What can a bone density report tell you?
Ask the doctor
Q. When given the results of a DEXA scan for osteoporosis, what do the numbers represent?
A. The DEXA scan is a type of x-ray that estimates bone mineral density, which is a measure of bone strength. It's the most common screening test for osteoporosis, a condition in which bone density is lower than normal and the risk of bone fracture is increased.
In most cases, the important numbers to know are your T-scores. These are measures of your bone density compared to young adults considered to have normal bone density. The lower your T-score, the higher your fracture risk. Here's how T-scores are reported:
- -1.0 and higher are normal
- between -1.0 and -2.5 indicate osteopenia (reduced bone density, but not as low as osteoporosis)
- -2.5 and below indicate osteoporosis (significantly low bone density).
Results can also be described as Z-scores. These are measures of bone density that compare your bone density with that of people your age and gender. If you're a 56-year-old woman, you could have low bone mineral density, but because so many women your age have low bone mineral density, your Z-score might be normal. That's why it's usually best to assess how low bone mineral density is compared with normal rather than compared with others your age. And that's why T-scores are generally most helpful.
Finally, your results may include the actual bone mineral density, which varies by which bones are tested and the type of machine used; normal scores typically range between .75 and 1.2 g/cm2. This is the readout from the DEXA machine. It is related to bone absorption of the x-ray energy. Although this number is used to create the T- and Z-scores, it isn't as helpful as T- and Z-scores scores in interpreting the results.
The risk of fracture increases with lower bone mineral density. An estimate of fracture risk can be calculated based on bone mineral density results and an individual’s other risk factors, such as
- age
- BMI
- history of prior fracture or certain medical conditions
- family history of osteoporosis
- calcium and vitamin D intake
- alcohol intake and smoking history
- medication use (especially corticosteroids).
Besides modifying these risk factors whenever possible (such as quitting smoking), doctors routinely recommend weight-bearing exercise and calcium and vitamin D supplements for post-menopausal women. Other medications such as alendronate (Fosamax) or risedronate (Actonel) may be recommended if bone density is low or falling over time. Talk to your doctor about your results and decide together on a treatment plan.
Image: © PonyWang/Getty Images
About the Author
Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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