Example Scenario:

The following example QC log shows normal and abnormal control results as well as patient results for a seven-day period.

The acceptable range for Level 1 (normal control) is 3.7–4.3 mmol/L. The acceptable range for Level 2 (abnormal control) is 6.7–7.3 mmol/L. When comparing the daily QC result obtained for the normal control to the range calculated for the normal control, each result is within the expected range. This indicates the analytical process is "in control" at the normal level on the day of testing.

When comparing the daily QC result for Level 2 (abnormal control) to the defined range for the abnormal control, the analytical process is "in control" for each day of testing except for the last day (Dec-07 in the example above). On December 1 through December 6, both levels were "in control," and the laboratory could report patient values reliably.

However, the laboratory was "out of control" for abnormal high potassium on December 7 because the value obtained for the QC material (8.0 mmol/L) was outside the acceptable range (6.7–7.3 mmol/L). This result indicates some type of error occurred which may have produced unreliable patient results which are abnormally high. In this scenario, the laboratory should not report any patient samples with an abnormally high potassium until resolving the error and re-testing the abnormally high sample.

From this example, it is apparent the range defined for each level of control is fundamental to the quality control system. The section "Basic QC Statistics" describes the calculations required to develop an acceptable control range.