A prospective study of discordance in diagnosis of osteoporosis using spine and proximal femur bone densitometry.

The T-score definition of osteoporosis, originally intended for epidemiologic research in this condition, is frequently used in making treatment decisions for individual patients. Discordance in classification depending on the site and type of measurement has been reported in retrospective and cross-sectional studies, but the impact of such discordance on clinical practice is unknown. This is potentially important in view of a recent proposal to confine osteoporosis diagnosis to densitometry at the hip. Having excluded those with degenerative changes in the lumbar spine, we compared the T-score classification of a prospective cohort of patients referred for their first dual-energy X-ray absorptiometry (DXA) scan, analyzing data for men and women in 10-year age groups. Total hip and neck of femur DXA identified significantly fewer osteoporotic patients than spine DXA, and this reduced sensitivity could not be improved by adjusting the T-score threshold without an unacceptable increase in non-osteoporotic cases. The majority of patients undetected by proximal femur DXA were at significantly increased risk of vertebral fracture. DXA at the lumbar spine had only moderate sensitivity and specificity for osteoporosis redefined by total hip densitometry, indicating differential rates of bone loss at the proximal femur and spine. We conclude that, as the most usual indication for bone densitometry is to aid the determination of an individual's fracture risk, both proximal femur and lumbar spine should continue to be assessed.