Proficiency Testing Schemes Flashcards
What are Proficiency testing schemes?
Means of assessing laboratory performance.
Objective & Independent
Required as part of ISO accreditation
Two types of scheme
-Assess the competence of a group of laboratories to perform a specific analysis.
-Evaluate a group of laboratories across a certain sector for a particular type of analysis
PT Standards
ISO 17043:2010 Conformity assessment - general requirements for proficiency testing
ISO 13528:2015 Statistical methods for use in proficiency testing by inter-laboratory comparison
Declared PTs
Labs know that the samples are part of a PT (trial)
Undeclared PTs
Labs do not know samples are part of a trial
How does a PT scheme work?
Providers prepare samples, undertake pre-submission analyses and set performance criteria
Participating labs sent identical samples
Results are submitted to the scheme provider and analysed
Trial results are reported back to the lab
Anonymity and confidentiality is maintained
Labs can seek advice to improve performance
Z-score
Results from a PT scheme are expressed as a Z-score.
Z = (reported result − assigned value)/
target standard deviation
Z scores & Normal distribution
Z scores are based on normal distribution of results.
95% of results will lie within +/-2 standard deviations of the mean.
Comparing multiple samples
Combine Z scores
Rescaled sum of Z scores (RSZ)
sum of Z/root (number of scores)
What RSZ results mean
- An acceptable RSZ score may hide poor individual Z scores (mask poor precision)
Discouraged by ISO 13528
Sources of error in PT schemes
During the setting of the PT
-preparation of sample or test item
-submission of samples or test items to labs (temperature-sensitive samples during transport)
At labs in the scheme:
-sample handling
-sample pre-treatment
-final measurement
-lab facilities and personnel
-incorrect data input
Blind trials test the whole process effectively.
