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How Bone-Density Tests Work: A family doctor explains

by James Hubbard, M.D., M.P.H.

bone_density_test.jpgLike something out of a science-fiction movie, the dual-energy X-ray absortiometry shoots a beam through your body, and out comes a reading that indicates whether you have osteoporosis. It doesn’t hurt; it’s not dangerous, and it could help keep you from shrinking. That’s right, shrinking.

But this is not sci-fi. This is modern medicine.


BONE-DENSITY TEST HELPS YOU CATCH OSTEOPOROSIS EARLY
Calcium and other minerals make your bones dense and strong. Not enough means they break more easily. The spinal bones may snap from everyday standing and walking, causing shortened bones and stooping. You could even become shorter overall.

But you may be able to prevent this worst-case scenario if you catch osteoporosis (porous bones) early. And the most common and accurate measurement for osteoporosis is the DEXA scan—dual-energy X-ray absortiometry (also called DXA).


WHAT HAPPENS DURING A BONE-DENSITY TEST

The test requires you to lie down on a padded table for a few minutes while a machine X-rays your hip and spine. A radiologist then reads the results. Some machines X-ray your finger or heel instead. Although this isn’t considered to be as accurate, it’s usually less expensive, and the machines are smaller and more mobile.

The amount of radiation from a DEXA scan can be one-tenth to one-one-hundredth of a chest X-ray. The lowest is “about as much as the average 24-hour exposure people get from cosmic rays, radon, TV screens, etc.,” says Donald Bachman, M.D., F.A.C.R., director of women’s imaging at MetroWest Medical Center in Framingham, Mass.

newsletter-graphic-free2The test works by shooting two forms of X-ray energy through your body to see what gets absorbed. “At higher energies, bone and soft tissue absorb similar amounts,” explains Dr. Bachman, “while at lower energies, bone absorbs relatively more than soft tissue.” So the test measures what comes out of the other side of the body at both energy levels. “By subtracting one measurement from the other, we can calculate how much was absorbed by the bone, and that can be used to calculate the density of the bone.”


WHAT IS A DEXA T SCORE?
After all this happens, the final analysis is called a T score, which is actually just a general statistical term referring to the results of using certain comparison methods. The DEXA T score compares your bone density to that of a normal young person.

The T score is a bell-shaped curve. The average is 0; -1 to -2.5 indicates osteopenia (not quite osteoporosis). Anything less than -2.5 means osteoporosis. “In practice, each number is about 10 percent above or below the average,” says Dr. Bachman. “For example, a T score of -3 would be 30-percent lower than the average for young normals.”

“For postmenopausal women and older men, each decrease in T-score number doubles the risk of fracture,” says Dr. Bachman. Your health-care provider will consider your score, along with risk factors you might have (past fractures, age, medications) to determine your treatment.


WHEN TO SCREEN FOR OSTEOPOROSIS
“Bone-density scans should be done by well-trained and experienced providers as the accuracy of the test can be poor if not done well,” Dr. Bachman warns. “In addition to the [American College of Radiology], which provides practice standards, the International Society of Clinical Densitometry provides training and certification and has many resources.”

Agencies such as the U.S. Preventive Services Task Force and the American College of Obstetricians and Gynecologists recommend a screening test for all women 65 and older, with a repeat test every two years unless your health-care provider recommends otherwise. (For example, you may need an earlier test if you’re a postmenopausal woman with recent bone fractures, who has a low body weight, who smokes or who has a strong family history of osteoporosis.)

Although osteoporosis occurs in older men, screening recommendations are fuzzy. Check with your health-care provider. Possible reasons for a scan in men include unexplained back pain, change in posture, loss of height and fractured bones.


JAMES HUBBARD, M.D.,M.P.H
, is the publisher of MyFamilyDoctorMag.com and a long-time family doctor. He still practices in Colorado Springs. Read more here .

Photo of Discovery DXA scanner courtesy Hologic Inc.

Article last updated and/or approved: March 2010. Bios current as of September 2007. Original article appeared in September/October 2007 former print magazine.


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written by James Hubbard, M.D., M.P.H. , August 30, 2008

Bonnie, each insurance is different so I would check with them. Some pay for certain routine exams, but yours might also pay because you are a higher risk taking the prednisone.

Thanks

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written by Bonnie Sayers , August 29, 2008

I want to get one soon due to taking prednisone often for my eczema. I am 48 and think by the time I turn 50 I should have one. Thanks for explaining the procedure. Is this a test that Drs will request so that insurance covers it or an extra patient has to pay?
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