Uncertainty in proficiency testing schemes: KBUDEK experience

Abstract

Objectives In Eurochem CITAC Guide CG4, uncertainty is defined as “a parameter associated with the result of a measurement that characterizes the dispersion of the values attributed to the measurand”. In proficiency testing schemes, the assigned value has a standard uncertainty which arises from the method used to determine it. In several schemes, consensus values from participants will be used (ISO/IEC Guide 43-1). In this situation, assigned value is based on the consensus value of the participants. As a result, calculated assigned values have an uncertainty originating from the testing conditions of laboratories and other factors. On the other hand, standard deviation of the proficiency test is used to estimate the size of the laboratory bias. In the case where the standard uncertainty of the assigned value is too large in proportion to standard deviation of the scheme, some of the participants would receive false action and warning signals. According to ISO/IEC Guide 13528, if the uncertainty of the assigned value is smaller than the 0.3 times of the standard deviation of the scheme, there is no need to include the uncertainty in the interpretation of the results of the proficiency scheme. Otherwise, uncertainty of the assigned value should be considered in the evaluation of the results. The way in which assigned value is obtained should be checked and En numbers or z'-score should be used instead of z -score (SDI). Design & methods In this study, we checked the standard uncertainty of the robust average of calcium results of KBUDEK, which is an external quality control program of KBUD (Association of Clinical Chemists) in Turkey. Results We found that assigned robust mean was 13.1 mg/dL, robust standard deviation was 0.63 mg/dL and standard uncertainty was 0.11mg/dL. So, 0.11 is smaller than 0.19 (0.3 x 0.63). Conclusion Thus, there was no need to use standard uncertainty of the assigned mean in the calculation of performance scores.