Bone density test: Measure your risk of osteoporosis

Years ago, the only time osteoporosis — a disease that causes bones to become more fragile and more likely to break — could be detected was after you broke a bone. By then, however, your bones might already be quite weak. Today a bone density test, also called densitometry or DEXA scan, can determine if you have osteoporosis or are at risk of osteoporosis before you break any bones.

Who is a bone density test for?

A bone density test uses special X-rays to measure how many grams of calcium and other bone minerals — collectively known as bone mineral content — are packed into a segment of bone. The higher your 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. Doctors use a bone density test to determine if you have, or are at risk of, osteoporosis.

Bone density tests are not the same as bone scans. Bone scans require an injection beforehand and are usually used to detect fractures, cancer, infections and other abnormalities in the bone.

The U.S. Preventive Services Task Force recommends routine bone density screening if:

• You're a woman age 65 or older
• You're 60 and at increased risk of osteoporosis

Research hasn't yet determined the optimal interval for repeat bone density screenings, or the right age to stop screening. However, two or more years may be needed between tests to reliably measure a change in your bone density. Your doctor can recommend the best screening interval for you based on your personal medical history and osteoporosis risk factors.

The older you get, the higher your risk of osteoporosis because your bones become weaker as you age. Your race also makes a difference. You're at greatest risk of osteoporosis if you're white or of Southeast Asian descent. Black and Hispanic men and women have a lower, but still significant risk. Other risk factors for osteoporosis include low body weight, a personal history of fractures, a family history of osteoporosis and using certain medications that can cause bone loss.

Bone density

Bone density

With bone loss, the outer shell of a bone becomes thinner and the interior becomes more porous. Normal bone (A) is strong and flexible. Osteoporotic bone (B) is weaker and subject to fracture.

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How do you prepare for a bone density test?

Bone density tests are easy, fast and painless. Virtually no preparation is needed. In fact, some simple versions of the bone density tests can be done at your local pharmacy or drugstore.

If you're having the test done at a medical center or hospital, be sure to tell your doctor beforehand if you've had recent oral contrast or nuclear medicine tests. These tests require an injection of radioactive tracers that might interfere with your bone density test.

How is a bone density test done?

Bone density tests are usually done on bones that are most likely to break because of osteoporosis. These sites include the lumbar vertebrae, which are in the lower region of your spine, the narrow neck of your femur bone adjoining the hip, and the bones of your wrist and forearm.

The equipment for bone density tests includes large machines on which you can lie down (central devices) as well as smaller, portable machines that measure bone density on the periphery of your skeleton, such as in your finger, wrist or heel (peripheral devices).

Central devices

• DEXA scan. Dual energy X-ray absorptiometry (DEXA) scans measure the bone density at your hip or spine. This test offers very precise results and is the preferred test for diagnosing osteoporosis. During this test, you lie on a padded platform for a few minutes while an imager — a mechanical arm-like device — passes over your body. It won't touch you. The test does, however, emit radiation, though your exposure during a bone density test is commonly about one-tenth of the amount emitted during a chest X-ray. This test usually takes five to 10 minutes to complete.
• Quantitative CT scan. This test uses a computerized tomography (CT) scanner combined with computer software to determine your bone density, usually at your spine. Quantitative CT (QCT) scans provide detailed, 3-D images and can take into account the effects of aging and diseases other than osteoporosis on your bones. QCT scans emit more radiation than DEXA scans do. For a QCT test, you lie on a movable table that's guided into a large tube-like area where images are taken. It typically takes less than 10 minutes.

Peripheral devices
Peripheral devices are found in pharmacies and are considerably less expensive than are central devices. But these smaller machines do have limitations.

Measurements taken at your hip and spine generally are considered more accurate assessments of your osteoporosis risk because these are the locations where major fractures tend to occur — fractures that can severely limit the quality and even the length of your life.

A test done on a peripheral location, such as your heel, may predict risk of fracture in your spine and hip as well. But because bone density tends to vary from one location to the other, a measurement taken at the heel usually isn't as accurate as a measurement taken at the spine or hip.

Measurements of bone density in your heel or finger still can be used to screen for osteoporosis, though. If your test is positive, your doctor might recommend a DEXA scan at your spine or hip to confirm your diagnosis.

Locations for bone density testing

Locations for bone density testing

Bone density tests are usually done on bones in the spine (vertebrae), hip, forearm, wrist, fingers and heel.

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Dual energy X-ray absorptiometry (DEXA)

Dual energy X-ray absorptiometry (DEXA)

During a DEXA test, you lie on a padded platform while an imager — a mechanical arm-like device — passes over your body.

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Results of a bone density test

Your bone density test results are reported in two numbers: T-scores and Z-scores.

T-score
Your T-score is your bone density compared with what is normally expected in a healthy young adult of your sex. Your T-score is the number of units — standard deviations (SD) — that your bone density is above or below the standard.

Keep in mind that these scores apply mostly to white postmenopausal women, who tend to have lower bone density as compared with other racial groups and men. Interpretations may vary if you're a woman of color or a man.

Z-score
Your Z-score is the number of standard deviations above or below what's normally expected for someone of your age, sex, weight, and ethnic or racial origin. This is helpful because it may suggest you have a secondary form of osteoporosis through which something other than aging is causing abnormal bone loss. A Z-score less than -1.5 might indicate these other factors are to blame. Your doctor would then try to determine if there's any underlying cause for the low bone mass. If he or she can identify a cause, that condition can often be treated and the bone loss slowed or stopped.

Interpreting T-scores and Z-scores

Interpreting T-scores and Z-scores

The T-score compares your bone density with that of an average healthy young adult of your sex. So if your T-score is -2.0, your bone density is lower than average by two standard deviations. Your Z-score, on the other hand, tells you how your bone mass compares with that of someone your age. So if your Z-score is -0.5, your bone density is less than the norm for people your age by one-half of a standard deviation.

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What are the pros and cons of a bone density test?

Bone density testing is a valuable tool in the diagnosis of osteoporosis and is a fairly accurate predictor of your risk of fractures. Significant differences in the various testing methods do exist, however. Central devices are more accurate, but cost significantly more than peripheral devices do.

Not all health insurance plans cover bone density tests, so ask your insurance provider whether it provides coverage that applies to your situation. Medicare pays for bone density tests only in the following instances:

• If you're postmenopausal and at risk of osteoporosis
• If you have primary hyperparathyroidism
• If you have certain spinal abnormalities that might indicate a fracture
• If you're on long-term corticosteroid therapy, such as prednisone
• If your doctor wants to assess your response to osteoporosis medications

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 complete medical evaluation, including a history and physical. This information can help your doctor better interpret the results of the bone density test.