What is a bone mineral density test?

A bone mineral density (BMD) test measures calcium and other minerals in bone. Bones containing more minerals are denser, so they tend to be stronger and less likely to break.

Bones can become less dense as we age or if we develop certain medical conditions. When too much bone is lost, osteoporosis can develop. Osteoporosis causes bones to become weak and brittle, which increases the risk of fractures (broken bones).

Bone mineral density testing can:

  • Identify and diagnose osteoporosis.
  • Measure the risk of fractures (broken bones).
  • Monitor the effectiveness of osteoporosis treatment.

What tests are used to measure bone mineral density?

The most common bone mineral density test is a central dual energy x-ray absorptiometry (DXA or DEXA). DXA uses radiation to measure how much calcium and other minerals are in a specific area of your bone. Because the weak bones that tend to break most often are the hip and spine, DXA usually measures bone mineral density in these bones.

Other tests can also measure bone mineral density or bone loss:

  • Quantitative ultrasound (QUS) of the heel: Shows pictures of your bone and can predict your risk of broken bones and osteoporosis. But it is not used to monitor response to osteoporosis treatment, and it does not measure bone mineral density or give as much information as DXA. If the QUS shows that you have a higher risk of osteoporosis or broken bones, your doctor may recommend a central DXA test to confirm the finding.
  • Peripheral DXA: Measures bone mineral density, usually in the wrist and heel, using a portable device. This test does not give as much information as central DXA, so it is less accurate. Results showing a higher risk of broken bones or osteoporosis may need to be confirmed with a central DXA test.

What is a T-score?

If you are a woman in postmenopause or a man who is age 50 or older, your bone mineral density test result will be a T-score.

A T-score is the difference between your bone mineral density and 0, which is the bone mineral density of a healthy young adult.

The lower your T-score, the higher your risk of bone fracture.

What is a Z-score?

If you are a premenopausal woman or a man younger than age 50, your bone mineral density test result will be a Z-score. Z-scores are also used for children.

The Z-score is the difference between your bone mineral density and the average bone mineral density for healthy people of your age, ethnicity, and sex.

Who should get a bone mineral density test?

The U.S. Preventive Services Task Force recommends that women over age 65 have a bone mineral density test. The Task Force also recommends this screening for women of any age who have factors, identified by a health care provider who uses a formal risk-assessment tool, that raise their chance of osteoporosis.

More research is needed before the U.S. Preventive Services Task Force can make a recommendation regarding osteoporosis screening in men or how often premenopausal women and women with risk factors should be screened.

What other tools can measure risk of fractures?

Bone mineral density measurement tests are not the only tools that doctors use to predict your risk of fractures.

Doctors may use the Fracture Risk Assessment Tool (FRAX) to estimate risk for fracture. This score uses your age, sex, medical history, country, and other factors. This information, along with your bone mineral density test results, can help health care providers understand your risk for fracture and can guide treatment. For people with osteoporosis or osteopenia, the FRAX score can predict the chances of a major fracture in the next 10 years. The FRAX score can also screen women in postmenopause younger than age 65 for osteoporosis risk.

Read more about osteoporosis and risk factors.

What steps should I take after my test?

It is important that you review your BMD test results with your doctor for a full explanation of what they mean for you. Any diagnoses or treatment recommendations would be based on your BMD test results, age, and other fracture risk factors that you may have. In some cases, your doctor may refer you to a specialist.

This content was created by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) with contributions from: