Sigma metrics used to assess analytical quality of clinical chemistry assays: Importance of the allowable total error (TEa) target.

Dave Armbruster1,*, Sten Anders Westgard2

1Global Scientific Affairs, Abbott Diagnostics, Lake Villa, IL, 2Westgard QC, Madison, WI, United States

Introduction: Introduction.  Six Sigma metrics were used to assess the analytical quality objectively and quantitatively.   Sigma metrics provide such a parameter, combining total error allowable (TEa), bias, and precision.                                                                                                 

Objectives: Objectives. Estimate Sigma metrics for clinical chemistry and immunoassay tests on multiple analyzers and compare quality among/between analyzers and assess the impact of different TEa goals.                                                                                                                              

Methods: Methods.  Sigma metrics estimate quality using TEa, bias, and precision.  Sigma metrics were calculated for 41 assays on five ARCHITECT c16000 analyzers.  Two controls were tested to calculate precision and bias, and Sigma metrics were calculated using three different TEa targets (Ricos biological variability, CLIA, and RiliBÄK) using the following equation: 
                                    Sigma = (TEa – bias)/CV (all values expressed as %)                                                                                                                                                         

Results: Results.  Sigma metrics varied with analyte concentration, TEa target, and between/among analyzers.  Sigma values identified assays requiring minimal QC and assays requiring more complex QC. 

Conclusion: Conclusions.  Sigma metrics measure quality and optimize QC rules.  Lack of TEa targets and variability of TEa from different sources can cause inconsistent estimates of Sigma for the same analyte.  Architect analyzers demonstrated generally high Sigma values and comparable analyzer-to-analyzer performance. Sigma metrics can compare the analytical quality of two or more analyzers to ensure comparability of patient test results.

Keywords: Laboratory organization