How Often Should I Get a Bone Mineral Density Test?

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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.

If a woman's bone density decreases during osteoporosis therapy, how do we know that the woman's bone density wouldn't have decreased even more if she wasn't taking therapy?

We can't compare the same woman taking and not taking the medication at the very same time (there is only one of her). Finally, studies are showing that most women who lose bone density during the first year of treatment with alendronate (Fosamax) or raloxifene (Evista) ,will gain bone density if the same treatment is continued for a second year. With estrogen therapy, the same scenario arises, where the (few) women who lost bone density in the first 12 months of use were not the same women who lost bone during the following 24 months of therapy. In studies of raloxifene therapy, no matter whether the bone density increased or decreased, there was still protection against fractures. Thus, we have a unified message on this issue for almost every approved osteoporosis medication. The 2003 American Association of Clinical Endocrinology guidelines say "No change or even a slight reduction of BMD, however, is not evidence of treatment failure and does not warrant alteration of therapy."

Conclusion

Don't be concerned if your physician doesn't order repeated bone density measurements every year. Due to the error of the machine, he or she may not expect your bone density to change in a way that is detectable in such a short time. If you're taking osteoporosis medications, don't rely too heavily on repeated bone density testing, because it may not tell you too much about your actual protection against fracture.