by Robert S. Dinsmoor
Dual-energy x-ray absorptiometry, a common test for measuring bone mineral density. DXA is used in the diagnosis of osteoporosis, a condition in which bones lose density and are at greater risk for fracture. It is used as a screening test, to check bone strength in individuals who are at risk for osteoporosis, for example, older men and postmenopausal women. And it is used to monitor treatments to see if they are making bones stronger.
DXA works by sending low-level x-ray beams through a person’s body and then assessing the beams that come out the other side. By comparing the x-ray energy that goes in with the x-ray energy that comes out, and taking into account what is absorbed by soft tissue, the DXA machine can determine how much of the x-ray energy the bone is absorbing. The more the bone absorbs, the denser it is.
There are two basic types of DXA: central DXA and peripheral DXA. Central DXA measures bone density in body areas such as the spine or hip, and peripheral DXA measures bone density in areas such as the forearm, wrist, heel, or finger. Central DXA is most appropriate for diagnosis and monitoring. Peripheral DXA is most appropriate for screening. For the central DXA scans, the individual lies on a padded table, which is positioned above an x-ray generator and below an imaging device. The imaging device moves slowly over the area to be examined, creating images on a computer monitor. In a peripheral DXA test, the individual’s finger, hand, forearm, or foot is placed in a small device that reads the bone density. Central DXA is usually done in a doctor’s office or hospital. Peripheral DXA may be done in a doctor’s office but may also be available in drug stores and other sites that cannot house a central DXA machine. Both procedures are quick and painless and use much less radiation than a traditional x-ray.
When the test is done, the DXA machine computes a person’s T-score. The T-score represents a person’s bone mineral density as compared with the ideal or peak bone mineral density of a healthy 30-year-old. A T-score of 0 means that the person’s bone mineral density is equal to that of a healthy young adult. The lower a person’s bone mineral density, the lower the T-score will be. A T-score between +1 and -1 is considered normal. A T-score between -1 and -2.5 indicates low bone mass and increased risk of osteoporosis. A T-score of -2.5 or lower means a person has osteoporosis.
The radiologist performing the DXA scan may also assign a Z-score, which compares the person’s bone mineral density with that of an average group of men or women of the same age, weight, and race. The Z-score is not used to diagnose osteoporosis, but it can sometimes help the doctor figure out whether an underlying disease or condition is causing bone loss.
The National Osteoporosis Foundation recommends routine DXA screening for all women 65 years or older, all men 70 and older, and all people 50 years or older with osteoporosis risk factors. Among the risk factors that doctors take into account when deciding to order a DXA test are low body weight, a medical condition such as lupus or rheumatoid arthritis, and the use of corticosteroid medicines such as prednisone.
Last Reviewed on May 18, 2011
Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.
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