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?
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| Which of the Westgard multi-rules does the following control data violate? | View Page |
| Westgard Multi-Rules Quality control charts are examined to see if there are problems in the procedure being tested. The Westgard rules are one tool that can help to determine whether there is a problem, and whether that problem is due to random or systematic error.The six Westgard multi-rules are: 12S rule: this rule applies when at least one result falls more than two standard deviations above or below the mean. This is a signal that the run must be examined in further detail, and does not in itself warrant discarding the run. However, if all of the results are with in 2s, the run should be accepted. 13s rule: this rule applies when a result falls outside of the 3s limit. The run is rejected, and a random error has probably occurred. 22S rule: this rule applies when two consecutive results exceed the +2 or the -2 standard deviation limit. The controls could be normal, abnormal, or one of each. 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. 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. 10x rule: this rule applies when 10 consecutive controls all fall on the same side of the mean, either above or below. This rule detects a systematic error. The run is rejected.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. | View Page |
| Tips on Using the Westgard Rules The Westgard rules can be very helpful in determining errors, but can be confusing. Here are some hints and guidelines on using the Westgard rules: Run at least two controls, one normal and one abnormal. Each should be plotted on its own chart. The Westgard rules call for accepting a run if the control measurements are within 2 standard deviations. However, it is still possible for all measurements to be within this limit, and still violate rules 10x or 41S. You may want to check for violation of these two rules, even if the run passes rule 12S. The 12S rule is meant to simplify and speed up error-checking, and using it may result in fewer errors detected. Visit the www.westgard.com for more information. For the 22S, 41S, and 10x rules, make sure you review the normal controls, the abnormal controls and a combination of the two. For example, the 10x rule applies if the past 3 normal controls and the past 7 abnormal controls have all been above their respective means. For the rules that look back over several runs, it may be necessary to look at the control charts for previous months. The rule that is broken provides a clue as to whether the error was systematic or random. This can aid in diagnosing the problem with the procedure. If any rule is broken, do not report patient results until the problem, if any, has been resolved. Once the problem has been resolved, it may be necessary to redo patient samples from previous runs, especially if the error was systematic. | View Page |
| Which of the Westgard rules have been broken in this control chart?
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| Which of the Westgard rules have been broken in this control chart?
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| Resources for Medical Error Prevention More information about laboratory-related medical errors and ways to prevent them can be found at the following Websites:
JCAHO 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
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| 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. At the left is the Gaussian or "bell-shaped" curve turned on its side to show the correlation of the curve to the chart (i.e. 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.
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| 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. | View Page |
| 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 rule 13s applies and the run is rejected. Troubleshooting must be performed before further testing can be done. | View Page |
| 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 E. 2.7 X SD), 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. | View Page |
| 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? | View Page |