A bone density test measures the density of minerals (such as calcium ) in your bones using a special X-ray. This information is used to estimate the strength of your bones.
We all lose some bone mass as we age. Bones naturally become thinner (called osteopenia ) as you grow older, because existing bone is broken down faster than new bone is made. As this occurs, our bones lose calcium and other minerals and become lighter, less dense, and more porous. This makes the bones weaker and increases the chance that they might break (fracture ).
With further bone loss, osteopenia leads to osteoporosis . So the thicker your bones are, the longer it takes to get osteoporosis. Although osteoporosis can occur in men, it is most common in women older than age 65.
If your bone density is lower than normal, you can increase bone density and strength by exercising, lifting weights or using weight machines, getting enough calcium and vitamin D. and taking certain medicines.
There are several different ways to measure bone density.
- Dual-energy X-ray absorptiometry (DXA). This is the most accurate way to measure bone density. It uses two different X-ray beams to estimate bone density in your spine and hip. Strong, dense bones allow less of the X-ray beam to pass through them. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to each other. DXA can measure as little as 2% of bone loss per year. It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to measure heel and forearm bone density, but SXA is not used as often as DXA.
- Peripheral dual-energy X-ray absorptiometry (P-DXA). P-DXA is a type of DXA test. It measures the density of bones in the arms or legs, such as the wrist. It can't measure the density of the bones most likely to break, such as the hip and spine. P-DXA machines are portable units that can be used in a doctor's office. P-DXA also uses very low doses of radiation, and the results are ready faster than standard DXA measurements. P-DXA is not as useful as DXA for finding out how well medicine used to treat osteoporosis is working.
- Dual photon absorptiometry (DPA). This test uses a radioactive substance to measure bone density. It can measure bone density in your hip and spine. DPA also uses very low doses of radiation but has a slower scan time than the other methods.
Ultrasound is a screening test that is sometimes offered at events such as health fairs. It is only used to look for problems. If results from an ultrasound test find low bone density, DXA is recommended to confirm the results. Ultrasound uses sound waves to measure bone density, usually in your heel. Ultrasound is quick, painless, and does not use potentially harmful radiation like X-rays. One disadvantage of ultrasound is it can't measure the density of the bones most likely to fracture from osteoporosis (the hip and spine). Ultrasound is not used to keep track of how well medicine for osteoporosis is working.
Before being screened for osteoporosis, you may want to think about what you will do if the tests show that you have a high chance of getting osteoporosis.
Osteoporosis: Should I Have a Dual-Energy X-Ray Absorptiometry (DXA) Test?
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Why It Is Done
A bone density test is suggested for:
- All women who are age 65 or older, and younger women who are at increased risk for broken bones caused by osteoporosis. 1
- Men with risk factors for osteoporosis, such as being older than 70.
- Men and women who have hyperparathyroidism.
- Men and women who have been taking corticosteroids, such as prednisone, for a long time.
- Follow-up of how well treatment for osteoporosis is working for men and women being treated for 2 years or longer.
How To Prepare
Avoid wearing clothes with metal buttons or buckles for the test. You also may want to remove any jewelry that might interfere with the scan, such as a bracelet if you are having the scan done on your wrist.
How It Is Done
A bone density scan is usually done in the special radiology department or clinic by a technologist. Peripheral dual-energy X-ray absorptiometry (P-DXA) machines are portable units that can be used in a doctor's office.
You will need to lie on your back on a padded table. You can usually leave your clothes on. 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. The DXA technique, which scans the hip and lower spine, takes about 20 minutes to perform. Other techniques may take 30 to 45 minutes.
Portable machines (P-DXA) can measure bone density in the wrist or forearm.
Testing at least two different bones (preferably the hip and spine) each time is the most reliable way of measuring bone density. It is best to test the same bones and to use the same measurement technique and bone density equipment each time.
How It Feels
A bone density test does not cause pain. If you have back pain, it may be uncomfortable to lie still on a table during the scan.
During a bone density scan, you are exposed to a very low dose of radiation. A bone density scan is not recommended for pregnant women because of the radiation exposure to the unborn baby.
A bone density test measures the density of minerals (such as calcium ) in your bones using a special X-ray. Results are usually available in 2 to 3 days.
Results of bone density tests can be reported in several ways.
Your T-score is your bone density compared to the average score of a healthy 30-year-old. It is expressed as a standard deviation (SD), which is a statistical measure of how closely each person in a group is to the average (mean) of the group. The average bone density is determined by measuring the bone density of a large group of healthy 30-year-olds (young adult reference range). Bone density values are then reported as a standard deviation from the mean of this reference group. Almost all 30-year-old people have a bone density value within 2 standard deviations of this mean.
- 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.
The following table contains the World Health Organization's definitions of osteoporosis based on bone density T-scores.