Dual-energy x-ray absorptiometry is an imaging test that measures a person’s bone mineral density. More commonly known as a DEXA scan, a bone density test, a bone density scan or bone densitometry, it is a painless, noninvasive test that helps measure how many grams of calcium and other minerals are present in a segment of bone.

Bone densitometry is considered to be the most accurate tool available to diagnose and monitor osteoporosis. This disease most often occurs in women after menopause but is also common in elderly men and in certain other populations, such as people who have used corticosteroids or abused alcohol for a long time. Osteoporosis involves a progressive loss of calcium that thins the bones and leaves them more susceptible to fractures. The lower a person’s bone mineral levels, the greater the risk of fractures. Bone densitometry tests also are used to monitor the effectiveness of treatment after osteoporosis has been diagnosed.

During a typical bone densitometry test, an invisible x-ray beam is sent through the bones. The beam is thin and contains a dose of radiation much lower than that used for standard x-rays. This beam has two energy peaks. Soft tissue absorbs one of these peaks, and bone absorbs the other peak. The amount of radiation absorbed by the soft tissue is subtracted from the total absorbed, and the remaining amount represents the measure of a patient’s bone density.

Though most bone densitometry devices use x-rays, others use ultrasound, an imaging technique that uses sound waves. Bone densitometry is not the same as a bone scan, a nuclear medicine test that involves an injection of radioactive dye. Bone scans can be used to detect bone disorders such as fractures, cancer or infections.

Bone densitometry is often used as a preventive measure that allows a physician to assess the health of a person’s bones, especially in people who are at high risk for osteoporosis. Some experts urge all women age 65 or older (or age 60 or older with increased risk factors) to undergo bone densitometry testing.

Other experts recommend that people with certain risk factors undergo this testing. This includes postmenopausal women who are not taking estrogen hormone replacement, who have a personal or maternal history of hip fracture or smoking, or who are tall (over 5 feet 7 inches, or 170 centimeters) or thin (less than 125 pounds, or 56.7). People from a Caucasian or Southeast Asian ancestry also have increased risk of osteoporosis. Men with medical conditions associated with bone loss, such as decreased testosterone (male hypogonadism) due to aging or some treatments for prostate cancer, are also advised to receive bone density testing. Some physicians now recommend routine bone density tests for men starting at age 70 or 80, or beginning at a younger age if there are additional risk factors.

In some cases, bone densitometry may be performed to determine the risk of fractures in people who have other conditions associated with bone loss, such as anorexia, asthma, arthritis, chronic obstructive pulmonary disorder (COPD), diabetes, kidney disease, liver disease, parathyroid conditions or thyroid disorders. People who use certain medications that may cause bone loss (e.g., corticosteroids, certain cancer treatments, antacids that contain aluminum, antiseizure drugs, thyroid replacement drugs, possibly the antidepressants known as selective serotonin reuptake inhibitors) also may be good candidates for bone densitometry. Even some young people, such as teen athletes who undereat and overtrain, can develop osteoporosis. Others who may be considered for this testing include people who:

• Have high bone turnover (revealed through excessive collagen in urine)
  
  
• Experience a fracture after a mild injury
  
  
• Show evidence of vertebral fracture or other signs of osteoporosis, such as a loss of height or a stooped posture
  
  

Bone densitometry assists physicians by determining how much thinning has already taken place, after which the physician can evaluate the patient’s vulnerability to bone fractures. After the health of the bones is known, the physician and patient can work together on a treatment plan to strengthen the bones and to help prevent further bone loss.

In addition, bone densitometry can be used to monitor a patient’s bone density and to track the effectiveness of treatments.

Bone densitometry may not be effective for all patients. The accuracy of testing may be questionable in people who have spinal deformities, have had spinal surgery, or who have vertebral compression fractures or osteoarthritis. In such circumstances, a CAT scan (computed axial tomography) may be preferred instead of bone densitometry.