How does a bone mineral density (BMD) test help diagnose osteoporosis?

David Slovik, MD
Endocrinology Diabetes & Metabolism
Osteoporosis used to be diagnosed only after a bone fracture. For many people, that diagnosis came too late to be of much use. Today, osteoporosis can be detected earlier with a bone mineral density (BMD) test. This test can also provide information regarding your risk of suffering a fracture and can help you and your doctor monitor your progress if you're taking bone-building medications.
BMD is the measurement of a bone's mineral content. In general, the lower your bone density, the higher your risk for fracture. Usually, BMD is measured by calculating the amount of mineralized tissue in grams per square centimeter in the area scanned (for example, the hip, spine, or heel). That number, like the figure that represents cholesterol level, is then placed on a spectrum created from the BMD measurements of people of all ages.
To decide what levels place people at risk, statisticians looked at the bone densities of thousands of women and noted who had osteoporotic fractures and who didn't. Since women who are in their 30s -- when bone mass is at its peak -- have the lowest fracture risk, their average bone mass was used as the reference point. Researchers then determined the levels of bone density that were associated with increased risk for osteoporosis.

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