Six Sigma in the Lab literature

Posted by Sten Westgard, MS

The most recent issue of Clinical Chemistry and Laboratory Medicine has an article on Sigma-metrics:

Practical application of Sigma Metrics QC Procedures in clinical chemistry, Christian HH Schoenmakers, Andre JM Naus, Henricus J Vermeer, Douwe van Loon and Gerard Steen, Clin Chem Lab Med 2011;49(11):1837-1843.

This study shows how a group of laboratories not only used Sigma-metrics to assess their chemistry tests and optimize their QC procedures, but also generated a table linking specific Sigma-metrics to the appropriate QC procedures.

One interesting finding is that, in a table showing the performance of 4 laboratories with the same instrument, the Sigma-metric results based on imprecision alone were very consistent. For example, if the Sigma-metric of CK for Lab 1 was 19.4, the other three labs had Sigma-metrics of 20, 19, and 18.8. And if the Sigma-metric for Total protein for Lab 1 was 2.4, the other three labs had Sigma-metrics of 2, 3, and 2.3. When bias was factored in, the difference between the Sigma-metrics of the laboratories grew larger, however.This tells us that instrument performance - as we hoped - is pretty consistent from instrument to instrument. However, it appears that there are bigger differences from lab to lab, which is reflected in their different bias values.

Another interesting part of the study is the included list of trouble-shooting actions. Every laboratory knows how to run QC, but once there is a real out-of-control situation, the guidelines on what to do next are not quite as helpful. The list of questions generated by the study include not only analytical considerations, but some practical issues of the lab and instrument:

• "Does the alarm concern one or more tests carried out on the same instrument?"
• "Is it a first alarm, or it it one in a series of alarms?"....
• "Is the correct QC material used?"
• "Is the QC material expired?"
• etc.

This study is an expanded discussion of findings that were first published in a poster at the 2010 Quality in the Spotlight conference, then later published on Westgard Web: Saving money by applying TEa and Six Sigma for internal QC.

Again,the full study is worth reading, not only for its Roadmap of Sigma-metrics to QC rules, but also for the in-depth discussion of the use of Sigma-metrics.