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Bone Health Overview

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Updated June 2006

Bone Mass Measurement: What the Numbers Mean

What Is a Bone Density Test?

A bone mineral density (BMD) test is the best way to determine your bone health. BMD tests can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. The most widely recognized bone mineral density test is called a dual-energy x-ray absorptiometry or DXA test. It is painless - a bit like having an x ray. It can measure bone density at your hip and spine.

What Does the Test Do?

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


Most commonly, your DXA 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 (SDs). 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

Bone density is within 1 SD (+1 or -1) of the young adult mean.

Low Bone Mass
Bone density is between 1 and 2.5 SD below the young adult mean (-1 to -2.5 SD).

Bone density is 2.5 SD or more below the young adult mean (-2.5 SD or lower).

Severe (established) osteoporosis
Bone density is more than 2.5 SD below the young adult mean and there have been one or more osteoporotic fractures.


Sometimes, your bone mineral density is compared to that of a typical individual whose age is matched to yours. This comparison gives you a Z-score. Because low BMD is common among older adults, comparisons with the BMD of a typical individual whose age is matched to yours can be misleading. Therefore, the diagnosis of osteoporosis or low bone mass is based on your T-score. However, a Z-score can be useful for determining whether there may be an underlying disease or condition that is causing bone loss.

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 could 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.

While 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 your future. Your doctor will want you to develop - or keep - healthy habits like eating foods rich in calcium and vitamin D, and doing weight-bearing exercise like walking, jogging, or dancing. In some cases, medication to prevent osteoporosis may be recommended.

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 United States Preventive Service Task Force recommends that women age 65 and older be screened routinely for osteoporosis. The task force also recommends that routine screening begin at age 60 for women who are at increased risk for osteoporotic fractures.

In addition, a panel convened by the National Institutes of Health in 2000 recommended that bone density testing be considered in people taking glucocorticoid medications for 2 months or more and in those with conditions that place them at high risk for an osteoporosis-related fracture.

However, the panel did not find enough scientific evidence upon which to base universal recommendations regarding when all women and men should obtain a bone density test. Instead, an individualized approach is recommended.

Also, various professional medical societies have established guidelines concerning when a person should get a bone density test. Many of these guidelines can be found by conducting a search in an online database established by the National Guideline Clearinghouse at

For Your Information

For updates and for any questions about any medications you are taking, please contact the U.S. Food and Drug Administration at:

U.S. Food and Drug Administration

Toll Free: 888-INFO-FDA (888-463-6332)

NIH Osteoporosis and Related Bone Diseases ~ National Resource Center

2 AMS Circle
Bethesda,  MD 20892-3676
Phone: 202–223–0344
Toll Free: 800–624–BONE
TTY: 202-466-4315
Fax: 202-293-2356

The NIH Osteoporosis and Related Bone Diseases ~ National Resource Center provides patients, health professionals, and the public with an important link to resources and information on metabolic bone diseases. The mission of NIH ORBD~NRC is to expand awareness and enhance knowledge and understanding of the prevention, early detection, and treatment of these diseases as well as strategies for coping with them.

The NIH Osteoporosis and Related Bone Diseases ~ National Resource Center is supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases with contributions from:

The National Institutes of Health (NIH) is a component of the U.S. Department of Health and Human Services (DHHS).

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