Quality control charts are examined to see if there are problems in the measuring system. The Westgard multi-rule approach can help to determine whether there is a problem, and whether that problem is due to random or systematic error.
If two controls are used, the Westgard Rules that may be considered for rejecting an analytical run are:
- 13s: This rule applies when a control result falls outside of the 3s limit, either above or below the mean. The run should be rejected. Usually, this indicates that a random error has occurred.
- 22s rule: This rule applies when two consecutive results exceed the +2 or the -2 standard deviation limit. The controls could be normal or abnormal (across runs), or one of each (within a run and both outside the same 2SD). 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 and only applies within a run (ie, not across runs) 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.
- 8xrule: This rule applies when 8 consecutive controls all fall on the same side of the mean, either above or below. The rule could also apply if 4 consecutive controls fall on the same side of the mean with both controls. This rule detects a systematic error. The run is rejected.