Following the bone density trail: a clinical perspective.

Low bone mineral density (BMD) has consistently been shown to be a major risk factor for future osteoporotic fractures. Despite the development of new technologies for imaging the inner components of the skeleton as well as assessing qualitative aspects of trabecular and cortical bone, dual-energy x-ray absorptiometry (DXA) has maintained its status as the “gold standard” for measuring BMD and fracture prediction due to its accuracy, precision, and ease of use. Virtually every society and government agency has endorsed screening for low bone mass in postmenopausal women, although the age at which this should be performed differs considerably. Over the last decade, the World Health Organization (WHO) fracture risk assessment tool (FRAX), which incorporates femoral neck BMD in its algorithm, has gained widespread utilization because it allows estimation of 10-yr major osteoporotic and hip fracture probability. Furthermore, recommendations from the U.S. Public Health Service Task Force (USPHSTF) and other organizations provide guidelines (1) for initiation of therapy to reduce fracture risk based on BMD thresholds in conjunction with risk assessment tools such as FRAX. What is less clear is how often bone density should be repeated in postmenopausal women in whom an initial BMD has indicated an absence of osteoporosis. This relates in part to uncertainty about whether the rate of bone loss, measured by repeated BMD measurements, predicts subsequent fractures, particularly in younger postmenopausal women. Despite this uncertainty, Medicare in the United States has reimbursed for DXA measurements performed every 2 yr independent of baseline BMD since the late 1980s, and there are no restrictions on the number of repeat tests. This leaves the clinician with a dilemma: should BMD be measured at regular intervals in postmenopausal women who have normal or modestly low BMD but without other clinical risk factors? A new paper by Leslie et al. (2) in this issue of the JCEM helps to answer this clinically relevant question. The Canadian group used registry data from Manitoba to examine a population of 4598 women with an average age of 65 yr who had repeated BMD measurements by their local physicians within the province over a defined period. The principal question addressed in this study was whether changes in BMD between two measurements added to the predictive value of a single BMD measurement. Although the cohort included patients from individual practices, the analysis was carefully controlled and was limited to those women for whom BMD was obtained on the same DXA machine using similar software for the initial and the follow-up measurements. Their principal outcome measure was the prediction of major osteoporotic fractures (hip, wrist, humerus, or clinical spine), an identical set of fractures used for calculations in the WHO FRAX model (3). Their study showed that BMD was strongly predictive of fracture, but change in BMD did not enhance fracture prediction. This was true even after adjustment for other predictive factors. Leslie et al. (2) concluded that a single BMD value can be relied upon to accurately predict fracture and that prediction is not improved by adding bone loss as another independent variable. Their conclusion reaffirms the use of the current FRAX model that includes BMD but does not include bone loss as determined by a second BMD. This paper also showed that the initial BMD measurement in these women was equally predictive to the follow-up measurement, obtained an average of 4 yr later,