INFLUENCE OF PATIENTS WEIGHT ON DUAL-PHOTON ABSORPTIOMETRY AND DUAL-ENERGY X-RAY ABSORPTIOMETRY MEASUREMENTS OF BONE-MINERAL DENSITY

Lumbar spine bone mineral density (BMD) was measured by dual-energy X- ray absorptiometry (DXA) (Hologic QDR 1000) and by Gd-153 dual-photon absorptiometry (DPA) (Novo Lab 22a) in 120 post-menopausal women. Thou gh a high correlation existed between the two techniques, the ratio be tween DXA and DPA values was not constant. Using DXA we observed a hig her dependence of BMD on weight than in the DPA measurements. To inves tigate the different behaviour of DXA and DPA machines with weight, we analysed the effects of increasing thickness of soft tissue equivalen ts on the BMD of the Hologic spine phantom and on the BMD equivalent o f an aluminium standard tube. Increasing tissue-equivalent thickness c aused the phantom BMD measured by DPA to decrease significantly but ha d not effect on the DXA measurements. The different behaviour of DPA a nd DXA equipment with regard to the phantoms could account for the dif ferences observed in the relations between BMD and weight in the patie nts. Using multiple regression we studied the influence of weight and body mass index on the relation between BMD measured by the two techni ques. The introduction of either of these variables into the regressio n resulted in an improvement of the prediction of the DXA values from the DPA values. However, the residual standard error of the estimate w as still higher than the combined precision errors of the two methods, so that no simple relation allows a conversion of BMD(DPA) into BMD(D XA). Our results confirm that BMD is positively correlated with weight in postmenopausal women; the influence of weight on BMD is blunted wh en the Novo Lab 22a DPA machine is used for measuring bone mineral.