What is a bone density test?
A bone density test is used to measure the bone mineral content and density. It may be done using X-rays, dual-energy X-ray absorptiometry (DEXA or DXA), or a special CT scan that uses computer software to determine bone density of the hip or spine.
This measurement tells the doctor whether there is decreased bone mass, a condition in which bones are more brittle and prone to break or fracture easily.
A bone density test is used mainly to diagnose osteoporosis and determine your fracture risk. The testing procedure typically measures the bone density of the bones of the spine, pelvis, lower arm, and thigh. Portable testing may use the radius (one of the two bones of the lower arm), wrist, fingers, or heel for testing, but is not as precise as the non-portable methods because only one bone site is tested.
Standard X-rays may show weakened bones. But at the point where bone weakness can be seen on standard X-rays, it may be too far advanced to treat. Bone densitometry testing can find decreasing bone density and strength at a much earlier stage when treatment can be beneficial.
Bone density test results
A bone density test determines the bone mineral density (BMD). Your BMD is compared to two norms--healthy young adults (your T-score) and age-matched (your Z-score).
First, your BMD result is compared with the BMD results from healthy 25- to 35-year-old adults of your same sex and ethnicity. The standard deviation (SD) is the difference between your BMD and that of the healthy young adults. This result is your T-score. Positive T-scores indicate the bone is stronger than normal; negative T-scores indicate the bone is weaker than normal.
According to the World Health Organization, osteoporosis is defined based on the following bone density levels:
- A T-score within 1 SD (+1 or -1) of the young adult mean indicates normal bone density.
- A T-score of 1 to 2.5 SD below the young adult mean (-1 to -2.5 SD) indicates low bone mass.
- A T-score of 2.5 SD or more below the young adult mean (more than -2.5 SD) indicates the presence of osteoporosis.
In general, the risk for bone fracture doubles with every SD below normal. Thus, a person with a BMD of 1 SD below normal (T-score of -1) has twice the risk for bone fracture as a person with a normal BMD. When this information is known, people with a high risk for bone fracture can be treated with the goal of preventing future fractures. Severe (established) osteoporosis is defined as having a bone density that is more than 2.5 SD below the young adult mean with one or more past fractures due to osteoporosis.
Secondly, your BMD is compared to an age-matched norm. This is called your Z-score. Z-scores are calculated in the same way, but the comparisons are made to someone of your age, sex, race, height, and weight.
In addition to bone densitometry testing, your doctor may recommend other types of tests, such as blood tests, which may be used to detect the presence of kidney disease, evaluate the function of the parathyroid gland, evaluate the effects of cortisone therapy, and/or assess the levels of minerals in the body related to bone strength, such as calcium.
What are the risks of a bone density test?
You may want to ask your doctor about the amount of radiation used during the procedure and the risks related to your particular situation. It is a good idea to keep a record of your radiation exposure, such as previous CT scans and other types of X-rays, so that you can inform your doctor. Risks associated with radiation exposure may be related to the cumulative number of X-ray examinations and/or treatments over a long period.
If you are pregnant or think that you may be, you should tell your health care provider. Radiation exposure during pregnancy can lead to birth defects.
There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.
Certain factors or conditions may interfere with a bone density test. These include, but are not limited to, the following:
- Metal jewelry or other metal objects
- Body piercing
- A barium X-ray within 10 days of testing
- Calcified arthritic sclerosis of the posterior vertebrae
- Calcified abdominal aortic aneurysm
- Healed bone fractures
- Metallic clips from previous abdominal surgery
- Recent bone scans
What happens during a bone density test?
This test may be done on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor's practices.
Generally, it follows this process:
- In some cases, you may stay dressed but will be asked to remove all metallic objects, such as belt buckles, zippers, coins, keys, jewelry, dental appliances, and eye glasses. In other cases, you will be given a gown to wear so that no buttons, zippers, or hooks will interfere with the imaging process.
- You will be positioned on an X-ray table, lying flat on your back. Your legs will be supported on a padded box which helps to flatten the pelvis and lumbar spine.
- Under the table, a photon generator will pass slowly beneath you, while an X-ray detecting camera passes above the table parallel to the photon generator beneath. Together, they project pictures of the lumbar spine (lower back) and hip bones onto a computer screen. You will be asked to stay very still and may be asked to hold your breath for a short time in order to get a very clear picture.
- After the scan of the lumbar spine and hip bones is complete, your foot will be put into a brace that rotates your non-dominant hip (the side you use the least) inward, then the imaging procedure is repeated.
- The next imaging procedure will involve the radius, one of the two bones of the lower arm. The non-dominant arm (the arm you use the least) is usually examined, unless there is a history of a fracture of that arm.
- The computer will calculate the amount of photons that are not absorbed by the bones to determine the bone mineral content. The bone mineral density will then be calculated by the radiologist.
The entire scan takes about 30 minutes. A portable scan that checks only your forearm, finger, hand, or foot, takes a bone density reading in a few minutes.
While the bone densitometry procedure itself causes no pain, the movements of the body parts being examined may cause some discomfort or pain, particularly if you have recently had surgery or an injury. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.