Long-term quality control of DXA: a comparison of Shewhart rules and Cusum charts.

Long-term clinical trials using bone mineral density (BMD) measurements are now commonplace. It is important to maintain a high standard of quality control. In this study two methods of monitoring quality control measurements of dual-energy X-ray absorptiometry (DXA) equipment have been compared using receiver operating curve (ROC) analysis: Shewhart multi-rule charts and Cusum charts. Computer-generated faults within daily spine phantom measurements were used. The Charts were then applied to 3 years of quality control data from one machine and the results related to hardware faults, random events and changes in the underlying BMD measurement. In the ROC analysis the Shewhart multi-rule chart performed as well or better than the cusum chart, but the stringency of the rules must be tightened to obtain optimal performance.