Levey-Jennings control charts

To use a Levey-Jennings control chart, follow these steps ... 

Figure 19-12 Levey-Jennings control chart having control limits set as the mean 3 s. Concentration is plotted on the y-axis versus time (run number) on the x-axis.

It is often helpful to record the results of control samples in a visible manner not only because of the greater impact of a visual display but also for the relative ease with which it is possible to forecast trends. A variety of styles of quality control charts have been suggested but the most commonly used are those known as Levey-Jennings or Shewart charts, which indicate the scatter of the individual control results about the designated mean value (Procedure 1.7). 

Levey S, Jennings ER (1950) The use of control charts in the clinical laboratory. Am J Clin Path 20 1059-1066... 

An example of a Levey-Jennings chart is shown in Figure 19-12, where control limits have been set as the mean +3s. Power functions for a Levey-Jennings chart having 3 s control limits, or a control rule, are shown in Figure 19-13. The probability for false rejection is seen to be less than 0.05 or 5% even when n is very large. The probability for error detection increases as n increases, but for an of 2 to 4, the procedure is not very sensitive for either random or systematic errors. 

Figure 19-10 shows the power functions for a Levey-Jennings chart having 2 s control limits, or the I2J control... 

It is important to recognize the seriousness of the false rejection problem and its relationship to the control Limits that are chosen for the Levey-Jennings chart. These false rejections are in effect an inherent property of the control procedure. They occur because of the control limits that have been selected, not because of any problems with the analytical method. Therefore the use of 2s control limits cannot be generally recommended. With the use of 3s control hmits. 

The multirule procedure developed by Westgard and associates uses a series of control rules for interpreting control data. The probability for false rejections is kept low by selecting only those rules whose individual probabilities for false rejection are very low (0.01 or less). The probability for error detection is improved by selecting those rules that are particularly sensitive to random and systematic errors. The procedure requires a chart having lines for control limits drawn at the mean 1 s, 2 s, and 3 s, and is adapted to existing Levey-Jennings charts by the addition of one or two sets of control limits. 

Comparison of the probability for error detection between the multirule procedure and the Levey-Jennings chart having 3s limits shows improved error detection for the multirule procedure. The rule improves the detection of random error and the 22s, 4is, and 10 rules improve the detection of systematic error. Elimination of the lO f rule does not cause much loss in error detection but does considerably reduce the amount of control data that must be inspected thus the simplification may malce the multirule procedure easier to use. The 4]s rule could possibly be elim-... 

Analyze the control material by the analytical method to be controlled on at least 20 different days, and calculate the mean and standard deviation of those results. (This is the same as the initial step required for a Levey-Jennings chart or for a multirule chart.)... 

The report may also include Levey-Jennings plots of the data, but because this information is not available in real time, it does not effectively serve the purposes of internal QC. Blank control charts that are set up for each analyte and each control material save the laboratory the time that is required when these charts are prepared manually. 

Figure 1.1 Examples of (a) Levey-Jennings and (b) cumulative sum (cusum) control charts using inter-assay quality control data from an alprazolam GC-MS assay. Since the cusum chart persents the cumulative sum of deviations from the mean, it is more sensitive to small biases that develop over time, whereas Levey-Jennings charts are most useful for detecting changes in the precision of the assay...

After a run is complete, the samples are reviewed and quantitated. Each peak is manually reviewed by a technologist, and quantitation is done using Waters QuanLynx software. Controls from each run are plotted on Levey-Jennings charts to track performance over time, and ensure the validity of each day s run. 

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