Assuring the validity of results
ISO/IEC 17025 clause 7.7 requires laboratories to monitor the validity of their results. Monitoring has two sides: internal quality control within the laboratory and external quality control through proficiency testing (PT) and interlaboratory comparisons (ILC).
Internal quality control
Control samples and control charts, certified reference materials, replicate analyses, blind samples, spike recoveries.
External quality control
PT schemes from providers accredited to ISO/IEC 17043, and interlaboratory comparisons.
Our proficiency testing support
- Scope analysis. Grouping tests, matrices and concentration ranges in your scope.
- Scheme selection. Identifying schemes and providers that best represent your scope.
- Multi-year plan. Every scope group represented at regular intervals.
- Result evaluation. z-scores (or En, zeta), trends and consistency with your uncertainty, following ISO 13528 principles.
- Unsatisfactory results. Root cause analysis, corrective action and verification of effectiveness, all documented.
Common interpretation: |z| ≤ 2 satisfactory, 2 < |z| < 3 questionable, |z| ≥ 3 unsatisfactory. The provider's stated criteria always take precedence.
Internal QC control charts
Tracking control sample results on Shewhart-type control charts catches performance drift early. We define warning and action limits, interpretation rules and what to do when limits are exceeded. The same data can feed the reproducibility component of your measurement uncertainty budget.
Deliverables
- Multi-year proficiency testing plan by scope group
- Result evaluation and trend tracking sheet
- Root cause analyses and corrective action records for unsatisfactory results
- Internal quality control plan and control chart templates
How QC results connect to the management system: laboratory quality management.