Using Shewhart p control charts of external quality-assurance program data to monitor analytical performance of a clinical chemistry laboratory.

We have investigated the application of Shewhart's p control charts in our external quality-assurance program to monitor the long-term performance of our laboratory's analytical quality. The p control charts have been able to detect long-term changes in our laboratory's analytical performance that would have been difficult to detect by more-conventional techniques. We have explored methods for interpreting these charts as well as some of their limitations, which include minimum subgroup size and dependence on constant specification limits. These charts may be not only a simple method for the long-term monitoring of analytical performance of a laboratory, but also of use to the organizers of external quality-assurance programs.