Proficiency Testing interpreting performance

This chapter deals with handling the data generated by analytical methods. The first section describes the key statistical parameters used to summarize and describe data sets. These parameters are important, as they are essential for many of the quality assurance activities described in this book. It is impossible to carry out effective method validation, evaluate measurement uncertainty, construct and interpret control charts or evaluate the data from proficiency testing schemes without some knowledge of basic statistics. This chapter also describes the use of control charts in monitoring the performance of measurements over a period of time. Finally, the concept of measurement uncertainty is introduced. The importance of evaluating uncertainty is explained and a systematic approach to evaluating uncertainty is described. 

This chapter has considered two of the types of interlaboratory comparison exercise in which your laboratory may participate. It is important to remember that proficiency testing schemes and collaborative studies have different aims. The former is a test of the performance of the laboratory, whereas the latter is used to evaluate the performance of a particular analytical method. Laboratories should participate in proficiency testing schemes (where an appropriate scheme is available) as this provides an independent check of the laboratory s performance. This chapter has described the key features of proficiency testing schemes and explained how the results from participation in a scheme should be interpreted. 

Some important assays commonly used in biochemical genetics laboratories do not provide quantitative data (e.g. MPS-EP, qualitative urinary organic acid analysis, AA-TLC). In addition, all successful investigations depend heavily upon selection of the correct analytes to measure and the appropriate interpretation of the quantitative or qualitative results in their clinical context. These challenges suggest a requirement for external quality assessment or proficiency testing schemes that can inform participants about their performance in these areas when compared with other centres. 

The following criteria are designed solely for the purpose of determining performance of laboratories in proficiency testing. Results from these tests will be used by the Director-General for designating laboratories. No such criteria shall be considered as constituting any interpretation of, or precedent for, related provisions of the Convention. 

Post-analytically, schemes are beginning to emerge specifically to compare practice and performance between laboratories pertaining to the interpretation of test results. For instance, in the UK, NEQAS in conjunction with the National Biochemical Genetic network, MetBio.Net, are offering a scheme that provides the opportunity, when given relevant clinical details, to interpret quantitative amino acid results. This proficiency scheme can compare interpretive skills without the need to circulate scarce clinical samples. 

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