What Happens When a Laboratory Fails Proficiency Testing?
Understanding what proficiency testing failures mean and how laboratories should respond under ISO/IEC 17025.
By Ashley Dennis • Laboratory Quality Systems Expert
Published March 2026
Category: Laboratory Accreditation | ISO/IEC 17025
Reading time: ~4 minutes
- Proficiency testing failures occur when a laboratory's results fall outside acceptable statistical limits.
- A failed PT result does not automatically mean a laboratory has lost accreditation.
- Laboratories must investigate the root cause and implement corrective actions.
- Proper documentation and corrective action are essential for maintaining ISO/IEC 17025 compliance.
Introduction
Proficiency testing is an essential component of laboratory quality systems operating under ISO/IEC 17025. These programs allow laboratories to compare their analytical performance with other laboratories analyzing the same sample.
Occasionally, laboratories may receive results indicating unsatisfactory performance in a proficiency testing program. While this can be concerning, a failed proficiency testing result does not automatically mean that the laboratory’s accreditation status is at risk.
What matters most is how the laboratory investigates the issue and implements corrective actions.
How Proficiency Testing Results Are Evaluated
Proficiency testing providers typically evaluate laboratory performance using statistical measures such as z-scores or other performance indicators. These scores compare a laboratory’s reported result to the assigned value established by the proficiency testing provider.
Results generally fall into three categories:
- Satisfactory – Results fall within acceptable statistical limits.
- Questionable or Warning – Results are outside optimal limits but not considered failures.
- Unsatisfactory – Results fall outside acceptable limits and indicate a potential problem.
An unsatisfactory result is typically considered a proficiency testing failure.
Does a Failed PT Result Affect Accreditation?
A single failed proficiency testing result does not automatically result in the loss of accreditation. Accreditation bodies recognize that analytical issues can occur and expect laboratories to address them through their corrective action processes.
However, repeated failures or a lack of appropriate investigation may raise concerns during accreditation assessments.
Investigating the Root Cause
When a laboratory receives an unsatisfactory proficiency testing result, the first step is to conduct a structured investigation to determine the root cause of the issue.
Possible causes may include:
- instrument calibration issues
- analytical method errors
- sample preparation mistakes
- data transcription or calculation errors
- analyst training or competency gaps
A thorough root cause analysis helps ensure that the underlying problem is identified rather than simply addressing the immediate issue.
Corrective Actions
Once the root cause has been identified, laboratories must implement corrective actions to prevent the problem from occurring again.
Corrective actions may include:
- recalibrating instruments
- retraining laboratory personnel
- revising analytical procedures
- improving quality control monitoring
- updating documentation or calculation procedures
The effectiveness of the corrective action should be verified to ensure the issue has been resolved.
Documentation Requirements
ISO/IEC 17025 requires laboratories to document investigations and corrective actions related to nonconforming work.
Records should include:
- description of the failed proficiency testing result
- root cause investigation
- corrective actions implemented
- verification of corrective action effectiveness
Clear documentation demonstrates to accreditation assessors that the laboratory maintains control over its quality system.
Conclusion
Proficiency testing failures can occur in any laboratory, but they should be treated as opportunities to evaluate and improve analytical processes. By conducting thorough investigations and implementing effective corrective actions, laboratories can strengthen their quality systems and maintain confidence in their testing results.
Properly addressing proficiency testing failures demonstrates a laboratory’s commitment to quality and supports continued compliance with ISO/IEC 17025 requirements.
Preparing for ISO/IEC 17025 Accreditation?
Lab Accreditation & Quality Partners (LabAQP) helps laboratories strengthen quality systems and prepare for accreditation assessments.
Contact LabAQPRelated Resources
- 7 Common ISO/IEC 17025 Nonconformities Found During Assessments
- What Happens When a Laboratory Fails Proficiency Testing?
- Proficiency Testing vs Internal Quality Control: Why Many ISO/IEC 17025 Laboratories Get This Wrong
About Lab Accreditation & Quality Partners
Lab Accreditation & Quality Partners (LabAQP) provides consulting services to help laboratories develop strong quality systems and achieve ISO/IEC 17025 accreditation.
