Quality control charts are examined to see if there are problems in the procedure being tested. The Westgard rules are one tool that can help to determine whether there is a problem, and whether that problem is due to random or systematic error.
The six Westgard multi-rules are:
- 12S rule: this rule applies when at least one result falls more than two standard deviations above or below the mean. This is a signal that the run must be examined in further detail, and does not in itself warrant discarding the run. However, if all of the results are with in 2s, the run should be accepted.
- 13s rule: this rule applies when a result falls outside of the 3s limit. The run is rejected, and a random error has probably occurred.
- 22S rule: this rule applies when two consecutive results exceed the +2 or the -2 standard deviation limit. The controls could be normal, abnormal, or one of each. A violation of this rule usually indicates a systematic error. The run is rejected.
- R4S rule: this rule applies when the difference between the highest and lowest result of a run exceeds 4 standard deviations. This rule detects random errors. The run is rejected.
- 41S rule: this rule applies when four consecutive control samples all exceed the +1 or the -1 limit. The controls could be normal, abnormal, or a combination of the two. This rule detects systematic errors. The run is rejected.
- 10x rule: this rule applies when 10 consecutive controls all fall on the same side of the mean, either above or below. This rule detects a systematic error. The run is rejected.
Some labs choose not to use all of the Westgard rules; however, it is recommended that all labs use at least two rules, one that can detect systematic error and one that can detect random error.
