Questions

Now that we've introduced a new set of simplified "Westgard Sigma Rules" with QC Frequency recommendations, it's not surprising questions have arisen.

Questions on QC Frequency

James O. Westgard, PhD
September 2018

Recently, on the AACC Artery Forum, there was a question on how to formulate the frequency of QC. There are some minimum guidelines set forth in CLIA regulations, but given the high volume of many laboratories today, a minimal QC frequency may not be often enough to reduce risk to patients.

Here is the response posted on AACC Artery, for those who aren't members:

"The issue of frequency of SQC is complicated and it isn't possible to discuss all the factors (plus a solution) in a short message. However, there is new information and guidance becoming available in the scientific literature.

General guidance has been to relate frequency of SQC to regulations (maximum time between SQC events of 24 hours or 8 hours, depending on analyte, according to CLIA), known events or changes occurring with the testing process, and the risk of unknown or unexpected events. Other practical factors include mode of operation (critical control points, batch operation, bracketed continuous production), workload, desired reporting interval, criticality of the test, and criticality of the service (e.g, stat vs screening). Specific failure-modes, if known, can be addressed with specific controls in a QC plan.

More quantitative guidance is emerging based on the CLSI C24-Ed4 document, it's "roadmap" for planning SQC strategies, and the utilization of Dr. Parvin's patient risk model. In our latest edition of Basic QC Practices, we cited the work of Yago and Alcover as the beginning of a practical methodology based on graphical tools that reduces the difficulties with applying the patient risk model and performing the risk calculations. Since that time, more information on graphical tools and their application has been provided in the following papers:

• Bayat H, Westgard SA, Westgard JO. Planning risk-based statistical quality control strategies: Graphical tools to support the new Clinical and Laboratory Standards Institute C24-ED4 guidance. J Appl Lab Med 2017;1:211-221.
• Westgard JO, Bayat H, Westgard SA. Planning risk-based SQC schedules for bracketed operation of continuous production analyzers. Clin Chem 2018;64:289-296.
• Westgard SA, Bayat H, Westgard JO. Selecting a risk-based SQC procedure for a HbA1c Total QC plan. J Diab Sci Tech 2018;12:780-785.

Even simpler tools will soon be available, such as Westgard Sigma Rules with Run Sizes that we introduced at the Chicago AACC meeting this summer. That tool provides a graphical diagram that relates the observed sigma quality of a testing process to the control rules, total number of control measurements, and the maximum number of patient samples between SQC events. You can find a download on our website (www.westgard.com). Determination of the sigma quality of your testing process is an essential step, so that's where you should begin if you want to make a more objective selection of SQC strategies.

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James O. Westgard

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