Use of surplus proficiency test items

Significance of the topic

Proficiency testing (PT) materials that remain available after a PT round represent a valuable resource for laboratories. Properly used, surplus PT items support method implementation, analyst training, internal quality control (IQC) and troubleshooting. They can also help laboratories anticipate performance in future PT schemes. Understanding their benefits and limitations is essential to avoid inappropriate use and to ensure reliable interpretation of results.

Objectives and overview of the guidance

This document summarizes practical guidance on the reuse of surplus PT items. It identifies potential uses, outlines general considerations for safe and effective application, describes how accompanying PT data can inform use, and illustrates implementation using a case study where surplus PT items were used to verify a standard method for cadmium determination in food by ICP-MS.

Methodology and general considerations

• Assess accompanying information: Confirm whether assigned values, uncertainties and any provider notes are available and suitable for the intended use.
• Evaluate matrix suitability: Determine whether the PT item is commutable or a fortified/synthetic material. Matrix match affects assessment of pre-analytical and analytical steps.
• Check quantity and availability: Surplus items are often few in number, limiting their use for routine IQC or repeated long-term monitoring.
• Verify stability: PT providers ensure stability for the PT round, but not necessarily for extended storage. Additional stability data from the provider or in-house stability studies may be required before reuse.
• Consider assigned-value metrology: If the assigned value is consensus-based, labs should consider how it was derived and whether it is fit to assess bias. Values based on primary methods or gravimetric spikes have clearer traceability.
• Align performance criteria: Provider performance evaluations (e.g., z scores) might be suitable for setting IQC limits if they match the laboratory’s fitness-for-purpose criteria; otherwise, establish internal criteria.

Usefulness of accompanying PT data

Accompanying PT results and performance evaluations (such as assigned values, uncertainties and participant z scores) add value to surplus PT items by providing context for expected results and performance acceptability ranges. Users should verify the basis of assigned values, the traceability of values, and whether the provider’s acceptability criteria align with the laboratory’s quality objectives before relying on these data for bias assessment or IQC limit-setting.

Case study — implementing a standard method

A laboratory adopted EN 15763 (cadmium in foodstuffs by ICP-MS following microwave digestion) and obtained five surplus PT materials of diverse matrices and concentration levels. Each item was measured in duplicate. Two checks were applied:

1. Mean result within the PT round’s satisfactory range (|z score| ≤ 2).
2. Difference between duplicates not exceeding the method’s repeatability limit (r).

Summary of key outcomes (textual):

• All five matrices (fish muscle, tomato paste, chocolate, bovine liver, seaweed) produced average values that lay within the provider’s satisfactory ranges for their respective PT rounds.
• Observed duplicate differences were all below the method-specific repeatability limits, indicating acceptable within-run precision.
• These checks combined supported that the laboratory had correctly implemented the standard method for these sample types and concentration ranges.

Main results and discussion

The guidance highlights that surplus PT items can effectively validate method implementation and analyst performance when used with appropriate caution. Key discussion points:

• Surplus PT items are particularly useful for one-off verifications (e.g., method adoption) and for training, where traceable assigned values and comparable matrices exist.
• Limited sample numbers and uncertain post-round stability constrain their use as long-term IQC materials unless stability is demonstrated.
• Consensus-based assigned values are informative for comparability but may not be adequate for absolute bias assessment without knowing the value derivation and traceability chain.
• Using PT-derived performance ranges (e.g., z score limits) for internal control requires alignment between the provider’s acceptability criteria and the laboratory’s fitness-for-purpose requirements.

Instrumentation used

The case study specifically references the EN 15763 workflow: microwave digestion for sample preparation followed by analysis using inductively coupled plasma mass spectrometry (ICP-MS) for cadmium mass fraction determination. These techniques address both digestion completeness and sensitive quantification of trace cadmium in diverse food matrices.

Benefits and practical applications

• Method verification: Rapid confirmation that a laboratory can reproduce expected results for a given method and concentration range.
• Analyst training: Real-world matrices with assigned values provide effective training materials for new staff.
• Troubleshooting: Surplus items can help determine whether poor PT performance stems from analytical errors or sample-handling steps.
• Supplemental IQC: Where appropriate and stable, surplus PT items can be integrated into short-term IQC schemes or used to set temporary control limits.
• Risk mitigation: Using surplus items before participating in a PT round can identify method or workflow issues in advance, reducing the risk of poor performance in official schemes.

Future trends and potential applications

• Greater transparency on post-round stability from PT providers could expand the utility of surplus items for longer-term IQC use.
• Standardized metadata for surplus PT items (traceability, value derivation, storage recommendations) would facilitate safer reuse across laboratories.
• Development of digital registries of available surplus items might improve accessibility and planning for method validation or training activities.
• Integration of surplus PT materials into blended quality strategies combining reference materials, in-house controls and PT-derived items for robust performance monitoring.

Conclusion

Surplus PT items are a pragmatic resource for analytical laboratories when used with awareness of their limitations. They are especially useful for method verification, training and troubleshooting. Key prerequisites for safe reuse include verification of matrix suitability, assessment of available assigned-value information and demonstration of material stability for the intended application. When these conditions are met, surplus PT items can strengthen laboratory quality systems and reduce risk prior to participating in formal PT rounds.

Reference

• Brookman B., Mann I. (eds.) Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes, 3rd ed., 2021.
• Eurachem Information Leaflet: How can proficiency testing help my laboratory?, 2022.
• 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.