The application of sigma metrics in the laboratory to assess quality control processes

Van Heerden, Marli
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Introduction: Laboratories monitor and evaluate analytical performance using quality control processes in terms of precision and bias. Sigma metrics can provide an objective manner to assess and compare quality using an additional parameter of total allowable error. The aim of this study was to calculate the sigma metric of analytes when using different sources of total allowable error and suggest Westgard multirules accordingly. Materials and methods: A retrospective analysis was performed on 19 general chemistry analytes at a large academic hospital in Johannesburg. Sigma metrics were calculated on two identical analyzers using internal quality control data and the total allowable error from the Ricos biological variation database as well as three alternative sources (Royal College Pathologists of Australia (RCPA), Clinical Laboratory Improvements Amendment (CLIA), and European Federation Laboratory Medicine (EFLM)). Results: Both analyzers had similar sigma performance. The sigma varied based on the source of total allowable error used with the CLIA guidelines resulting in the best sigma metrics (46% and 53% of analytes achieving acceptable sigma on each analyzer, respectively) and the RCPA guidelines being the most stringent (21% and 23% respectively). Sodium and chloride performed poorly across all guidelines (sigma 23% respectively). Sodium and chloride performed poorly across all guidelines (sigma despite poor performance during certain months. Conclusion: The sigma varies greatly depending on the source of the total allowable error. In a busy laboratory, it could prove to be a valuable tool to save time and decrease costs. There is a clear need for standardization of sigma metrics protocols
A research report submitted to the Faculty of Health Sciences, University of Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Medicine, 2021