Comparison of rates of bone loss from the spine measured using two manufacturers' densitometers

Measurements of bone mineral density of the lumbar spine (LS-BMD) by dual-energy X-ray absorptiometry (DXA), made using different manufacturers' densitometers, differ by approximately 15%. The aim of this study was to compare the rate of bone loss from the lumbar spine measured using two manufacturers' densitometers. LS-BMD (vertebrae L2-L4) was measured using both a Lunar DPX and a Hologic QDR 1000/W in 17 postmenopausal women at baseline and 2 years. The mean rate of change of LS-BMD made using the two densitometers did not differ (DPX -0.54% per year; QDR -0.17% per year). although rates of change of LS-BMD measured using the two densitometers correlated (r = 0.47, p = 0.05), the variance of the rates of loss differed such that, in individual subjects, the difference between the rates of loss of LS-BMD, measured using the two densitometers, was related to the average rate of loss (r = 0.61, p = 0.009). For both densitometers, as bone mineral content (BMC) increased, the measured area also increased, leading to an underestimate of change in bone mineral density (BMD). This relation differed between the two densitometers such that the change in BMD was underestimated to a greater extent by QDR. We conclude that in the context of multicenter studies it may not be possible to directly combine data on the rate of bone loss acquired using different manufacturers' densitometers.