How to investigate poor performance in proficiency testing

Significance of the topic

Proficiency testing (PT) is a key element of laboratory quality assurance. A single poor PT result can indicate weaknesses in analytical practice, reporting, or the appropriateness of a PT scheme itself. Prompt, systematic investigation preserves confidence in results, prevents recurrence, and reduces the risk of reporting incorrect data to clients or regulators.

Objectives and overview of the guidance

This guidance explains how a laboratory should respond to unsatisfactory PT performance. It describes a pragmatic, stepwise investigation process to identify root causes, implement corrective actions, and document outcomes. The approach balances depth of investigation with the significance and frequency of the failure, and encourages discussion with PT providers when relevant.

Investigative methodology

The recommended investigation follows a structured sequence that involves the analysts and, where appropriate, laboratory management. Key elements are:

1. Review raw analytical data and internal quality control (IQC) records, including trends from previous PT rounds and the distribution of participant results in the current round.
2. Determine whether the PT report offers an explanation; if not, proceed with a stepwise root-cause analysis (see below).
3. Develop a corrective action plan and assess potential implications for previously reported patient or client results.
4. Implement and record corrective actions.
5. Verify effectiveness of actions and document closure of the investigation.

Recommended root-cause tools include Fishbone (Ishikawa) diagrams and the 5 Whys technique. The depth of analysis should reflect the analytical importance (e.g., clinical vs. non-critical assays), the frequency of problems, and evidence of trends.

Common causes of poor PT performance

Causes typically fall into three categories:

• Clerical errors: transcription mistakes, mislabelling, incorrect units or decimal-point errors. These do not necessarily reflect technical incompetence but do indicate weaknesses in result handling and reporting procedures.
• Technical problems: errors in any step of the analytical process (sample handling, preparation, incubation conditions, reagent preparation, calibration, instrument settings). When the immediate cause cannot be found, review of method validation and procedures is recommended.
• PT scheme or sample issues: inappropriate scheme selection, matrix incompatibility, or problems with PT items themselves. Laboratories should liaise with their PT provider if PT materials or scheme design may have contributed to the result.

Case studies

Example 1: A clinical laboratory reported tumor marker results that deviated by ~20% from the assigned value despite satisfactory daily IQC. Investigation identified an incorrect unit conversion introduced during an IT system update that affected PT reporting only; patient reports used a different unit and were not impacted. The corrective action centered on IT validation and change-control for result conversion logic.

Example 2: A food laboratory underestimated Listeria counts because an analyst incubated cultures at incorrect time/temperature. A 5 Whys analysis traced the problem to use of an outdated procedure that had not been redistributed following a revision; the document control workload had allowed the revision to be missed. Corrective measures included updating document distribution, training, and reinforcing procedure version checks.

Main results and discussion

The guidance emphasizes that each unsatisfactory PT score warrants investigation, while recognizing that not every questionable indicator requires the same depth of analysis. Effective investigations focus on data review, verification of procedures and IT systems, assessment of personnel competencies, and communication with PT providers. Documentation and verification of corrective actions are critical to demonstrate continuous improvement and to satisfy accreditation requirements.

Practical recommendations and workflow

Practical steps for laboratories:

1. Establish threshold criteria for initiating investigations (e.g., unsatisfactory scores, trends, or recurring borderline results).
2. Immediately secure and review raw data, IQC records, instrument logs, and relevant SOPs.
3. Engage the analyst(s) and supervisor in a root-cause analysis using structured tools.
4. Consider whether previous reported results could be affected and plan notifications or re-analyses if necessary.
5. Define, implement, and record corrective and preventive actions (training, procedure updates, IT change control, instrument maintenance).
6. Monitor subsequent PT rounds and IQC to confirm effectiveness.

Benefits and practical applications

Systematic handling of poor PT performance reduces the risk of recurring errors, strengthens confidence in reported results, and supports accreditation compliance. Laboratories benefit from improved document control, clearer responsibilities for IT changes, better training and competency management, and enhanced dialogue with PT providers to ensure scheme suitability.

Future trends and potential applications

Emerging directions that can improve PT investigations include:

• Digital change-control and automated alerts to prevent IT-induced reporting errors.
• Stronger integration of PT outcomes with laboratory information management systems (LIMS) and IQC dashboards for faster trend detection.
• Use of inter-laboratory data analytics and machine learning to detect subtle patterns and predict risk of poor performance.
• More flexible PT schemes and commutable materials tailored to modern methods (e.g., molecular assays, mass spectrometry).

Conclusion

Poor performance in proficiency testing is an opportunity to reinforce quality systems. A measured, documented investigation that considers clerical, technical, and scheme-related causes will identify appropriate corrective measures, protect past and future reporting, and contribute to continual improvement.

Reference

B. Brookman and I. Mann (eds.), Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes, 3rd ed., 2021.