Antiresorptive treatment for osteoporosis is usually prescribed for a minimum of five years. Although it reduces the risk of fracture, it does not relieve symptoms caused by existing fractures and may have side effects. Understandably, therefore, patients and their doctors seek reassurance that the treatment is working. The most common way to monitor response is repeated measurement of bone mineral density using dual energy x ray absorptiometry (DXA), an approach endorsed by recent guidelines.1 2 Routine monitoring is costly, however, and—if it does not reduce disease burden—may divert healthcare resources away from more deserving causes. In the linked study (doi:10.1136/bmj.b2266), Bell and colleagues assess the need to monitor the response to bisphosphonate treatment by estimating how much the effects of alendronate differ between individuals.3

Treatment aims to reduce the rate of fractures; therefore, monitoring should detect whether treatment will reduce the risk of fracture in individual patients. The effectiveness of repeated measurement of bone mineral density to monitor treatment depends on two prerequisites. Firstly, the test used (DXA) should be able to detect a significant change in bone mineral density within a time scale …