How Often Should I Get a Bone Mineral Density Test?

Medical Author: Carolyn Janet Crandall, MD, MS, FACP
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

This question sounds like it should have an easy answer. After all, a person taking blood pressure medication certainly checks blood pressure readings to adjust the dose of the medicine, right? Unfortunately, repeating bone mineral density tests (called serial bone mineral density testing ) is not that simple. In fact, it's quite controversial. Here are some reasons why. First, the usual rate of decline in bone density in postmenopausal women who are not taking therapy is about 1% per year. This is less than the error of the machine, which is about 3%. So, you wouldn't be able to tell whether a decrease of 2% over a year was "real" or possibly just the error of the machine.

What about the issues in women already taking osteoporosis therapy?

In postmenopausal women taking osteoporosis medication, the decrease in risk of fracture is almost totally due to factors other than bone mineral density changes. For example, one study showed that percentage changes in bone density with one osteoporosis medication (raloxifene, Evista) accounted for 4% of the observed protection against fracture, and the other 96% of the risk reduction remained unexplained. This is probably due to the fact that osteoporosis medications work in other ways, besides changing bone mineral density, such as improving the strength of the bone. This improvement in strength of bone cannot be measured on a bone density scan. In other words, decrease in bone density predicts fracture risk reliably in postmenopausal women who are not taking therapy, but does not reliably predict fracture risk in postmenopausal women who are taking osteoporosis therapy. This is proven repeatedly in recent research and is mis-understood even by most physicians.