Bone density test
A bone density test or scan measures the strength of an individual's bones and determines the risk of fracture. It 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 in both men and women. It often results in serious fractures. These fractures notonly 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 scanscan 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) are provided at some drugstores. Thesetests are not as accurate as those that measure density in the total body, spine, or hip where most fractures occur.
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 afan beam that rotates around the patient. During the test, the scanner movesto capture images of the patient's spine, hip or entire body. A computer thencompares 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 30-35 and then start to lose mass. The test takes about 20 minutes to do and is painless. The DEXA bone scan costsabout $250. Some insurance companies and Medicare cover the cost. pDEXA wrist bone scans in drugstores are available for about $30.
Not all doctors routinely schedule this test. However a patient may need a bone density scan if she is at risk for osteoporosis, is near menopause, has broken a bone after a modest trauma, has a family history of osteoporosis, usessteroid or antiseizure medications, or 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 orabout one-fiftieth that of a chest x ray, or about the amount you get from taking a cross-country airplane flight.
The patient's bone density when compared with people at "young normal bone density" is called the T-score. T scores above 1 mean that a patient has a healthy bone mass. Scores from 0 to -1 mean that the patient has borderline bonemass and should repeat the test in two to five years. If a patient's T scoreranges from -1 to -2.5 she has low bone mass and are at risk for osteoporosis. If the T score is below - 2.5 she has osteoporosis and have lost a significant amount of bone. These people have a significantly greater chance of breaking a bone than others with T scores above 0 and should consult with their doctor on ways to slow osteoporosis.