The standard technique for determining bone density is a form of bone densitometry called dual-energy X-Ray absorptiometry (DEXA). DEXA is simple and painless and takes 2 - 4 minutes. The machine measures bone density by detecting the extent to which bones absorb photons that are generated by very low-level X-Rays. (Photons are atomic particles with no charge.) Measurements of bone mineral density are generally given as the average concentrations of calcium in areas that are scanned.
Bone density is usually measured at the hip rather than the spine or wrist, which appears to be the most predictive of hip fracture. (Hip fractures are the most dangerous fractures, particularly in women older than sixty.)
Evidence suggests that screening for osteoporosis can help prevent fractures. Expert groups now recommend bone density screening for the following people:
- All women over age 60
- Any postmenopausal women under 60 years with risk factors for osteoporosis
- Any older men or women who suffer a fracture
A bone density test determines if you have osteoporosis — a disorder characterized by bones that are more fragile and more likely to break.
The test uses X-rays to measure how many grams of calcium and other bone minerals are packed into a segment of bone.
Why it's done
Doctors use bone density testing to:
- Identify decreases in bone density before you break a bone
- Determine your risk of broken bones (fractures)
- Confirm a diagnosis of osteoporosis
- Monitor osteoporosis treatment
The higher your bone mineral content, the denser your bones are. And the denser your bones, the stronger they generally are and the less likely they are to break.
Bone density tests differ from bone scans. Bone scans require an injection beforehand and are usually used to detect fractures, cancer, infections and other abnormalities in the bone.
Although osteoporosis is more common in older women, men also can develop the condition. Regardless of your sex or age, your doctor may recommend a bone density test if you've:
- Lost height. People who have lost at least 1.5 inches (3.8 centimeters) in height may have compression fractures in their spines, for which osteoporosis is one of the main causes.
- Fractured a bone. Fragility fractures occur when a bone becomes so fragile that it breaks much more easily than expected. Fragility fractures can sometimes be caused by a strong cough or sneeze.
- Taken certain drugs. Long-term use of steroid medications, such as prednisone, interferes with the bone-rebuilding process — which can lead to osteoporosis.
- Had a drop in hormone levels. In addition to the natural drop in hormones that occurs after menopause, women's estrogen may also drop during certain cancer treatments. Some treatments for prostate cancer reduce testosterone levels in men. Lowered sex hormone levels weaken bone.
Limitations of bone density testing include:
- Differences in testing methods. Devices that measure density of the bones in the spine and hip are more accurate but cost more than do devices that measure density of the peripheral bones of the forearm, finger or heel.
- Previous spinal problems. Test results may not be accurate in people who have structural abnormalities in their spines, such as severe arthritis, previous spinal surgeries or scoliosis.
- Radiation exposure. Bone density testing uses X-rays, but the amount of radiation exposure is usually very small. Even so, pregnant women should avoid these tests.
- Lack of information about the cause. A bone density test can confirm that you have low bone density, but it can't tell you why. To answer that question, you need a more complete medical evaluation.
How you prepare
Bone density tests are easy, fast and painless. Virtually no preparation is needed.
Be sure to tell your doctor beforehand if you've recently had a barium exam or had contrast material injected for a CT scan or nuclear medicine test. Contrast materials might interfere with your bone density test.
Food and medications
Avoid taking calcium supplements for at least 24 hours before your bone density test.
Clothing and personal items
Wear loose, comfortable clothing and avoid wearing clothes with zippers, belts or buttons. Leave your jewelry at home and remove all metal objects from your pockets, such as keys, money clips or change. In some cases, you may be asked to change into an examination gown.
Locations for bone density testing
Bone density tests are usually done on bones that are most likely to break because of osteoporosis, including:
- Lower spine bones (lumbar vertebrae)
- The narrow neck of your thighbone (femur), next to your hip joint
- Bones in your forearm
Your test will be done on a device where you lie on a padded platform while a mechanical arm passes over your body. The amount of radiation you're exposed to is very low, much less than the amount emitted during a chest X-ray.
Because bone density can vary from one location in your body to another, a measurement taken at your heel usually isn't as accurate a predictor of fracture risk as a measurement taken at your spine or hip. Consequently, if your test on a peripheral device is positive, your doctor might recommend a follow-up scan at your spine or hip to confirm your diagnosis.