Lecture 21 – Bone Density

What is a Bone Density Test?  What do the numbers mean?

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

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

Your DXA test results are compared to the ideal or peak mineral density of a 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’s)  The more standard deviations below zero, indicated as negative numbers, the lower the BMD and the higher the risk of fracture.

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.  In conclusion, the greater the number the more severe the osteoporosis.

World Health Organization Standards


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.


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

Low Bone Mass Versus Osteoporosis

The information provided by a BMD 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 referred as osteopenia.  Low bone mass can be caused by many factors such as:


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, or

abnormally accelerated bone loss.

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

A person with low bone mass can take steps to slow down bone loss and prevent osteoporosis in the future.  Physicians will encourage healthy habits, such as eating foods that are rich in calcium and vitamin D.  Weight-bearing exercises, such as walking, jogging, or dancing will help increase strength.  In some cases doctors may recommend medication to prevent osteoporosis.  These medications may slow or reverse bone loss.  Future BMD tests will be necessary to check the progress.

The U.S. Preventative Services Task recommends that women age 65 and older should be screened routinely for osteoporosis.  This task force also recommends that routine screening begin at age 60 for women who are at increased risk for osteoporotic fractures.  A panel convened by the National Institutes of Health in 2000 recommended that bone density testing be considered in people taking glucocorticoids medications for two months or more.  This also holds for people with other conditions that place them at high risk for an osteoporosis-related fracture.

The panel did not find enough scientific evidence upon which to base universal recommendations about when all men and women should receive a BMD test.  Each person is analyzed on a case by case basis.

For more information you may contact the Centers for Disease Control and Prevention’s National Center for Health Statistics

3311 Toledo Road

Hyattsville, MD 20782




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Filed under The Skeletal System

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