Bone Density Test
Definition, Purpose, Description, Preparation, Risks, Normal results, Abnormal results
A bone density test, or scan, is designed to check for osteoporosis, a disease that occurs when the bones become thin and weak. Osteoporosis happens when the bones lose calcium and other minerals that keep them strong. Osteoporosis begins after menopause in many women, and worsens after age 65, often resulting in serious fractures. These fractures may not only bring disability, but may affect longevity. As many as one-fourth of women who fracture their hip after age 50 die within one year.
Most people today will get a bone density scan from a machine using a technology called Dual Energy X-ray Absorptiometry or DEXA for short. This machine takes a picture of the bones in the spine, hip, total body and wrist, and calculates their density. If a DEXA machine is not available, bone density scans can also be done with dual photon absorptiometry (measuring the spine, hip and total body) and quantitative computed tomography scans (measuring the spine). Bone density scanners that use DEXA technology to just measure bone density in the wrist (called pDEXA scans) provide scans at some drugstores. Yet these tests are not as accurate as those that measure density in the total body, spine or hip— where most fractures occur.
A bone density scan measures the strength of an individual's bones and determines the risk of fracture. An observation of any osteoporosis present can be made.
To take a DEXA bone density scan, the patient lies on a bed underneath the scanner, a curving plastic arm that emits x rays. These low-dose x rays form a fan beam that rotates around the patient. During the test, the scanner moves to capture images of the patient's spine, hip or entire body. A computer then compares the patient's bone strength and risk of fracture to that of other people in the United States at the same age and to young people at peak bone density. Bones reach peak density at age 30 and then start to lose mass. The test takes about 20 minutes to do and is painless. The DEXA bone scan costs about $250. Some insurance companies and Medicare cover the cost. pDEXA wrist bone scans in drugstores are available for about $30.
The patient puts on a hospital gown and lies on the bed underneath the scanner. Not all doctors routinely schedule this test. If the following factors apply to a patient, they may need a bone density scan and can discuss this with their doctor. The patient:
- is at risk for osteoporosis
- is near menopause
- has broken a bone after a modest trauma
- has a family history of osteoporosis
- uses steroid or antiseizure medications
- has had a period of restricted mobility for more than six months
The DEXA bone scan exposes the patient to only a small amount of radiation—about one-fiftieth that of a chest x ray, or about the amount you get from taking a cross-country airplane flight.
The patient, when compared with people at "young normal bone density" (called the T-score) has the same or denser bones than a healthy 30-year-old. T scores above 1 mean that a patient has a healthy bone mass. Scores from 0 to 1 mean that the patient has borderline bone mass and should repeat the test in two to five years.
The patient has two to four times the risk of a broken bone as other people in the United States at the same age and those at peak bone density. If a patient's T score ranges from 1 to 2.5 they have low bone mass and are at risk for osteoporosis. A T score below 2.5 means osteoporosis is already evident. These patients should have a repeat bone density scan every year or two.
Bonnick, Sydney Lou. The Osteoporosis Handbook. Taylor Publishing, 1994.
Brown, Susan E. Better Bones, Better Body: A Comprehensive Self-Help Program for Preventing, Halting and Overcoming Osteoporosis. New Canaan, CT: Keats Publishing, 1996.
Bilger, Burkhard. "Bone Medicine." Health, May/June 1996, 125-128.
"Bone Density Testing: Should You Be Checked?" Mayo Clinic Health Newsletter 15 (June 1997): 6
Hanlon, Toby, et al. "Size Up Your Bones Now." Prevention, Feb. 1996, 75.
National Osteoporosis Foundation. 1150 17th St., NW, Suite 500, Washington, DC 20036-4603. (800) 223-9994. <http://www.nof.org>.
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