Sigma Metric as an Essential Tool for Quality Management: Case Study from Serum Total Carbon-dioxide

Abstract

The Six Sigma concept is accepted worldwide as a tool for implementation and improvement of quality management system in the laboratory. The aim of this study was to assess whether Sigma-metric could provide further insights and add more strength of error detection, particularly in the interference sensitive testing area which some errors are not easily identified by traditional quality control practices. Building 1 and SDMC laboratories of Ramathibodi Hospital have participated in Westgard Sigma Verification of Performance Program for 32 clinical chemistry tests. Acceptable analytical performance in medical laboratory should be over or equal to 4-Sigma. All methods had performances of at least 4-Sigma, with the exception of serum total carbon-dioxide (TCO₂) test, which showed the acceptable performance for Building 1 (5.1-Sigma) but not for SDMC (2.2-Sigma). All internal quality control (IQC) and external quality assessment (EQA) data of TCO₂ test were shown within the control limits. After reviewing the possible critical control point events, the laboratory environment, particularly atmospheric CO₂, might be a potential cause of poor performance of TCO₂ measurement in SDMC. Atmospheric CO₂ concentrations over a one-day period of Building 1 and SDMC laboratories ranged from 528 to 831 ppm and 776 to 1917 ppm, respectively. Repeated measurement of TCO₂ in control materials and atmospheric CO₂ from Building 1 were quite stable. Contradictory, the concentration of TCO₂ in control materials at SDMC were clearly increasing during 10am to 4pm with the peak at 12pm which were respectively related to the increase in atmospheric CO₂. In conclusion, Sigma-metric is an essential tool to identify the potential risk of error and to monitor the residual risks of measurement procedure which may not be discovered by the IQC and EQA procedures. Sigma-metric should be more broadly used in clinical laboratories, to further improve and monitor testing performance across total quality management system.