Proficiency testing – How much and how often?

Importance of the topic

The selection of appropriate proficiency testing (PT) schemes and the frequency of participation are central elements of a laboratory's quality assurance strategy. PT participation provides independent external verification of measurement competence, complements internal quality tools (RMs/CRMs, IQC, method validation) and supports accreditation requirements. Determining the right level and cadence of PTs reduces the risk of undetected systematic errors, improves stakeholder confidence in results used for regulatory, forensic or commercial decisions, and helps demonstrate ongoing technical competence during audits.

Objectives and overview of the guidance

This guidance outlines a structured approach for laboratories to define which PT schemes to join (level) and how often (frequency). It advocates an individualized strategy based on the laboratory’s specific technical areas, existing QA measures, risk assessment and operational context. The document emphasizes complementarity of QA tools, the need to identify limitations, regulatory constraints on minimum PT frequency, and the requirement to justify PT decisions during assessments.

Methodology and decision-making framework

Key steps for establishing a PT strategy:

• Map areas of technical competence in terms of: measurement procedure, measurand/characteristic, and product/matrix.
• Inventory existing QA tools: regular use of reference materials (RMs/CRMs), independent method comparisons, involvement in method validation or RM characterisation studies, internal quality control (IQC), and inter-/intralaboratory comparisons.
• Perform a simple risk assessment considering methodological limitations, degree of validation/verification, staff competence and turnover, availability and representativeness of RMs, intended use of results and consequences of erroneous reporting, test workload between PT rounds, and changes in regulatory or performance limits.
• Choose PT schemes that match each area of competence; if equivalent procedures or multiple matrices exist, define separate PT participation for each comparable area.
• Set frequency taking into account risk, workload, staff experience, and availability of suitable PT providers and schemes.

Important practical notes:

• PT schemes may offer flexible participation schedules (e.g., 2, 4, 6 or 12 rounds per year); laboratories should select frequency according to risk and operational needs.
• Where PT participation is impractical, document the reasons and strengthen alternative QA measures.
• For accredited labs, the PT strategy should cover at least the interval between full reassessments and be reviewed annually. Decisions must be technically justified during audits.

Used instrumentation

The guidance discusses instrument-specific contexts through examples rather than prescribing instrumentation. Example techniques mentioned include:

• Quantitative real-time PCR (qPCR) for DNA target detection in food matrices.
• Liquid chromatography–mass spectrometry (LC-MS) and gas chromatography–mass spectrometry (GC-MS) for multi-residue pesticide analysis in fruits and vegetables.
• Inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) for mineral and trace element determinations in biological and food matrices.

These techniques illustrate how differences in analytical procedure or sample preparation (e.g., high vs low water-content matrices, meat/fish vs cereals) define separate areas of technical competence and therefore distinct PT requirements.

Main results and discussion (key points and case studies)

Core conclusions and illustrative cases:

• Areas of technical competence should be defined narrowly enough to ensure PT schemes are relevant and comparable; each area typically requires at least one related PT participation.
• Different measurement procedures, characteristics, or products (e.g., LC-MS vs GC-MS; high vs low water content matrices) may necessitate multiple PT enrolments even for the same laboratory scope.
• Risk factors such as staff turnover or methodological complexity justify higher PT frequency. In the provided case studies, a pesticide laboratory separates four competence areas (LC-MS/GC-MS × high/low water content) and prioritizes PT frequency for the dominant product group. A company with two ICP-MS labs assigns higher PT frequency to the site with less experienced staff.
• Limitations of other QA tools (e.g., non-representative CRMs or unstable IQC material) mean they cannot substitute for PT entirely; the suite of QA measures must be evaluated holistically.

Benefits and practical applications

Practical advantages of a risk-informed PT strategy include:

• Targeted external verification where it most affects result reliability and customer impact.
• Efficient allocation of QA resources—focusing PT participation on high-risk areas and high-volume or high-consequence tests.
• Improved defensibility of laboratory competence during accreditation assessments and audits.
• Enhanced detection of inter-method biases, matrix effects, or training gaps that internal controls alone may miss.

Future trends and potential applications

Emerging considerations likely to affect PT strategies:

• Increased availability of modular and matrix-specific PT schemes (including digital and blinded sample distribution) enabling finer granularity in PT selection.
• Greater use of commutable RMs and synthetic blind samples to better reflect routine sample composition and reduce PT representativeness issues.
• Integration of PT results with laboratory information management systems (LIMS) and statistical process control to automate trend detection and corrective action triggers.
• Growing regulatory expectations and harmonization may set minimum PT frequencies for additional sectors, requiring labs to reassess participation plans.
• Remote or proficiency testing-as-a-service models and interlaboratory data-sharing platforms may broaden access and reduce costs.

Conclusion

Defining how much and how often to participate in proficiency testing requires a structured, risk-based approach tailored to each laboratory’s technical scope, QA portfolio and operational risks. Laboratories should: identify distinct areas of technical competence, evaluate complementary QA tools and their limitations, perform a risk assessment that includes staff competence and consequences of erroneous results, and document a PT strategy that is reviewed regularly and defendable during audits. Appropriate PT participation strengthens confidence in measurement results and supports continual improvement and accreditation compliance.

References

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

Produced by the Eurachem Proficiency Testing Working Group, Second English edition, July 2022.