Proficiency testing – How much and how often?

Importance of the Topic

A clear, risk-based strategy for participation in proficiency testing (PT) is essential for laboratories to demonstrate technical competence, maintain accreditation and ensure reliability of measurement results. PT complements other quality assurance measures (reference materials, internal quality control, method validation and interlaboratory comparisons) and provides an independent check on analytical performance that is often required by regulators and customers.

Objectives and Overview of the Guidance

This guidance describes how a laboratory should decide which PT schemes to join (level) and how often (frequency). It explains how to define areas of technical competence, perform a simple risk assessment to set PT priorities, and integrate PT participation into the laboratory quality plan. The guidance emphasizes that selection of PT schemes should be tailored to the laboratory’s methods, products and the intended use of results.

Methodology and Approach

The recommended approach is practical and risk-based: list areas of technical competence, evaluate complementary QA measures, assess risks specific to each area, and then select appropriate PT schemes and participation frequency. Key steps include:

• Define areas of technical competence by three parameters: measurement procedure, characteristic (analyte/parameter) and product (matrix).
• Review current QA tools: use of RMs/CRMs, internal QC, independent procedures, method validation and blind sample checks.
• Identify limitations and gaps in existing QA measures (e.g., lack of matrix-matched CRMs, unstable IQC material).
• Conduct a simple risk assessment considering method limitations, validation extent, staff competence and turnover, consequences of reporting wrong results, number of routine tests between PT rounds, and changes in regulatory limits or method complexity.
• Choose PT schemes that match each defined area of technical competence and set frequency based on risk and operational context.

Instrumentation Used

The document includes typical analytical techniques as examples of measurement procedures laboratories might list when planning PT participation. Examples mentioned are:

• Quantitative real-time PCR (qPCR) for DNA sequence determination.
• Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) for elemental analysis.
• Liquid chromatography–mass spectrometry (LC-MS) and gas chromatography–mass spectrometry (GC-MS) for pesticide residue analysis.

Main Results and Discussion

The guidance does not present experimental data but provides practical conclusions and workflow recommendations: laboratories should participate in at least one PT relevant to each area of technical competence when suitable schemes are available. Frequency should reflect risk and operational priorities: laboratories that test more frequently, handle higher-risk matrices, or have higher staff turnover should participate more often. PT schemes often offer flexible round frequencies (e.g., 2, 4, 6 or 12 rounds/year), allowing alignment with identified risk levels. Legislative requirements may impose minimum PT frequencies in some domains, which must be followed.

Two illustrative case studies highlight application:

• Pesticide testing laboratory dividing competence into four areas based on two analytical platforms (LC-MS, GC-MS) and two matrix types (high vs low water content), choosing higher PT frequency for the predominant matrix type.
• Two-site company using ICP-MS for elemental analysis with different sample preparation by matrix type; the site with less experienced staff adopts a higher PT participation frequency.

Benefits and Practical Applications

Implementing a structured PT strategy delivers multiple benefits:

• Independent verification of measurement performance and comparability across laboratories.
• Targeted allocation of QA resources where measurement risk is highest.
• Support for accreditation and readiness for reassessment audits through documented technical justification of PT choices.
• Early detection of method or personnel-related issues, enabling corrective actions before customer impact.

Future Trends and Applications

Expected developments and opportunities for PT strategies include:

• Greater availability of matrix-matched and commutable reference materials and PT samples to reduce methodological bias.
• More flexible and modular PT offerings (digital reporting, tailored matrix/analyte panels, variable frequency cycles) that better match laboratory risk profiles.
• Integration of PT data with laboratory information systems and statistical dashboards for continuous performance monitoring.
• Increased regulatory alignment and clearer sector-specific guidance on minimum PT requirements.
• Expanded use of proficiency testing in emerging methods (e.g., molecular diagnostics, high-resolution mass spectrometry), requiring development of new schemes.

Conclusion

A laboratory should adopt a documented, risk-based PT strategy that identifies areas of technical competence and selects appropriate PT schemes and frequencies informed by other QA measures and the consequences of erroneous results. The PT plan should be reviewed at least annually and be defensible during audits. Flexible PT offerings and targeted participation ensure efficient use of resources while maintaining measurement credibility and compliance.

References

1. B. Brookman and I. Mann (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.