What do the results of a bone density test mean?

Nidal Boutros, MD
Internal Medicine
Bone density testing results have three parts:
  • Your overall bone mineral density score reflects the amount of calcium in your bones.
  • The T-score is created by comparing your bone density measurement to that of a healthy young adult at the peak of bone mass, which is achieved in the late 20s or early 30s.
  • The Z-score compares your bone mineral density to the average value for someone who is the same age and gender as you.
Chad Kurzynske, MD
Internal Medicine
Bone density testing helps determine if a person has osteopenia or osteoporosis. Both osteopenia and osteoporosis put people at greater risk for bone fractures. A normal bone density scan indicates good bone health and a decreased risk for bone fractures as a person ages.
Yvonne J. Braver, MD
Internal Medicine
A bone density test can predict your risk of osteoporosis. In this video, Yvonne Braver, MD, of Brandon Regional Hospital describes how the test determines your risk.
Sam Diasti, MD
Internal Medicine
A bone density test determines if the structure of your bone is strong or not, indicating your risk for osteoporosis. In this video, Sam Diasti, MD, an internist at Memorial Hospital of Tampa, says when women should get a baseline bone density test. 
Terry W. Smith, MD
Family Medicine
Bone density test results are an objective way to measure bone density, which then can be used to assess whether you have osteopenia (sub-normal bone density) or osteoporosis (a condition characterized by decrease in bone mass). A bone density test can give you a baseline reading as to whether you need treatment or not. If treatment is needed, a bone density test should be done every two years to make sure that the treatment is actually working.

The results of bone density tests are reported in units called standard deviations (SDs). This is a unit devised by statisticians so they can describe how far a particular value (in this case, your bone density) is from the average value.

Two different scores are used to report the results of bone density tests. First, a T-score compares your particular bone density with the average bone density for a "reference population." In theory, this group consists of "normal young adult women" (as determined by certain criteria set up by researchers). Second, a Z-score compares your particular bone density with that for all women your age.

So, who are these people you're being compared with? The reference population that represents "normal young adult women" with regard to bone density consists of white non-Hispanic women age 20 to 29 who participated in a particular study between 1988 and 1994. These women had dual-energy X-ray absorptiometry (DXA) bone density measurements of the hips, and it is against their values that your values are being compared.

Having bone density that's lower than average is not a disease. It only means you're more likely to have fractures. Having a low bone density measurement is a reason to continue with diagnostic testing for osteoporosis or other bone diseases. It does not, in itself, establish a particular diagnosis or suggest which treatment (if any) you should get.

Who makes up the group of women who have a T-score below -2.5 SDs? Many -- but not all -- of them will have osteoporosis. Some will have osteoporosis because of other medical conditions that require their own treatment plan. Some will have osteomalacia, a metabolic bone disease caused by lack of vitamin D. Some will have no bone disease at all. They have small, thin bones just because that's their normal physiologic makeup.

Who makes up the group of women who have a T-score above -2.5 SDs? Most of these women will not have osteoporosis, but some will. For example, a woman might have above-average bone mass at age 45 but then experience rapid bone turnover. As a result, her bone mass at age 80 may be well below average but not low enough for a T-score of -2.5 SDs.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.