Proficiency testing – How much and how often?

Importance of the topic

A clear, evidence-based strategy for participation in proficiency testing (PT) is a cornerstone of laboratory quality assurance. PT provides an independent benchmark of a laboratory’s analytical performance, complements internal controls and reference materials, and is often required by regulators and accreditation bodies. Deciding which PT schemes to use (level) and how frequently to participate (frequency) affects confidence in results, regulatory compliance, and the laboratory’s ability to detect methodological weaknesses or personnel training needs.

Objectives and overview of the guidance

This guidance explains how a laboratory should define a PT participation strategy tailored to its technical activities. It outlines a framework for identifying areas of technical competence, assessing risk factors that influence PT need and cadence, and integrating PT into the laboratory’s overall quality plan. The aim is to ensure laboratories participate in PT schemes that are relevant and sufficient to demonstrate competence for each defined testing area.

Methodology and approach

The recommended approach combines scope definition, capability mapping and a simple risk assessment:

• Define areas of technical competence using three parameters: measurement procedure, measured characteristic, and product/matrix. Equivalent procedures or diverse matrices should be mapped as separate competence areas when they affect analytical performance.


• Assess existing QA measures: routine use of reference materials (RMs/CRMs), internal quality control (IQC), alternative independent methods, method validation/verification, and participation in other interlaboratory comparisons or blind-sample testing.


• Perform a risk assessment addressing method limitations (instrument stability, matrix interferences), extent of validation, staff experience and turnover, RM availability and representativeness, intended use of results (regulatory, forensic, public health), number of routine tests between PT rounds, and complexity or changing requirements that may lower decision/conformity limits.

Instrumentation used

The guidance references typical analytical techniques used as examples of measurement procedures that define PT areas; laboratories should map the specific instruments and sample preparation variants they employ. Representative instruments/procedures cited include:

• Quantitative real-time PCR (qPCR) for nucleic acid targets.
• Liquid chromatography–mass spectrometry (LC-MS) and gas chromatography–mass spectrometry (GC-MS) for multi-residue pesticide analysis.


• Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) for elemental determinations.

Main results and discussion

The guidance emphasizes that PT selection must be proportional to both technical diversity and residual risk. Key points:

• Each distinct area of technical competence should be covered by at least one suitable PT scheme when available. If multiple equivalent procedures cover the same analyte/matrix, a single PT may suffice if it appropriately challenges all relevant procedures.


• Where matrices, preparation techniques or instrumentation differ materially (for example, high-water vs low-water content produce, or different digestion/extraction methods for food vs cereals), the laboratory should treat these as separate competence areas and participate in PT accordingly.


• Frequency should be informed by risk: higher risk (complex methods, high staff turnover, critical public-health or legal consequences) warrants more frequent PT participation. Some providers offer flexible rounds (e.g., 2, 4, 6 or 12 per year) to accommodate different needs.


• Legislation or accreditation requirements may mandate minimum PT frequency for certain tests; these external requirements must be integrated into the laboratory strategy.

Two illustrative case studies are discussed: a pesticide-testing laboratory that splits activities by analytical technique (LC-MS vs GC-MS) and by matrix water content, resulting in four competence areas with variable PT frequency aligned to workload; and a multi-site company using ICP-MS where sites with higher staff turnover adopt higher PT frequency to mitigate increased risk.

Benefits and practical applications

A formal PT strategy provides transparent justification for PT choices during audits, contributes to continuous improvement, and helps ensure fit-for-purpose results. Practical benefits include:

• Independent verification of analytical accuracy and comparability across methods and sites.


• Early detection of method drift, training needs or procedural deficiencies.
• Evidence for regulatory compliance and accreditation assessments.


• Optimized allocation of resources by prioritizing PT participation where risk and impact are greatest.

Future trends and potential applications

Anticipated developments and opportunities include:

• More flexible and modular PT offerings that mirror real-world methodological diversity (e.g., matrix-specific or method-specific rounds).


• Enhanced remote or digital PT approaches, including blinded electronic data challenges and synthetic/sample surrogates, to increase accessibility where physical PT is infeasible.


• Integration of PT results with laboratory information management systems (LIMS) and IQC data to enable automated trend analysis and risk-based scheduling of PT rounds.


• Greater emphasis on multi-technique PTs and commutable reference materials that better reflect routine sample matrices and analytical challenges.

Conclusion

A risk-based, documented PT participation strategy is essential to demonstrate ongoing analytical competence. Laboratories should define competence areas precisely, evaluate their internal QA measures, and select PT schemes and frequencies that reflect methodological diversity, staff competence and the consequences of erroneous results. The strategy should be reviewed regularly and justified during audits; it should also be flexible enough to respond to changing methods, regulatory requirements and operational risk.

References

1. Brookman B., Mann I. (eds.) Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing Schemes, 3rd ed., 2021.
2. EA-4/18 G:2021 Guidance on the level and frequency of proficiency testing participation.