The long-term predictive value of bone mineral density measurements for fracture risk is independent of the site of measurement and the age at diagnosis:: results from the Prospective Epidemiological Risk Factors study

Although the utility of bone mass measurements has been the subjects of extensive investigations, the number of studies comparing the predictive value of bone mass measurement at different skeletal sites in the same cohort with a large number of clinically verified endpoints is limited. Furthermore, scant information is available on how age at the time of diagnosis influence the risk of future fractures posed by low bone mineral density (BMD). We have followed 5,564 Danish postmenopausal women for a mean period of 7.3 years. Bone mineral content (BMC) at the forearm and BMD at the spine and hip were assessed at baseline. Vertebral fractures were assessed on digitalized images of lateral X-rays of the thoracic and lumbar spine, whereas non-vertebral fractures were self-reported. At follow-up, 17.6% of the women revealed an incidental vertebral fracture and 14.2% reported a new non-vertebral fracture. The absolute risk per 1,000 person-years of osteoporotic fracture increased significantly with decreasing bone mass at all three skeletal sites (P < 0.001). Osteoporotic BMD (T-score <=-2.5) had similar predictive values of fractures regardless of the skeletal site of measurement. Furthermore, the absolute risk of osteoporotic fractures increased significantly with increasing age at the same level of bone mass. Interestingly, the relative risk (RR) of vertebral fracture accompanying 1 SD decrease in spine BMD was similar across different age groups: < 55 years (RR:2.1, 95% CI 1.3-3.3), 55-64 years (RR:2.3, 95%CI 1.7-3.2), 65-74 years (RR:2.0; 95%CI 1.5-2.6). Furthermore, women with any prior osteoporotic fracture had a 2.4-fold (95% CI 2.01-2.75, P < 0.001) increased risk of a new vertebral fracture. Both age and prior fracture are strong predictors of future fractures. The long-term predictive value of bone mass measurement is independent of the site of measurement and the age at diagnosis.