The interchangeability of radioisotope and X-ray based measurements of bone mineral density

Lumbar spine and femoral neck bone mineral density (BMD) were measured with a Novo radioisotope based dual photon densitometer and with a Lunar X-ray densitometer in 94 subjects attending a Metabolic Bone Disease Clinic. There was a strong correlation between results obtained from each machine for the same skeletal site. The correlation coefficients for the spine and femoral neck were 0.97 and 0.88, respectively. The differences between results from each machine were normally distributed with a mean bias of 37.5% for the spine and 27.8% for the femur, which arise principally from differences in machine calibration. In each case the BMD was greater when measured by X-ray absorptiometry. The range for the bias was approximately 25-50% for the spine and 10-45% for the femoral neck. The results from these two machines are not interchangeable. When subjects who are participating in long term studies using a radioisotope densitometer are transferred to an X-ray densitometer, an individual conversion factor must be measured at each site for each subject.