Diagnosis of osteoporosis by planar bone densitometry: can body size be disregarded?

Bone densitometry using dual energy X-ray absorptiometry (DXA) is frequently used to diagnose osteoporosis and to identify patients at risk of later fractures. The parameters of interest are bone mineral content (BMC) and bone mineral areal density (BMD). Bone densitometry results have a large overlap between normals and patient with fractures. This would suggest that other factors are important for the development of fractures or that bone densitometry is not used optimally. It is generally believed that the conversion of BMC to BMD by division of the former by the projected bone area is a good normalization procedure. Other normalization procedures have been attempted in the past with little success. We hypothesized that this might be due to a blurring effect of time since menopause, and that body size could be demonstrated to have an effect on measured BMC and BMD, if this time effect could be eliminated. The results of this study, comprising 1625 early post-menopausal women studied at virtually the same time since menopause, confirm that this is the case. Body surface area was the parameter among conventional body size variables showing the highest correlation with BMC and BMD. It was clearly shown that low values of BMD were seen more often in the lowest than in the highest body surface area quartile. The difference between quartiles was statistically significant. Simple division of BMC by actual body surface area or division of BMD by the square root of body surface removed the uneven distribution between the body surface area quartiles for lumbar spine and femoral neck measurements, and reduced it at peripheral measuring sites. It is suggested that BMC and BMD of the lumbar spine and the femoral neck should be normalized as described to avoid overdiagnosis of osteoporosis in persons of petite body stature and underdiagnosis in tall ones.