Selecting the right proficiency testing scheme for my laboratory

Significance of the topic

Quality assurance through participation in Proficiency Testing (PT) is a cornerstone of laboratory credibility, regulatory compliance and continual improvement. Choosing the most appropriate PT scheme minimises costs and administrative burden while maximising the relevance of the external assessment to routine laboratory work. A considered selection process supports meaningful interpretation of PT outcomes, informs corrective actions, and strengthens confidence among customers and accreditation bodies.

Objectives and overview of the guidance

The source guidance helps laboratories decide which PT schemes best match their analytical scope and needs. It highlights key selection criteria: match of PT materials to routine samples, statistical design and data collection methods, performance evaluation procedures, provider competence, and quality of communication and support. The document uses practical examples to illustrate common pitfalls and choice trade-offs.

Methodology and recommended decision factors

The guidance outlines decision factors grouped into thematic areas:

• Relevance of PT parameters: assess whether PT matrices, analytes and concentration ranges reflect the laboratory’s routine samples. Consider matrix effects and concentration ranges that could change method performance.
• Data collection and statistical design: evaluate the PT provider’s statistical approach (design, assigned value derivation), number of test items and replicates, procedures for submitting results (web portal, email, etc.), and how comparisons across different methods are handled.
• Performance evaluation method: verify that scoring algorithms (e.g. z-scores) and performance criteria are clearly described and align with the laboratory’s needs, including whether measurement uncertainty is considered in assessment.
• Provider competence and trustworthiness: check accreditation to ISO/IEC 17043, provider track record, transparency and robustness of assigned values, and whether performance criteria are fit for purpose.
• Communication and customer support: ensure the provider offers clear documentation in an accessible language, prompt reporting, ongoing communications (e.g. user meetings), and adequate technical support or customization options.

Main results and discussion

The guidance’s primary conclusion is that not all PT schemes are equally useful for every laboratory; selection must be deliberate. Key discussion points include:

• Matrix and analyte relevancy: Examples show that schemes suitable for drinking water may not be appropriate for industrial waste due to different contaminant levels and matrices. For DNA sequencing PTs, the provider may supply tissue samples or DNA extracts, which assess different steps of the workflow.
• Method comparability: A PT scheme must consider operationally defined methods. For instance, fat determination in complex matrices (milk powder, cereals, feed) can yield different results depending on the extraction or hydrolysis method used; a PT scheme must either separate methods or ensure comparability between them.
• Accounting for measurement uncertainty: Laboratories may want PT evaluations to incorporate their reported measurement uncertainties. If a provider’s scoring does not allow for this, the laboratory must understand the implications for performance assessment.
• Regulatory and accreditation needs: Some laboratories (e.g. running confirmatory analyses) may require stricter performance criteria than screening labs. Check whether scheme criteria meet customer, regulatory and accreditation expectations.

Practical recommendations and benefits

The guidance offers pragmatic steps and benefits:

• Use a decision checklist when evaluating PT schemes: matrix and analyte match, concentration range, statistical design, number of items/replicates, method-specific comparisons, performance scoring and whether uncertainty is used, provider accreditation and experience, reporting turnaround, language and support levels.
• Where no single PT exactly matches routine samples, consider participating with documented limitations or choosing schemes that test critical failure modes of your procedures.
• Selecting the right PT improves the relevance of external quality evidence for accreditation, helps target method development and training, and reduces wasted effort on irrelevant assessments.

Future trends and applications

Emerging directions and potential uses of PT scheme selection include:

• Greater tailoring of PTs to method families and matrices, including method-specific comparisons and harmonised assigned-value approaches for operationally defined methods.
• Increased use of digital data submission platforms and automated statistical feedback to speed results and provide richer diagnostics.
• Integration of uncertainty-aware performance evaluation and Bayesian approaches to better reflect laboratory knowledge and reduce false non-conformances.
• Development of PT schemes for next-generation techniques (e.g. high-throughput sequencing, non-targeted screening) with appropriate design for complex data types.

Conclusion

Choosing an appropriate PT scheme requires aligning scheme design and provider competence with the laboratory’s matrices, methods and regulatory context. Laboratories should evaluate PTs systematically using the factors described: relevance of test items, statistical design, comparability across methods, clarity of performance criteria (including treatment of uncertainty), provider accreditation and communication quality. Doing so increases the value of PT participation for accreditation, quality improvement and customer confidence.

References

• B. Brookman and I. Mann (eds.) Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes (3rd ed. 2021). (Referenced in source material.)