How to investigate poor performance in proficiency testing

Investigating Poor Performance in Proficiency Testing: Practical Guidance for Laboratories

Importance of the topic

Proficiency testing (PT) is a cornerstone of laboratory quality assurance and accreditation. A single unsatisfactory PT result may indicate isolated human error, a method or instrument problem, or deficiencies in the quality management system. Prompt, structured investigation of poor PT performance protects patient and customer outcomes, prevents repeat failures, and preserves accreditation status while reducing time and cost from undiagnosed problems.

Objectives and overview of the guidance

This guidance outlines a pragmatic, stepwise approach for investigating unsatisfactory or questionable PT results. It describes criteria for initiating an investigation, recommended analytical and management activities to find root causes, and how to plan and verify corrective actions. The aim is to help laboratories identify technical, clerical or PT-scheme-related causes and to integrate findings into the quality management system.

Methodology for investigating poor PT performance

• Trigger criteria: Investigate every unsatisfactory PT score and establish laboratory-specific thresholds for follow-up on borderline or trending results.
• Tiered, stepwise approach: Use a structured investigation (for example, root-cause tools such as fishbone diagrams or the 5 Whys) and involve the analyst(s) and relevant management or quality personnel.
• Data review: Examine raw analytical data, instrument logs, internal quality control (IQC) records, previous PT rounds and inter-laboratory performance in the same scheme round.
• Planning corrective action: After identifying likely causes, document corrective and preventive actions, assess potential impact on previously reported results and assign responsibility and timelines.
• Execution and verification: Implement actions, record them in the management system, and verify effectiveness through follow-up QC, repeat analysis or subsequent PT participation.

Recommended investigation steps (practical sequence)

1. Collect and review all relevant data: raw results, calibration and maintenance records, reagent lot numbers, analyst notes, and IQC performance around the PT date.
2. Compare with historical performance: check for trends across PT rounds or internal QC deviations that might indicate gradual drift or intermittent failure.
3. Check reporting and clerical processes: transcription, unit conversions, sample labeling and IT system settings that could alter reported values.
4. Assess method and technical execution: review SOP version used, sample preparation, incubation conditions, instrument settings, calibrations and any deviations from validated procedures.
5. Consider PT scheme factors: evaluate commutability of PT items, assigned value appropriateness and possible distribution errors; discuss anomalies with the PT provider when needed.
6. Use root‑cause analysis tools: apply fishbone diagrams or iterative ‘Why?’ questioning to distinguish primary causes from secondary symptoms.

Causes of poor performance (grouped)

• Clerical and reporting errors: unit conversion mistakes, transcription errors, mislabeling, incorrect units or misplaced decimal points. These often reflect weaknesses in reporting or IT integration rather than technical competence.
• Technical and procedural failures: incorrect sample handling, wrong incubation temperature/time, expired reagents, wrong instrument settings, or use of an outdated/incorrect SOP. Where the cause is unclear, review method validation and operator training.
• PT scheme or material issues: non-commutable samples, inappropriate assigned values or PT provider errors. In such cases, engage the PT provider and accreditation contacts to evaluate scheme suitability.

Case studies and illustrative examples

• Example A — Unit-conversion error: A medical laboratory reported tumor marker results 20% off the assigned value while internal QC passed. Root-cause analysis found an incorrect unit conversion introduced during an IT system update; patient reporting used different units so clinical results were unaffected. This highlights the need to validate IT changes affecting result units and reporting templates.
• Example B — Outdated procedure causing microbiology underestimation: A food testing lab underestimated Listeria counts. A 5 Whys sequence traced the issue to an analyst using an out-of-date SOP, because the revised procedure had not been distributed due to overload in document control. The finding pointed to document control workload and distribution processes as corrective targets.

Used instrumentation (summary of items mentioned and typical elements to check)

• Laboratory information systems and IT interfaces: configuration for units, result formatting, and data transfer. Validate any software updates affecting result transformation or reporting.
• Incubators and temperature-controlled equipment: ensure correct temperature and timing controls, calibration and maintenance records.
• Analytical instruments and consumables: instrument calibration, maintenance logs, reagent lot tracking and expiry management.
• Microbiology supplies and counting methods: media batch records, colony enumeration procedures, and timing protocols.

Main results and discussion: what investigations typically reveal

Investigations commonly identify preventable causes: reporting/transfer errors, SOP version control failures, operator deviations from validated methods, or inadequate equipment calibration. Less frequently, PT material problems or scheme design issues are responsible. Effective investigations integrate technical review with quality-management checks (document control, training records, and IT change management) to prevent recurrence. Engaging PT providers early can clarify whether the scheme or sample material contributed to poor results.

Benefits and practical use of the method

• Protects data integrity and client safety by promptly addressing deviations.
• Prevents repeated PT failures and reduces risk to accreditation status.
• Improves laboratory processes through targeted corrective and preventive actions (SOP distribution, IT change control, training, equipment maintenance).
• Provides documented evidence for accreditation bodies and customers showing due diligence in quality management.

Future trends and potential applications

• Digital integration: more robust LIMS and automated QC monitoring will reduce manual transcription and unit-conversion errors and provide real-time alerts for anomalous results.
• Advanced analytics and AI: anomaly detection across QC and PT data streams can flag subtle trends earlier than manual review.
• Improved PT design: greater emphasis on commutability and realistic matrixes will reduce false attribution of poor performance to participants.
• Collaborative networks and data sharing: benchmarking and rapid consultation between laboratories and PT providers will speed identification of systemic PT-scheme issues.

Conclusion

Every unsatisfactory PT result warrants a structured investigation proportionate to the impact and frequency of the problem. A stepwise approach — review raw data and IQC, check clerical/reporting steps, assess technical execution and method validity, and consult the PT provider — maximizes the chance of finding the root cause. Documented corrective actions and verification close the loop and strengthen the laboratory’s quality system, reducing risk to clients and accreditation status.

References

• Brookman B, Mann I (eds). Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes, 3rd ed., 2021.
• Information on PT providers and schemes can be obtained from national accreditation bodies, EPTIS and other national or international organisations.