The Quest for Quality: Westgard Rules of Quality control

In 1981, Dr. James Westgard from the University of Wisconsin wrote an article on laboratory quality control that set the basis for evaluating analytical run quality for medical laboratories. The elements of the Westgard system are based on principles of statistical process control used in industry nationwide since the 1950s. There are six basic Westgard rules in the Westgard scheme. These rules are utilized individually or in combination to know the quality of analytical runs.

Westgard devised a shorthand notation for expressing quality control rules. Most of the quality control rules can be expressed as NL where N stands for the number of control observations to be evaluated and L stands for the statistical limit for evaluating the control observations. Thus 13s represents a control rule that is violated when one control observation is beyond the ±3s control limits.

 The Westgard rules are outlined as follows:

1. Rule 1-2s Definition:

The 1-2s Control Rule talks about one control result going beyond the established mean +/- 2SD range. This is a actually “warning rule,” which does not indicate an “out-of-control” condition, but is used to initiate further testing.

Remember that in the absence of added analytical error, about 4.5% of all quality control results will fall between the 2s and 3s limits. This rule just warns that random error or systematic error might be present in the test system.

The connection between this value and other control results within the current and previous analytical runs must be analyzed. If no relationship can be found and no source of error can be identified, it must be assumed that a single  control value outside the ±2s limits is allowed random error. Patient results can be reported.

Interpretation:

If no other control rule is violated, then the warning is attributed to normal random error. Patient results are acceptable.

Corrective Action:

No corrective action is needed here. But the “warning” simply suggests a system error may be coming up. A comprehensive check of the routine maintenance schedule and review of the quality control ampoule handling and sampling technique is recommended.

2. Rule 1-3s Definition:

The 1-3s Control Rule shows that one control result has gone beyond the established mean +/- 3SD range. This is a “rejection rule,” which is sensitive to random error.

Interpretation:

Excessive random error exists. The analyzer is “out-of-control.” Patient results arecannot  and should be re-analyzed after corrective actions have solved the problem.

Corrective Action:

Re-run the quality control level that is in question, emphasizing proper technique. If the repeated level is within +/- 2SD range then the problem can be because of random error. If the repeated level exceeds the +/- 2SD range, then further corrective action should be conducted. The following are probable causes:

• Inadequate or wrong +/- 2SD range.
• Improper storage temperature correction of quality control results.
• Improper technique when handling the quality control.
• Change of quality control batch.
• Inadequate maintenance of the instrument.

 3. Rule 2-2s Definition:

The 2-2s Control Rule shows that two consecutive control results have are beyond the same mean +/- 2SD limit. This is a “rejection rule,” which is sensitive to systematic errors.
This rule identifies systematic error only. The criteria for violation of this rule are: Two consecutive QC result Greater than 2s on one same side of the mean;

There are two applications to this rule:

• Within-run
• Across runs.

Within-run application:

This affects all control results obtained for the current analytical run. For example, if a normal (Level I) and abnormal (Level II) control are assayed in this run and both levels of control are greater than 2s on the same side of the mean, this run violates the within-run application for systematic error.

Across run application for systematic error is violated:

if Level I is -1s and Level II is +2.5s (a violation of the 12s rule), the Level II result from the previous run must be examined. If Level II in the previous run was at +2.0s or greater, the across run application for systematic error is violated. Violation of the within-run application indicates that systematic error is present and that it affects potentially the entire analytical curve. Violation of the across run application indicates that only a single portion of the analytical curve is affected by the error

Interpretation:

A systematic error exists. The analyzer is “out-of-control.” This may be an early indicator for a “shift” in the mean value. Patient results are not acceptable and should be re-analyzed after corrective action has solved the problem.

Corrective Action:

To resolve systematic errors, corrective action must be conducted to address the following probable causes:

• Inadequate or wrong +/- 2SD range.
• Improper technique when handling the quality control.
• Improper storage temperature correction of the quality control results.
• Change of the quality control batch.
• Inadequate maintenance of the instrument

4. Rule R-4s Definition:

The R-4s Control Rule shows that one result is beyond the mean +/- 2SD limit and the adjacent result has exceeded the mean +/- 2SD limit. This is a “rejection rule,” which is sensitive to random error.

This rule identifies random error only, and is applied only within the current run. If there is at least a 4s difference between control values within a single run, the rule is violated for random error. For example, assume both Level I and Level II have been assayed within the current run. Level I i +2.8s above the mean and Level II is -1.3s below the mean. The total difference between the two control levels is greater than 4s (e.g. [+2.8s – (-1.3s)] = 4.1ss). 

Interpretation:

Excessive random error exists. The analyzer is “out-of-control.” Patient results are not acceptable and should be re-analyzed after corrective action has solved the problem.

Corrective Action:

Repeat the quality control level that is questionable, emphasizing proper technique. If the repeated level is within +/- 2SD range then the problem can be because of random error. If the repeated level is beyond the +/- 2SD range, then further corrective action should be conducted. The following are probable causes:

• Improper technique when handling the quality control.
• Change of quality control batch.
• Inadequate maintenance of the instrument.Inadequate or wrong +/- 2SD range.
• Improper storage temperature correction of quality control results.

5. Rule 4-1s Definition:

The 4-1s Control Rule shows four consecutive control results are beyond the same mean +/- 1SD limit. This is a “rejection rule,” which is sensitive to systematic errors.

Interpretation:

A systematic error exists. The analyzer is “out-of-control.” This may be an early indicator for a “shift” in the mean value. Patient results are not acceptable and should be re-analyzed after corrective action has solved the problem.

Corrective Action:

To resolve systematic errors, corrective action should be conducted to address the following probable causes:

• Inadequate or wrong +/- 2SD range.
• Improper storage temperature correction of the quality control results.
• Change of the quality control batch.
• Inadequate maintenance of the instrument
• Improper technique when handling the quality control.

6. Rule 10-x Definition:

The 10-x Control Rule shows ten consecutive control results have fallen on the same side of the mean. This is a “rejection rule,” which is sensitive to systematic errors.

Interpretation:

A systematic error exists. The analyzer is “out-of-control.” Patient results are not acceptable and should be re-analyzed after corrective action has solved the problem.

Corrective Action:

See Rule 2-2s.

Other Westgard rules

Violation of any of the following rules may not necessarily require rejection of the analytical run. These violations typically identify smaller systematic error or analytical bias that is not often clinically significant or relevant. Analytical bias may be omitted by performing calibration or instrument maintenance.

Rule 3-1s and 4-s

The criteria which must be met to violate this rule are:

• Three consecutive results Greater than 1s On the same side of the mean3
•  Four consecutive results Greater than 1s On the same side of the mean

There are two applications to the 31s and 41s rule. These are within control material (e.g. all Level I control results) or across control materials (e.g.,Level I, II, and III control results in combination). Within control material violations indicate systematic bias in a single area of the method curve while violation of the across control materials application indicates systematic error over a broader concentration