If 3 controls are used, these rules should be considered for rejecting an analytical run:
13s: One control value outside 3 SD.
2 of 3 22s: 2 of the last 3 values across runs are outside the same 2 SD or within a run, 2 of 3 values are outside the same 2 SD.
R4s: This applies if any 2 of the 3 controls within a run exceed 4 SD (One control is >+2 SD and another control within the same run is >-2 SD)
31s: 3 consecutive control values are on the same side of the mean and exceed 1 SD. This could be either across runs for the same control or for 3 controls within the same run.
6x: 6 consecutive values on the same side of the mean.
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. The most commonly used rules are the first three that are listed in both the "2 controls" list on the previous page and the "3 controls" list, above (13s, 22s, and R4s).