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How dense are you? A guide to testing your bone mass

What Lisa Wilson found most surprising about having a bone mineral density test was how quickly it was over.

"That's it?" she remembers asking the radiology technologist, somewhat incredulously.

A bone mineral density test, or BMD, does exactly what its name suggests—it analyzes your bone mass. It can tell you and your doctor how strong your bones are. It can find out if you have, or are at risk for, the disease called osteoporosis.

Having a BMD test, notes the National Institutes of Health (NIH), can reveal how likely it is that your future will include fractures.

What is a BMD test?

The standard method for measuring bone density is dual-energy x-ray absorptiometry—DXA, for short.

DXA is a painless, low-dose x-ray of the areas of the body most prone to osteoporotic fracture. That includes the hip, spine and sometimes the wrist.

In most cases, you'll lie on a padded table while an x-ray beam passes over you. Images are sent to a computer, which analyzes your bone mass and compares it with an established norm.

The test is usually completed within a half hour, according to the Radiological Society of North America (RSNA).

"It was extremely quick," Wilson says.

What the numbers mean

You might get two different sets of results, according to the NIH:

T-score. This number is a comparison of your bone density with that of a healthy young adult. The difference is measured in units of standard deviation, or SD, with the norm being zero. A T-score that is within one standard deviation of zero—between +1 and -1—is considered normal. Osteoporosis is diagnosed if a T-score is -2.5 or below.

Z-score. This number compares your bone mass with that of a typical person your age, size and sex. Since bone density lessens with age, a Z-score is less precise for diagnosing osteoporosis in older people, according to the National Osteoporosis Foundation (NOF). It is primarily a tool to diagnose the disease in premenopausal women, children and teens, and younger men. 

Who should have a BMD test?

Guidelines from both the NOF and the U.S. Preventive Services Task Force recommend routine BMD screenings for women starting at age 65.

Both groups also suggest testing younger women who are at increased risk for osteoporosis.

The NOF, for example, suggests testing postmenopausal women under age 65 who:

  • Have a family history of osteoporosis.
  • Fracture a bone.
  • Smoke.
  • Have a low body weight.

Your doctor also might recommend a BMD test if you take certain medications linked to bone loss—such as anti-seizure medication, corticosteroids, or hormones for thyroid disease. Other risk factors include low testosterone levels in men, excessive alcohol intake and recurrent falls.

In addition, the NOF recommends BMD testing for men ages 70 and older and men ages 50 to 70 who have an increased risk for osteoporosis.

Talk with your doctor

BMD tests are not 100 percent accurate. However, they are the best way to measure your bone health, according to the NIH.

The radiation dose delivered by DXA is minimal, according to the RSNA. Still, women who may be pregnant should alert the technician before having a BMD test.

Ask your doctor if a BMD screening is appropriate for you. He or she can also suggest ways to reduce your risk for osteoporosis.

reviewed 5/2/2019

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