Use of surplus proficiency test items

Significance of the topic

Surplus proficiency testing (PT) items—leftover test materials provided by PT organizers after a PT round—represent a cost-effective resource for laboratories. When used appropriately they support method verification, analyst training, troubleshooting, internal quality control (IQC) and pre-evaluation of laboratory performance in future PT schemes. Understanding their limitations (quantity, stability, matrix suitability and the nature of assigned values) is essential to avoid incorrect conclusions about measurement performance.

Objectives and overview

This guidance explains potential uses, benefits and limitations of surplus PT items and provides practical considerations for their application. It outlines how accompanying PT data (assigned values, consensus information and performance criteria) can be interpreted and used, and illustrates application with a case study where surplus PT items supported implementation of a standard method for cadmium determination in foodstuffs.

Methodology and practical considerations

• Intended uses: method verification, training of new analysts, troubleshooting, assessing likely PT performance and possible use as IQC material.
• Pre-use checks: availability of assigned values and uncertainties; matrix compatibility (natural vs. synthetic/spiked materials); sample quantity relative to intended repeated use; and evidence of post-round stability or requirements for in-house stability testing.
• Assigned-value interpretation: determine whether the assigned value is a consensus result or traceable to a primary/known spike; consensus-derived values may require caution when assessing method bias.
• Performance criteria: compare PT provider evaluation rules with the laboratory’s fitness-for-purpose criteria. If compatible, provider-based acceptability ranges (e.g., z score thresholds) may be used to set IQC limits; otherwise set independent control limits.

Used instrumentation

Where instrument information was provided by the source, the example procedure combines microwave-assisted digestion followed by inductively coupled plasma mass spectrometry (ICP-MS) for determination of cadmium mass fractions in food matrices, following standard EN 15763.

Main results and discussion (case study summary)

A laboratory implementing EN 15763 analysed five surplus PT items (varied matrices and concentration levels) in duplicate to verify method performance prior to routine use. The evaluation criteria were: (i) mean of duplicates within the PT round’s satisfactory performance interval (corresponding to |z| ≤ 2) and (ii) the observed difference between replicates not exceeding the standard method’s repeatability limit (r). All five materials—representing fish muscle, tomato paste, chocolate, bovine liver and seaweed—met both criteria, supporting correct operation of the digestion and ICP-MS procedure. This demonstrates how surplus PT items can provide effective real-sample checks across matrices and concentration ranges when accompanied by relevant PT statistics.

Benefits and practical uses

• Cost-effective verification: reuse of PT materials reduces the need to purchase or prepare separate verification samples.
• Method implementation: multi-matrix PT items help demonstrate method robustness across real sample types and concentration ranges.
• Training and troubleshooting: identical materials to those used in PT allow reproducible exercises for new analysts and targeted investigation when PT results are questionable.
• IQC potential: when assigned values are metrologically traceable and sufficient quantities exist, surplus items can serve as supplemental IQC materials or to help set control limits.

Limitations and risks

• Limited quantity: often only small numbers of items remain, restricting repeated use or long-term IQC applications.
• Matrix and preparation differences: fortified or synthetic PT items may not exercise all analytical steps equally compared with routine samples.
• Uncertain post-round stability: PT providers ensure stability for the round duration but not necessarily for extended storage; additional stability data or in-house testing may be required.
• Assigned-value origin: consensus values have limited metrological traceability; using them to assess bias requires understanding how they were derived.
• Compatibility of performance criteria: PT acceptability criteria may not match a laboratory’s fitness-for-purpose requirements, necessitating bespoke control limits.

Future trends and potential applications

• Greater documentation: PT providers may increasingly supply post-round stability data and traceability information to extend the utility of surplus items.
• Standardized reuse policies: harmonized guidance on permissible reuse and reporting could expand safe applications of surplus PT materials for IQC and method transfer.
• Digital support: improved metadata (matrix description, assigned-value provenance, uncertainty and commutability indicators) in digital formats will ease risk assessment by laboratories.
• Extended collaborative programs: sharing surplus PT items among accredited labs for method validation networks could become more common, reducing waste and costs.

Conclusion

Surplus PT items are a valuable resource when used with awareness of their limitations. They can efficiently support method verification, analyst training, troubleshooting and, in some cases, IQC implementation. Laboratories must assess matrix suitability, quantity, stability and the provenance of assigned values, and align any use with their fitness-for-purpose criteria. When these checks are performed, surplus PT materials can strengthen routine analytical quality workflows and reduce costs.

References

1. Brookman B., Mann I. (eds.). Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes. 3rd ed., 2021.
2. Eurachem Information Leaflet. How can proficiency testing help my laboratory? 2022.
3. Magnusson B., Örnemark U. (eds.). Eurachem Guide: The Fitness for Purpose of Analytical Methods – A Laboratory Guide to Method Validation and Related Topics. 2nd ed., 2014. ISBN 978-91-87461-59-0.