What is a dual-energy X-ray absorptiometry (DXA) test?
Dual-energy X-ray absorptiometry (DXA) is a test that uses two different X-ray beams to check bone thickness (density) in your spine and hip. This information is used to estimate the strength of your bones.
Why is a bone density test done?
Bone density testing is often done for:
- People who are at risk for osteoporosis, including:
- Women who are age 65 and older.
- Older men.
- People who take some medications, such as steroids.
- People who have certain medical conditions, such as hyperparathyroidism.
- People who have osteoporosis, to see how well treatment is working.
How is a dual-energy X-ray absorptiometry (DXA) test done?
- You will lie on your back on a padded table. You probably can leave your clothes on, but you will remove any metal buttons or buckles for the test.
- You may need to lie with your legs straight or with your lower legs resting on a platform built into the table.
- The machine will scan your bones and measure the amount of radiation they absorb. During this test you are exposed to a very low dose of radiation.
How do you prepare for a bone density test?
Avoid wearing clothes with metal buttons or buckles. Remove any jewelry that might cause a problem with the scan. For instance, don't wear a bracelet if you are having the scan done on your wrist. Some doctors recommend not taking calcium supplements for 24 hours before the test.
What do the results of a bone density test mean?
Results of bone density tests can be reported in several ways.
T-score
Your T-score is your bone density compared to the average score of a healthy 30-year-old. (This is called the young adult reference range). It is expressed as a standard deviation (SD).
- A negative (–) value means that you have thinner bones (lower bone density) than an average 30-year-old. The more negative the number is, the less bone density you have compared with an average 30-year-old.
- A positive (+) value means that your bones are thicker and stronger than an average 30-year-old's.
The following table contains the World Health Organization's definitions of osteoporosis based onbone densityT-scores.
| T-score | |
|---|---|
| Normal: | Less than 1 standard deviation (SD) below the young adult reference range (a score of –1 or higher) |
| Low bone density (osteopenia): | Between 1 and 2.5 SDs below the young adult reference range (a score between –1 and –2.5) |
| Osteoporosis: | More than 2.5 SDs below the young adult reference range (a score of –2.5 or less) |
If your bone density test result is low:
- You may haveosteoporosis. Doctors usually use the lowest T-score to diagnose osteoporosis. For example, if your T-score at your spine is –3 and your T-score at your hip is –2, the spine T-score would be used to diagnosis osteoporosis.
- Your chance of breaking a bone is higher than average. The more negative your T-score, the greater your chances of breaking a bone during a fall or from a minor injury.
Low bone density values may be caused by other problems, such as:
- Taking certain medicines.
- Cancer, such asmultiple myeloma.
- Cushing's syndrome,hyperthyroidism, andhyperparathyroidism.
- Diseases of the spine, such asankylosing spondylitis.
- Earlymenopause.
- Low levels of vitamin D.
- Heavy alcohol use.
Z-score
Your bone density value may also be compared to other people of your age, sex, and race. This is called your Z-score. It is given in standard deviations (SD) from the average value for your age group.
- A negative (–) value means that your bones are thinner (lower bone density) and weaker than most people in your age group. The more negative the number is, the less bone density you have compared with others in your age group.
- A positive (+) value means that your bones are thicker and stronger than most people in your age group.
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