Use of surplus proficiency test items

Use of surplus proficiency test items

Importance of the topic

Proficiency testing (PT) items made available after a PT round can be valuable resources for laboratories. They offer real-sample or representative-materials that help verify method implementation, train staff, support troubleshooting, and in some cases serve as internal quality control (IQC) material. Understanding the benefits and limitations of surplus PT items is important to ensure they are used appropriately and that conclusions drawn from them are valid for routine analytical practice.

Objectives and overview of the guidance

The document provides practical advice to laboratories on how to evaluate and use surplus PT items. It outlines potential applications, key considerations before reuse, the value of accompanying PT data, and presents a case study demonstrating practical implementation for method verification.

Methodology and general considerations

Before using surplus PT items laboratories should perform a focused assessment covering several aspects:

• Check accompanying information: determine whether assigned values, uncertainties and any documentation meet the laboratory’s needs for the intended use (e.g., method verification, IQC).
• Evaluate matrix suitability: identify whether the item closely resembles routine samples or is synthetic/spiked; this affects which stages of the analytical procedure the item can meaningfully challenge.
• Confirm availability and quantity: surplus items are often limited in number, restricting use for repeated monitoring or long-term IQC.
• Assess post-round stability: PT providers ensure stability for the round duration but do not necessarily provide long-term stability data. If no additional stability information is available, laboratories must test stability under intended storage and use conditions prior to extended use.

Usefulness of accompanying PT data

Surplus PT items are more informative when accompanied by PT outcomes and assigned values. Key points to evaluate in the PT data are:

• How the assigned value was derived: consensus values reflect the aggregate of participants and may carry method-dependent biases; values based on known spikes or primary methods provide stronger metrological traceability for bias assessment.
• Performance evaluation criteria: the PT provider’s acceptability thresholds (for example z scores) may or may not match the laboratory’s fitness-for-purpose requirements. If they align, these criteria can inform IQC limits; if not, laboratories must define their own control limits.

Case study — implementing a standard method (EN 15763)

Summary of the example: A laboratory implementing EN 15763 (determination of cadmium mass fraction in foodstuffs by ICP-MS after microwave digestion) obtained five surplus PT items with different matrices and concentration levels. Each item was analysed in duplicate to: (i) check that the mean replicate result lay within the satisfactory performance interval reported for the respective PT round (|z| ≤ 2), and (ii) verify that the difference between replicates did not exceed the repeatability limit r specified in the standard method.
Key outcomes from the PT items (summary):

• Matrices tested included fish muscle (frozen), tomato paste (liquid), chocolate (pellets), bovine liver (frozen) and seaweed (freeze-dried), covering a wide range of sample matrices and cadmium mass fractions (approximately 0.076 to 1.84 mg/kg).
• For each item the laboratory’s average fell within the PT satisfactory range and the intra-sample replicate differences were below the method’s repeatability limits, indicating correct implementation of the method across diverse matrices.
• The case demonstrates how a small panel of surplus PT items can provide practical evidence of method performance for method adoption and staff competence verification.

Main results and discussion

Surplus PT items can effectively support several laboratory activities when their provenance and limitations are considered. They are particularly useful for:

• Verifying correct implementation of standard or in-house methods across representative matrices and concentration ranges.
• Training new analysts with materials that mimic external assessment samples.
• Predicting likely PT performance by comparing results with the PT’s assigned values and reported performance intervals.
• Troubleshooting and reassessing analytical procedures after discrepant PT outcomes.
• Occasionally serving as IQC material if stability and quantity permit.

Limitations and risks:

• Limited sample quantity restricts repeated use and long-term monitoring applications.
• Assigned values based on consensus may not be fully traceable to primary standards, affecting their suitability for bias estimation.
• Post-round stability is not guaranteed; without additional data, long-term use risks sample degradation and misleading control results.

Benefits and practical applications

Practical advantages of using surplus PT items include cost-effective access to well-characterised or representative samples, enhanced confidence during method adoption, improved training realism, and augmented capability for diagnosing method problems. When accompanied by PT statistics (assigned values, performance ranges), surplus items can also help set or adjust control limits and inform method performance expectations.

Used instrumentation

Where instrument details are provided, document them explicitly. In the case study, the analytical technique was inductively coupled plasma mass spectrometry (ICP-MS) following microwave-assisted digestion. This combination addresses trace-level metal determination in complex food matrices and is consistent with the EN 15763 standard.

Future trends and potential uses

Potential developments and opportunities for surplus PT items include:

• Better documentation from PT providers on post-round stability and storage recommendations to extend the utility of surplus items.
• Production of small-volume, matrix-matched IQC materials derived from PT items or PT providers offering dedicated surplus-IQC packs.
• Improved traceability statements for assigned values (e.g., linking consensus values to reference methods or reference materials) to expand the items’ use for bias assessment and method validation.
• Use of surplus items in inter-laboratory method harmonisation exercises and training programs, especially where routine matrix diversity is needed.

Conclusion

Surplus PT items are a useful resource for laboratories when their characteristics, assigned values, and stability are adequately assessed. They are particularly valuable for method verification, analyst training, and targeted troubleshooting. Laboratories must however account for limitations such as limited quantity, uncertain long-term stability, and the origin of assigned values. When used judiciously and with appropriate supplementary evaluation, surplus PT items can enhance quality assurance activities without replacing robust internal QC and method validation practices.

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.