Westgard Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Westgard and links to relevant pages within the course.
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|How many standard deviations from the mean would be accepted as being within control limits on a control chart utilizing the Westgard rules:||View Page|
|Which one of the following would not result in a rejection of an analytical run under the Westgard rules:||View Page|
|Suppose you had the following charts for the normal and abnormal controls for a given month:Normal Abnormal Which of the Westgard multi-rules do these measurements break?||View Page|
|Which of the Westgard multi-rules does the control data violate? ||View Page|
|Which of the Westgard multi-rules does the following control data violate? ||View Page|
|Westgard Multi-Rule Approach|
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.
|Responding to Out-of-Control Results|
If any QC rule required by your laboratory's QC program is violated, do not report patient results until the unacceptable result has been investigated and resolved. Once the problem has been resolved, it may be necessary to retest patient samples from previous runs, especially if the error proved to be a systematic error.It is important to document all steps that were taken to resolve the QC error.
|Which of the Westgard rules have been violated according to the control charts shown below that represent 2 levels of daily controls for 10 days? (choose all that apply)||View Page|
|Which of the Westgard rules have been broken in this control chart?||View Page|
|Resources for Medical Error Prevention|
More information about laboratory-related medical errors and ways to prevent them can be found at the following Websites: Joint Commission at http://www.jointcommission.org ASCP at http://www.ascp.org ASCLS at http://www.ascls.org IOM at http://www.iom.edu Westgard.com at http://www.westgard.com
|What is a Levey-Jennings Chart?|
The Levey-Jennings chart usually has the days of the month plotted on the X-axis and the control observations plotted on the Y-axis. On the right is the Gaussian or "bell-shaped" curve turned on its side to show the correlation of the curve to the chart (ie, fewer data points should appear on the upper and lower extremities of the chart, since the "bell" is thinner farther from the mean). By observing the data plotted in the L-J chart, we can determine if test results are in control and accurate, or if test results are not in control and consequently unacceptable. Use of the Westgard, Cumulative Summation Rules, and the Youden plots will help establish an effective error-detecting scheme.
|Using Westgard Rules|
In 1981, Dr. James Westgard and his associates developed a multi-rule procedure for interpreting control data. Since then, a number of sophisticated quality control schemes or analogues based on this multi-rule logic have evolved. To show how the Westgard Rules may be applied in quality control, three of the most common rejection limits will be illustrated in the following pages.
|Westgard Rule 13S|
Westgard rule 13s states that if a control is greater than ± 3 standard deviations from the mean, it should be rejected and rerun. This is because either a random error or a very large systematic error has occurred, as less than 1% of all test values exceed ± 3SD. In the accompanying example, the control for Day 13(noted by the arrow) is greater than +3SD from the mean. Consequently, the 13s rule applies and the run is rejected. Troubleshooting must be performed before further testing can be done.
|What is a Cumulative Summation Limit?|
Like the Westgard Rules, the Cumulative Summation Limit or Rule (CUSUM for short) has different approaches. The CUSUM type used on the following pages is more sensitive to systematic than random error. Nevertheless, it does provide an easy means to detect impending problems. CUSUM is calculated on worksheets like the one below. Basically CUSUM works in the following manner: a decision limit is predetermined (See section E. on the right side of the chart, where CUSUM limit is defined as SD x 2.7), and when the CUSUM of control observations exceed this limit, one must look for error in the testing process. The right side of the worksheet is used to determine the mean, standard deviation (SD), and CUSUM limit.
|CUSUM and Westgard Rules|
Looking at the Levey-Jennings chart you will notice that the plots correspond with the Westgard rule 41s. What type of movement and error do you think this reflects?