Interpretation of bone mineral density measurement and its change

Bone mineral density (BMD) is measured with random error, but it is used as a surrogate definition of "osteoporosis" and as an endpoint in clinical tr ials. A Bayesian model was formulated to address two questions: given an ob served level of BMD for an individual, what is the individual's "true" leve l, and to what extent does an observed BMD change reflect a real change. Fo r an individual, there is a good agreement between observed and "true" BMD values. In individuals with low measured BMD values, however, the 90% confi dence interval for the true level is particularly wide, which leads to high (as high as 20%) false-positive and false-negative rates of diagnosis of o steoporosis. In a clinical trial with an overall average increase in BMD of 2%, no conclusion of significant change for an individual could be drawn u ntil an observed increase of at least 5.5% or an observed decrease of at le ast 7.5%. It is proposed that the current practice of informing individuals about their t- and z-scores should be replaced with a report system in whi ch their osteoporosis probability risk category is conveyed. Also, assessme nt of change in an individual should take into account the overall change i n a population.