VALIDATION OF A DUAL-ENERGY X-RAY ABSORPTIOMETER - MEASUREMENT OF BONE MASS AND SOFT-TISSUE COMPOSITION

We investigated the reproducibility of total and regional body composi tion measurements performed on a dual energy X-ray absorptiometer (DXA ). A group of 38 women aged 21-81 (mean 52.4) years was scanned twice with repositioning to determine intraobserver reproducibility of measu rements of bone mineral density (BMD, g.cm(-2)), bone mineral content (EMC, g), lean mass (LM,kg) and fat mass (FM, kg) of the total body an d of the major subregions of the body. In addition, the ability of the DXA machine to detect changes in LM and FM (simulated by placing 11.1 and 22.3 kg porcine lard on the body of 11 subjects) was examined, Co efficients of variations calculated from the root mean square averages of individual standard deviations were as follows (BMD, BMC, FM, LM): 1.4%, 1.1%, 1.4%, 1.7% (total body), 2.2%, 2.1%,-,- (head), 2.8%. 2.8 %, 2.0%, 3.2% (trunk), 3.6%, 3.9%, 4.0%; 4.9% (alms), 2.7%, 1.3%, 2.6% , 2.8% (legs). Percentage fat (%fat) of exogenous lard was 81.3(SD 3.5 )% as assessed by the absorptiometer which corresponded well with the result of chemical analysis (82.5%). Estimated %fat of exogenous lard was not influenced by initial body mass or percentage body fat. Percen tages of expected mean values with 11.1 kg lard placed on the body wer e 99.9 (SD 0.3) for body mass, 100.5 (SD 2.1) for LM, and 99.5 (SD 3.5 ) for FM. BMD was overestimated by 3.2% (P<0.005) with 11.1 kg lard on the body. BMD as well as BMC increased significantly with 22.3 kg lar d on the body (P < 0.005). The results showed that BMD, BMC, LM, and F M of the total body were precisely estimated by the DXA machine used. Regional measurements were less precise. Changes in total body soft ti ssue composition were precisely and accurately estimated. The lard pla ced on the body falsely affected BMD and BMC measurements. Changes in body mass could have a similar effect.