This is an online calculator which can be used as part of the QC: The Levey-Jennings Control Chart lesson in the Basic QC Practices series.
Let us take an example where two sets of control limits are needed to implement QC rules. The first set uses 2s control limits (for implementation of the 1_{2s} rule) calculated as the mean plus or minus 2 times the standard deviation. The second set uses 3s control limits (for implementation of the 1_{3s} rule) calculated as the mean plus or minus 3 times the standard deviation.
For this example, Control 1 has a mean of 200 and a standard deviation of 4 mg/dL. The upper control limit would be: 200 + 2*4, which is 208 mg/dL. The lower control limit would be: 200 - 2*4, or 192 mg/dL.
You should end up with 3s control limits of 188 and 212 for Control 1. For Control 2, you should have 2s control limits of 240 and 260 and 3s control limits of 235 and 265.
Once the control charts have been set up, you start plotting the new control values that are being collected as part of your routine work. The idea is that, for a stable testing process, the new control measurements should show the same distribution as the past control measurements. That means it will be somewhat unusual to see a control value that exceeds a 2s control limit and very rare to see a control value that exceeds a 3s control limit. If the method is unstable and has some kind of problem, then there should be a higher chance of seeing control values that exceed the control limits. Therefore, when the control values fall within the expected distribution, you classify the run to be "in-control," accept the results, and report patient test results. When the control values fall outside the expected distribution, you classify the run as "out-of-control," reject the test values, and do not report patient test results.
Return to the lesson on QC: The Levey-Jennings Control Chart in the Basic QC Practices series.