课件：使用骨密度仪进行病情监测PPT课件.ppt

* In most clinical centers the precision error is worse than what is reported by the manufacturer. This is because precision is determined not only by the machine (DXA scanner), but also by the skill of the operator and the condition of the patient. Because manufacturer’s often use young normal volunteers and highly trained operators, their precision is better than seen at most clinical centers. This stresses the need for determining precision error at each center rather than relying on the manufacturer. NOTE: These are not ISCD Benchmark values. * Stress that patient must be repositioned between scans with patient getting off the table in between. This approach to determining short term precision using DXA. The minimum number of patients to scan is shown. It is ok to scan more. The numbers were chosen to satisfy the statistical requirements for the required degrees of freedom. Need to ensure the patients scanned for the precision study are representative of your patient population (not office staff). Use BMD results to calculate average BMD for each patient, SD, and root mean square (RMS) Further explained in next few slides. * Example of precision experiment in 14 subjects scanned 3 times each. * Root mean square method for determining precision has become standard in densitometry. * Continued example of precision experiment in 15 subject scanned 3 times each. (minimum of 15 scanned 3 times or minimum of 30 scanned 2 times) * To get RMS-%CV: RMS-%CV = (RMS SD)/Mean Absolute amount is less affected by BMD than RMS-%CV. Machine measures grams/cm2. Same grams/cm2 at low BMD is a higher percent than at higher BMD. CV Bonnicks book p86 tech p173 * Continued example of precision experiment in 15 subject scanned 3 times each. Used with the advance calculator on the ISCD web site to calculate precision as RMS-%CV Although the calculator calculates the CV and % CV using the individual CV, you can calculate it for the entire group by dividing RMS SD by the Mean of the