Bone Mass Measurements: What the Numbers Mean

A bone mineral density (BMD) test can provide a snapshot of your bone health.
Bone Mass Measurements: What the Numbers Mean - Articles


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What Is a Bone Density Test?

A bone mineral density (BMD) test can provide a snapshot of your bone health. The test can identify osteoporosis, deter-mine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. The most widely recognized BMD test is called a central dual-energy X-ray absorptiometry, or central DXA test. It is pain-less—a bit like having an X-ray. The test can measure bone density at your hip and spine.

Peripheral bone density tests measure bone density in the lower arm, wrist, finger, or heel. These tests are often used for screening purposes and can help identify people who might benefit from additional bone density testing.

What Does the Test Do?

A BMD test measures your bone mineral density and compares it to that of an established norm or standard to give you a score. Although no bone density test is 100 percent accurate, the BMD test is an important predictor of whether a person will have a fracture in the future.

The T-score

Most commonly, your BMD test results are compared to the ideal or peak bone mineral density of a healthy 30-year-old adult, and you are given a T-score. A score of 0 means your BMD is equal to the norm for a healthy young adult. Differences between your BMD and that of the healthy young adult norm are measured in units called standard deviations (SD). The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.

As shown in the table below, a T-score between +1 and -1 is considered normal or healthy. A T-score between -1 and -2.5 indicates that you have low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of -2.5 or lower indicates that you have osteoporosis. The greater the negative number, the more severe the osteoporosis.

World Health Organization Definitions Based on Bone Density Levels

NormalBone density is within 1 SD (+1 or -1) of the young adult mean.
Low bone massBone density is between 1 and 2.5 SD below the young adult mean (-1 to -2.5 SD).
OsteoporosisBone density is 2.5 SD or more below the young adult mean (-2.5 SD or lower).
Severe (established) osteoporosisBone density is more than 2.5 SD below the young adult mean, and there have been one or more osteoporotic fractures.

Low Bone Mass Versus Osteoporosis

The information provided by a BMD test can help your doctor decide which prevention or treatment options are right for you. If you have low bone mass that is not low enough to be diagnosed as osteoporosis, this is sometimes referred to as osteopenia. Low bone mass can be caused by many factors, such as:

  • Heredity
  • The development of less-than-optimal peak bone mass in your youth
  • A medical condition or medication to treat such a condition that negatively affects bone
  • Abnormally accelerated bone loss

Although not everyone who has low bone mass will develop osteoporosis, everyone with low bone mass is at higher risk
for the disease and the resulting fractures.

As a person with low bone mass, you can take steps to help slow down your bone loss and prevent osteoporosis in the future. Your doctor will want you to develop—or keep—healthy habits such as eating foods rich in calcium and vitamin D and doing weight-bearing exercise like walking, jogging, or dancing. In some cases, your doctor may recommend medication to prevent osteoporosis.

Osteoporosis: If you are diagnosed with osteoporosis, these healthy habits will help, but your doctor will probably also recommend that you take medication. Several effective medications are available to slow—or even reverse—bone loss. If you do take medication to treat osteoporosis, your doctor can advise you concerning the need for future BMD tests to check your progress.

Who Should Get a Bone Density Test?

The U.S. Preventive Services Task Force recommends that all women over age 65 have a bone density test. Women who are younger than age 65 and at high risk for fractures should also have a bone density test.

Due to a lack of available evidence, the task force did not make recommendations regarding osteoporosis screening in men.

Various professional medical societies have established guidelines concerning when a person should get a BMD test. For more information about these guidelines, go to the Osteoporosis page of MedlinePlus.


Bone Mass Measurement: What the Numbers Mean.” National Institutes of Health Osteoporosis and Related Bone Diseases National Resource Center, June 2015. Accessed March 1, 2018.

Prepared and updated by Dori Owczarzak, extension educator. Reviewed by Lynn James, senior extension educator.