Westgard et al have proposed a series of multiple rules (multirules) for interpreting quality control data. The rules are sensitive to random and systemic errors, and they are selected to keep the probability of false rejection low.
Procedure:
(1) Starting with a stable testing system and stable control material, a control material is analyzed for at least 20 different days. This data is used to calculate a mean and standard deviation for the control material.
(2) Usually 2 control materials are analyzed (one with a low value, one with a higher value in the analytical range). Sometimes 3 or more control materials may be used, and rarely only 1.
(3) The controls are included with each analytical run of the test system.
(4) A Levey-Jennings control chart is prepared to graphically represent the data for each control relative to the mean and multiples of the standard deviation.
(5) With each analytical run, the pattern of the current and previous control results are analyzed using all of the selected Westgard control rules.
(6) If none of the rules fail, then the run is accepted. If one or more rules fail, then different responses may occur. This may include rejecting the run, adjusting the stated mean, and/or recalibrating the test.
|
Westgard Control Rule |
Definition |
|
1:2S |
control result is outside +/- 2 standard deviations of the mean |
|
1:3S |
control result is outside +/- 3 standard deviations of the mean |
|
2:2S |
2 consecutive control results are more than 2 standard deviations from the mean |
|
R:4S |
either (a) one control is more than 2 SD above mean and other is more than 2 SD below the mean; or (b) the range between 2 controls exceeds 4 SD |
|
4:1S |
the last 4 consecutive control results are all either 1 SD above or below the mean |
|
10:X |
the last 10 consecutive control results all lie on the same side of the mean |
|
Rule Failure |
Systemic Error |
Random Error |
|
1:3S |
yes |
yes |
|
2:2S |
yes |
|
|
R:4S |
|
yes |
|
4:1S |
yes |
|
|
10:X |
yes |
|
