Monitoring changes in bone density.

The relationship between declining bone density and increasing fracture risk is firmly established; the relationship between increasing bone density and decreasing fracture risk is less clear. Because of this, the clinical utility of assessing the therapeutic efficacy of prescription therapies to reduce fracture risk by measuring changes in bone density has been called into question. However, there is substantial clinical trial data to support this approach. Nevertheless, an apparent increase or decrease in the bone density may be misinterpreted without an understanding of the statistical concepts of precision and least significant change. These concepts are not difficult and are of profound clinical importance. If the least significant change is not known, serial measurements of bone density cannot be interpreted. These concepts will be discussed and illustrated, and the rationale for the importance of changes in bone mineral density on therapy will be explored.