Evidence-based assessment of the application of Six Sigma to infectious disease serology quality control

Recently, four publications have applied Sigma metrics to infectious disease serology testing. Three studies applied Sigma metrics to assays in a blood bank setting, whereas one used anti-toxoplasma testing in a diagnostic laboratory. One of the four studies applied the approach to microtitre plate enzyme immunoassays, whereas the other three were on fully automated chemiluminescent assay. The approaches used by each study differed in the approach used to calculate imprecision, measured as a percentage coefficient of variation, and bias. All four studies used the difference between the mean of the quality control result and the assay cut-off to describe the total allowable error.

This paper compares and critically reviews the methods used to calculate the Sigma value for each assay in the four studies and provides an evidence-based assessment of these approaches using real-world quality control data obtained from the National Serology Reference Laboratory, Australia (NRL) QConnect™ quality control program.

The assessment found that there was a lack of standardisation in the application of Sigma metrics for infectious disease testing, including examples of inappropriate assumptions being used. Westgard Sigma Rules suggest that assays found to have a Sigma value of six or greater can be monitored using two quality control measures and applying a single mean ± three standard deviation rule.

Although this approach will significantly reduce the number of false rejections generated by using the full range of Westgard rules, we provide evidence that even this approach can potentially cause true errors to be missed.