may determine s_pt from desirable performance criteria, regulatory requirements, or from the observed distribution of participant results. Laboratories may justify using an alternative s_pt when the provider’s choice is not appropriate for their intended use, and re-calculate scores accordingly.

Corrective actions

An unsatisfactory score indicates a need for investigation and corrective action. Typical steps include:

• Check for clerical, transcription or calculation errors.
• Review method trueness and precision, including sample preparation and calibration.
• Assess recent quality control charts and instrument maintenance logs.
• Implement corrective measures such as method recalibration, staff retraining, or procedural revisions, and document follow-up verification.

Participation in PT yields limited value if identified problems are not investigated and corrected.

Evaluation of results over time

Regular PT participation allows trend monitoring and early detection of gradual shifts in performance. Plotting successive PT scores on a control chart reveals systematic drift, intermittent problems, or effects of method changes. Combined use of internal QC and PT improves overall surveillance of measurement quality.

Method comparisons

When PT schemes collect method details from participants, reports can be used to benchmark a laboratory’s method against alternatives used by peers. This supports informed choices about method selection, validation needs and potential advantages or limitations of different analytical approaches in routine settings.

Use of PT data to estimate bias and contribution to measurement uncertainty

Although certified reference materials or reference-method comparisons are preferred for establishing bias, PT provides a pragmatic opportunity to evaluate bias under realistic matrix and concentration conditions when CRMs are unavailable or unrepresentative. Repeated PT participation yields information on bias magnitude and its variability, which can contribute to the laboratory’s assessment of measurement uncertainty.

The zeta (ζ) score provides a way to check the plausibility of a laboratory’s reported measurement uncertainty. It compares the laboratory result and its uncertainty with the assigned value and its uncertainty, using the combined standard uncertainty in the denominator. If |ζ| exceeds the usual acceptance limits, this suggests the laboratory’s uncertainty estimate may be underestimated or that the result is inconsistent with the assigned value.

In many cases the reproducibility observed across PT participants will exceed a single laboratory’s internal uncertainty estimate; a large discrepancy should prompt review of the uncertainty model and its components.

Benefits and practical use of PT

• Objective performance benchmarking against peers and acceptance criteria.
• Evidence of competence for customers, accreditation bodies and regulators when satisfactory results are demonstrated.
• Detection of systematic errors, matrix effects and method-specific limitations that may not appear in internal QC alone.
• Input to measurement uncertainty estimation and validation of uncertainty budgets via zeta-score checks.
• Educational value: helps evaluate staff training effectiveness and target further competence development.

Future trends and possibilities for application

PT is likely to evolve along several directions that increase its utility for laboratories:

• More frequent and matrix-diverse PT schemes tailored to emerging analytes and complex sample types.
• Improved statistical methods for assigned value and s_pt estimation, including robust techniques to limit the influence of outliers and heterogeneous method groups.
• Integration of PT results with laboratory information management systems and QC dashboards to automate trend detection and corrective-action workflows.
• Expanded use of PT data in measurement uncertainty frameworks, including formal incorporation of PT-derived bias variability into uncertainty budgets.
• Broader use of interlaboratory comparisons to benchmark novel methods (e.g., high-resolution mass spectrometry, non-target screening) and to support standardisation efforts.

Conclusion

Proficiency testing is a practical, external tool that complements internal quality control, supports accreditation, and helps laboratories maintain and demonstrate measurement quality. Proper interpretation of PT scores, timely corrective actions, and regular participation are essential to realize its full benefits. PT also provides valuable data for assessing bias, validating uncertainty estimates and comparing method performance across laboratories.

References

• Brookman B., Mann I. (eds.): Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes. 3rd edition, 2021.
• Eurachem Proficiency Testing Working Group: Proficiency testing leaflet, Second English edition, July 2022.