Proficiency testing and interlaboratory comparisons: how should laboratories use PT and ILC?

What is laboratory proficiency testing and what does it evaluate?

Proficiency testing, or PT, evaluates participant performance against criteria established for a scheme. It can provide external information about laboratory results in the activity examined. Before participation, understand the item, measurand, reporting arrangements, and performance evaluation method. One exercise represents its defined conditions and does not, by itself, prove that every laboratory method works correctly. Its professional value comes from reviewing the report, understanding limitations, and using the findings within the quality program. Participation should therefore support confidence in results rather than end with filing a certificate or displaying a favorable score without explaining what that evidence means for the laboratory's routine work.

What is the difference between PT and an interlaboratory comparison?

An interlaboratory comparison involves measurements or tests by different laboratories on suitable items under predetermined conditions. Proficiency testing uses such comparisons to evaluate performance against defined criteria. Other comparisons can serve different purposes, so the terms are not always interchangeable. When selecting an activity, examine whether it meets the technical objective and applicable expectations. Identify the organizer, study design, and conclusions the activity can support. Its name alone is insufficient. Ask about the comparison basis, item handling, and treatment of differences between methods or conditions that may affect participant results. This helps establish what information the exercise can legitimately add to the laboratory's existing evidence.

How often should an ISO 17025 laboratory participate in proficiency testing?

Do not select a universal frequency without reviewing applicable requirements and the laboratory's circumstances. Consider disciplines, risks, activity volume, changes, available schemes, and prior performance. Check the relevant accreditation body's policy. A participation plan should explain coverage, gaps, and the reasoning behind decisions. If a suitable scheme is unavailable, investigate appropriate alternatives and acceptance requirements with the relevant parties, retaining the rationale. A calendar alone is not an assurance program. Connect participation with the information needed to evaluate the validity of routine results across the laboratory's actual activities. Review the plan when scope or operating conditions change rather than repeating the previous year's schedule automatically.

How should a suitable PT provider and scheme be selected?

Review the discipline, item or matrix, measurement levels, evaluation approach, and timing. Consider provider competence and available scheme information in line with applicable requirements. An exercise that does not represent the laboratory's work may provide limited evidence even when performance is satisfactory. Connect selection with the application established through method validation and verification. Before registering, confirm that items can be handled correctly and records retained. Assign execution, reporting review, and final interpretation responsibilities. This prevents a familiar gap in which staff complete the technical work and submit a result, but nobody is responsible for examining the report or acting on its implications.

How should PT results be produced and reported under controlled conditions?

Follow scheme instructions and use arrangements representative of the relevant laboratory activity. Retain identification, handling conditions, raw data, calculations, and method version. Review units, significant figures, and uncertainty information where requested. A transcription error can affect evaluation, so define a reporting check before submission. Do not adjust results using unauthorized information from another participant or alter records to obtain a better score. The purpose is to evaluate actual performance. Record unusual events and follow the scheme's communication process. This allows later interpretation to distinguish an analytical issue from a reporting mistake or an item handling condition that differed from the stated instructions.

What should happen after an unsatisfactory proficiency testing result?

Read the report and evaluation method before deciding what went wrong. Review reporting, units, calculations, item handling, equipment, and method performance. Consider whether routine customer results might also be affected. Respond appropriately to the evidence, including the laboratory's arrangements for nonconforming work where relevant. Simply promising to participate again is not a cause analysis. Record investigation, action, and effectiveness review. The article on ISO 17025 internal audit and management review explains how findings become tracked decisions. If the evidence suggests a broader problem, evaluate consequences and any necessary communication under the applicable agreements and requirements rather than treating the exercise as an isolated administrative issue.

How do PT and ILC relate to measurement uncertainty and result validity?

Comparison data can inform performance evaluation, but using them in uncertainty estimation requires technical justification and understanding of limitations. A score should not automatically become a numerical uncertainty contribution. The guide to measurement uncertainty budgets explains connecting data with models and assumptions. Examine trends alongside internal quality control and other relevant evidence. Consistent results can strengthen the picture, while a change may justify targeted investigation. The objective is to understand what each source contributes. External comparison does not replace employee competence, equipment control, or evaluation of method suitability for the actual activities included in the laboratory's service and accreditation scope.

How can PT planning support accreditation and laboratory management?

The accreditation body evaluates participation under its applicable policy and the wider evidence of competence. An unsatisfactory result does not automatically determine every accreditation outcome, and a satisfactory one does not guarantee recognition. Demonstrate suitable planning, participation, interpretation, and response. Bring together schemes, reports, and actions to identify areas requiring support. The safety consultant of D.S.Consults assists laboratories with quality systems and accreditation preparation. Contact the safety consultant to review PT and ILC arrangements within your result validity program. A focused review can connect participation with decisions about training, methods, and resources, giving management a clearer understanding of what the available evidence supports and which gaps remain.