Westguard-rules.doc

What are the "Westgard rules"?
For convenience, we adopt a short hand notation to abbreviate different decision criteria or control rules, e.g., 12s to indicate 1 control measurement exceeding 2s control limits. We prefer to use subscripts to indicate the control limits, but other texts and papers may use somewhat different notation (e.g. 1:2s rather than 12s) Combinations of rules are generally indicated by using a "slash" mark (/) between control rules, e.g. 13s/22s.
The individual rule are defined below. The "thumbnail" graphic next to a rule shows an example of control results that violate that rule. You can click on a graphic to get a larger picture that more clearly illustrates the application of each control rule.
13s refers to a control rule that is commonly used with a Levey-Jennings chart when the control limits are set as the mean plus 3s and the mean minus 3s. A run is rejected when a single control measurement exceeds the mean plus 3s or the mean minus 3s control limit.

12s refers to the control rule that is commonly used with a Levey-Jennings chart when the control limits are set as the mean plus/minus 2s. In the original Westgard multirule QC procedure, this rule is used as a warning rule to trigger careful inspection of the control data by the following rejection rules.

22s - reject when 2 consecutive control measurements exceed the same mean plus 2s or the same mean minus 2s control limit.

R4s - reject when 1 control measurement in a group exceeds the mean plus 2s and another exceeds the mean minus 2s.

41s - reject when 4 consecutive control measurements exceed the same mean plus 1s or the same mean minus 1s control limit. 

10x - reject when 10 consecutive control measurements fall on one side of the mean.

In addition, you will sometimes see some modifications of this last rule to make it fit more easily with Ns of 4:
8x - reject when 8 consecutive control measurements fall on one side of the mean.

12x - reject when 12 consecutive control measurements fall on one side of the mean.

The preceding control rules are usually used with N's of 2 or 4, which means they are appropriate when two different control materials are measured 1 or 2 times per material.
What are other common multirules?
In situations where 3 different control materials are being analyzed, some other control rules fit better and are easier to apply, such as:
2of32s - reject when 2 out of 3 control measurements exceed the same mean plus 2s or mean minus 2s control limit;

31s - reject when 3 consecutive control measurements exceed the same mean plus 1s or mean minus 1s control limit.

6x - reject when 6 consecutive control measurements fall on one side of the mean. 

In addition, you will sometimes see some modification of this last rule to include a larger number of control measurements that still fit with an N of 3: 
9x - reject when 9 consecutive control measurements fall on one side of the mean. 

A related control rule that is sometimes used, particularly in Europe, looks for a "trend" where several control measurements in a row are increasing or decreasing:
7T - reject when seven control measurements trend in the same direction, i.e., get progressively higher or progressively lower.

How do you perform multirule QC?
You collect your control measurements in the same way as you would for a regular Levey-Jennings control chart. You establish the means and standard deviations of the control materials in the same way. All that's changed are the control limits and the interpretation of the data, so multirule QC is really not that hard to do! For manual application, draw lines on the Levey-Jennings chart at the mean plus/minus 3s, plus/minus 2s, and plus/minus 1s. See QC - The Levey Jennings chart for more information about preparing control charts.
In manual applications, a 12s rule should be used as a warning to trigger application of the other rules, thus anytime a single measurement exceeds a 2s control limit, you respond by inspecting the control data using the other rules. It's like a warning sign at the intersection of two roads. It doesn't mean stop, it means look carefully before proceeding.
How do you look carefully? Use the other control rules to inspect the control points. Stop if a single point exceeds a 3s limit. Stop if two points in a row exceed the same 2s limit. Stop if one point in the group exceeds a plus 2s limit and another exceeds a minus 2s limit. Because N must be at least 2 to satisfy USA CLIA QC requirements, all these rules can be applied within a run. Often the 41s and 10x must be used across runs in order to get the number of control measurements needed to apply the rules. A 41s violation occurs whenever 4 consecutive points exceed the same 1s limit. These 4 may be from one control material or they may also be the last 2 points from a high level control material and the last 2 points from a normal level control material, thus the rule may also be applied across materials. The 10x rule usually has to be applied across runs and often across materials.
Computer applications don't need to use the 12s warning rule. You should be able to select the individual rejection rules on a test-by-test basis to optimize the performance of the QC procedure on the basis of the precision and accuracy observed for each analytical method and the quality required by the test.