Proficiency testing (PT) plays a central role in laboratory quality assurance. It provides an external and independent assessment of performance by comparing the results of one laboratory with those of other participants under predefined conditions. Its main objective is to help the participant assess the validity of its measurements, while also reinforcing confidence in technical competence, continuous improvement and, where applicable, compliance with accreditation requirements.
For laboratories working under standards such as ISO/IEC 17025 or ISO 15189, participation in appropriate PT schemes, or in suitable interlaboratory comparisons where no PT is available, is an important part of demonstrating competence. But PT should not be treated as a simple box-ticking exercise. Its real value lies in selecting the right scheme, using it properly and interpreting the results in a way that supports real improvement, not just apparent compliance.
Why proficiency testing is important
A laboratory needs more than internal controls to demonstrate that its results remain valid over time. PT adds an external and objective layer of assurance by enabling comparison with peer laboratories and with assigned values or other performance criteria. In addition, PT schemes may cover pre-analytical and post-analytical phases, not only the analytical phase, which makes them a broader quality tool than many laboratories initially assume.
PT can help laboratories in different ways, including:
- identifying measurement problems that internal control may not detect;
- comparing methods, operators or analytical systems;
- supporting staff training and learning;
- reviewing aspects related to measurement uncertainty;
- building confidence among management, customers, regulators and accreditation bodies;
- and improving performance over time through investigation and corrective actions.
That last point matters much more than laboratories sometimes like to admit. A PT result is not just a score. It is evidence, and evidence tends to become very interesting when someone questions the competence of the laboratory.
How to select proficiency testing schemes
Choosing a PT scheme does not simply mean finding one with a familiar name and signing up. Before participating, laboratories should define a PT participation strategy, including the level and frequency of participation, based on their areas of technical competence and the risks associated with their activity.
Some of the key questions a laboratory should ask are:
- What level of PT participation does it need?
- How often should it participate?
- Is the PT scheme relevant to its routine work?
- Is the PT provider competent, ideally in accordance with ISO/IEC 17043?
- Is the scheme independent from manufacturing or marketing interests in the relevant field?
PT participation should be considered together with other quality assurance measures, such as certified reference materials, internal quality control, method validation or other interlaboratory comparisons. In some sectors, PT may only be feasible for part of the measurement procedure, which makes an overall quality strategy even more important.
What makes a PT scheme fit for its purpose
A PT scheme should resemble the laboratory’s routine work as closely as possible. This includes factors such as the matrix, analyte, concentration levels, reporting units and the nature of the measurement procedure. The closer the scheme is to real work, the more meaningful the results will be.
When assessing whether a PT scheme is fit for purpose, the laboratory should consider issues such as:
- whether the PT items are relevant to its routine samples;
- whether the concentration levels are appropriate;
- whether the participant base and peer group are suitable;
- whether the distribution schedule and frequency fit its operational needs;
- whether result reporting is practical and aligned with routine practice;
- and whether the statistical design and evaluation system are clearly described and appropriate.
A structured approach to this selection helps review key blocks such as the type of PT item, the participant profile, distribution, results, reports and provider competence. It is the kind of checklist laboratories should always use, instead of deciding based on whatever happens to seem available first.
How to use proficiency testing results
Participating in PT only has real value if the laboratory uses the result properly. PT is not about “passing” or “failing”, but about learning from the result and using that information to improve the quality of measurements. A satisfactory result does not automatically demonstrate a high level of competence, and a single unsatisfactory result does not automatically prove the opposite. What matters is the quality of the interpretation and the response.
Therefore, each PT round should be reviewed carefully. The laboratory should not focus only on the performance score, but also on the wider context, including:
- the behaviour of the peer group;
- the suitability of the statistical model used;
- whether the PT result is consistent with the laboratory’s own quality control data;
- and whether the PT scheme is truly relevant to the measurement in question.
In continuous PT schemes, performance over time is especially important. Monitoring trends across several rounds makes it possible to identify persistent bias, imprecision problems or human errors that an isolated result may not reveal.
Understanding PT scores and performance
PT providers may use different approaches to evaluate performance, but in quantitative results there are two basic elements: the assigned value and the standard deviation for proficiency assessment (SDPA). It is also common to use scores such as z, z’, zeta and En, each with slightly different implications depending on whether uncertainty is taken into account.
For z, z’ and zeta scores, the usual interpretation is:
- |score| ≤ 2.0: satisfactory
- 2.0 < |score| < 3.0: questionable
- |score| ≥ 3.0: unsatisfactory
This sounds very neat on paper, but interpretation should not be made automatically. A score only makes sense if the PT scheme is fit for purpose and if the evaluation model used is genuinely appropriate for the laboratory’s activity.
Investigation of questionable or unsatisfactory PT results
Every laboratory will from time to time receive a questionable or unsatisfactory PT result. What matters is not ignoring it, but investigating it properly. Every unsatisfactory result should be analysed, and repeated questionable results or consistent bias should also trigger a review.
Possible causes may include:
- administrative errors, such as incorrect transcription or wrong units;
- technical problems in sample preparation, calibration, equipment, reagents or data processing;
- failures in internal quality control;
- inadequate selection of the PT scheme;
- stability or homogeneity problems in the PT items;
- or incidents originating from the PT provider itself.
A poor result, if well managed, becomes a risk management and improvement tool. If badly managed, it becomes a recurring problem with documentation attached.
2026 agenda: current programmes
As part of SHAPYPRO’s planning for 2026, several proficiency testing exercises are already scheduled. To consult the full calendar, the 2026 agenda can be downloaded at the following link: https://shapypro.com/es/2026-agenda-esp/
The programmes currently in progress are:
- EN1656 – P. aeruginosa
- EN1657 – C. albicans
- EN1276 – S. aureus
- EN1650 – A. brasiliensis
- EN13704 – B. cereus
Integrating this type of calendar into the PT participation strategy is important because participation is not only about choosing the right scheme in theory, but also about planning in time, allocating resources and ensuring that the laboratory can generate, review and use the results within its quality cycle.
How SHAPYPRO can help
In this context, SHAPYPRO can help laboratories in the selection, use and interpretation of PT schemes, assessing whether a scheme is truly relevant to their scope, technical competence and quality objectives.
This may include reviewing the participation strategy, identifying the most appropriate schemes, interpreting PT results from a technical and regulatory perspective, and supporting the investigation of questionable or unsatisfactory results. In practice, this means turning PT participation into a meaningful quality tool, rather than just another deadline the laboratory drags along until an auditor asks about it.
Conclusion
The selection of and participation in a PT scheme should never be reduced to a compliance routine. PT is most valuable when laboratories choose schemes that are fit for purpose, interpret results with judgement, properly investigate poor performance and use the process to improve their competence over time. Proficiency testing is not simply about obtaining a score, but about reinforcing confidence in the validity of measurements and strengthening the overall management system.
For laboratories working in regulated or accredited environments, that difference matters. A PT scheme is not useful just because it exists. It is useful when it is relevant, well chosen and used intelligently. Which, unfortunately for human convenience, requires judgement.