SPC Rules

In 1981 Dr. James Westgard of the University of Wisconsin published an article on laboratory quality control setting the basis for evaluating analytical run quality for medical laboratories. The Westgard system is based on the principles of statistical process control used in manufacturing nationwide since the 1950s. There are six basic rules in the Westgard scheme: 1-3s, 2-2s, R-4s, 1-2s, 4-1s, and 10-x. Use these rules individually or in combination (multi-rule) to evaluate the quality of analytical runs.

The rationale for applying these rules is:

• Reduce false rejections made when applying just the 1-2s rule for run rejection.
• Increase error detection more than provided when applying just the 1-3s rule for run rejection.
• Include rules to detect and distinguish random and systematic error
  (1-3s and R-4s to detect random error and 2-2s, 4-1s, and 10-x to detect systematic error).

Westgard devised a shorthand notation for expressing quality control rules. Most quality control rules can be expressed as N^L, where N represents the number of control observations to be evaluated and ^L represents the statistical limit for evaluating the control observations. Therefore, 1-3s represents a control rule violation when one control observation exceeds the ±3SD control limit.