Proficiency testing (PT) is a system of regular inter-laboratory comparisons that evaluates the performance of individual laboratories for specific tests or measurements. It is a requirement for ISO 17025 accreditation and is integral to laboratory quality assurance programs.
In a typical PT scheme, a reference organization distributes homogeneous, stable samples to participating laboratories. Each laboratory analyzes the samples using its routine methods and reports results within a specified timeframe. The organizing body compares all results statistically and assigns a performance score.
The z-score is the most common performance statistic. It is calculated as the difference between the laboratory’s result and the assigned value, divided by the standard deviation for proficiency assessment. A z-score within ±2 is considered satisfactory, between ±2 and ±3 is questionable, and exceeding ±3 is unsatisfactory.
The assigned value may be determined by consensus of participants after outlier removal, by reference laboratory measurement, or by formulation of the sample preparation. The standard deviation for proficiency assessment may be derived from participant results, from precision models (Horwitz equation), or set as a fixed value based on regulatory requirements.
Internal quality control uses control materials analyzed with each batch to monitor day-to-day consistency. Levey-Jennings charts plot control results over time, with Westgard rules for interpreting control failures. External quality assessment (EQA) includes PT schemes and also encompasses on-site assessments and comparison with reference laboratories.
Unsatisfactory PT results trigger investigation of root causes, corrective action, and re-testing. Common causes include calibration errors, calculation mistakes, reagent deterioration, and operator variation. Laboratories must demonstrate successful performance in subsequent rounds.
PT is available from organizations such as LGC Standards, FAPAS, and national accreditation bodies, covering clinical chemistry, microbiology, food analysis, environmental testing, and many other fields.
