Bone density test: Measure your risk of osteoporosis

Years ago, the only time osteoporosis — the disease that causes bones to become more fragile and 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, can determine if you have osteoporosis or are at risk of osteoporosis before you break any bones.

What is a bone density test?

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 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

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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Who should have a bone density test?

The U.S. Preventive Services Task Force recommends a bone density test if:

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

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.

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.

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 (DXA)

Dual energy X-ray absorptiometry (DXA)

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

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What can you expect after the test?

No recovery is needed after the test.

What are the results of a bone density test?

The results of your bone density test 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 the 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.