DXA LONGITUDINAL QUALITY-CONTROL - A COMPARISON OF INBUILT QUALITY ASSURANCE, VISUAL INSPECTION, MULTIRULE SHEWHART CHARTS AND CUSUM ANALYSIS

The performance of dual-energy X-ray absorptiometry (DXA) instruments can be monitored using various quality control (QC) procedures. It has not been established which of these is most appropriate. The aim of t his study was to determine which of four VC procedures is the best to use for longitudinal monitoring. Eighteen centres with DXA instruments scanned an aluminium spine phantom weekly for up to 16 months, and th e bone mineral density data were used for the VC procedures. The metho ds investigated were the instrument's inbuilt quality assurance (QA) p rocedure, visual inspection, multi-rule Shewhart control charts, and C usum analysis using a truncated-V mask. True and false positive fracti ons (TPF and FPF) of each method were calculated, including those for a range of action levels for the Shewhart charts and dimensions of the Cusum mask. For Shewhart, the action levels giving the most desirable TPF and FPF were whole multiples of the standard deviation (SD). For Cusum, the most desirable mask dimensions were 3.6 SD for the total he ight of the vertical section and 0.9 SD per data point for the gradien t of the wings. Predictive power of each method as a means of fault de tection was decided by the number of faults detected out of a total of 8 non-mechanical faults subsequently diagnosed. The inbuilt QA detect ed 2, visual inspection 7, Shewhart chart 7 and Cusum analysis 7. The FPFs were: visual inspection 0.09, Shewhart 0.04, Cusum 0.08. At these levels of FPF, the average time in days (range) from onset of a fault to detection was 39 (6-82) for visual inspection, 39 (4-116) for Shew hart and 21 (1-49) for Cusum. All three ''phantom'' methods are excell ent for DXA QC, with modified Cusum analysis being the most effective. The inbuilt QA appears of little use on its own for longitudinal QC.