QUALITY-CONTROL OF BONE DENSITOMETRY IN A NATIONAL-HEALTH SURVEY (NHANES-III) USING 3 MOBILE EXAMINATION CENTERS

A quality control (QC) program for bone mineral measurements at the pr oximal femur by dual-energy x-ray absorptiometry (DXA) was designed fo r the osteoporosis component of the Third National Health and Nutritio n Examination Survey (NHANES III). Major elements of the QC program ar e (1) a QC center for review of all scans, (2) setup procedures for an d continuous monitoring of daily QC procedures, (3) reference standard s for cross-calibration, (4) longitudinal studies for assessment of in strument stability, (5) monitoring of technologist performance, and (6 ) training. This report describes the results of the QC program of the first half of this 6 year study, which began in 1988. Measurements we re performed on 7376 subjects in three mobile examination centers, whi ch traveled to a new location about every 3 months, a total of 44 loca tions. A small percentage (3.5%) of all scans were rejected, mostly be cause of patient motion during scanning, but 33% of the remaining scan s required reanalysis at the QC center to refine the location of the r egions of interest. Precision in spine and hip phantoms was below 1% a t all ROIs. In 535 subjects with duplicate scans, age 20-91 of both se xes, examined in a blinded review, precision for BMD at the femur neck ROI was 3.2% (CV) and 5.1% for Ward's triangle BMD. The central revie w improved scan quality, increased the number of usable scans, and red uced significantly the range of the mean percentage difference in the duplicate scans. Minor alterations in machine function were observed a nd corrected. QC results on phantoms were similar to those obtained wi th stationary instruments in dedicated laboratories. Retrospective ana lysis of the regression slopes from QC records did not show a need for correction of the data base, indicating that the instruments were sta ble during the course of the study.