A bone density test is used to measure 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. For various reasons, the DEXA scan is considered the "gold standard" or most accurate test.
This measurement tells the healthcare provider whether there is decreased bone mass. This is a condition in which bones are more brittle and prone to break or fracture easily.
A bone density test is used mainly to diagnose osteopenia and osteoporosis. It's also used to determine your future fracture risk. The test typically measures the bone density of the bones of the spine, lower arm, and hip. Portable testing may use the radius (1 of the 2 bones of the lower arm), wrist, fingers, or heel for testing. But it is not as precise as the nonportable methods because only 1 bone site is tested.
Standard X-rays may show weakened bones. But at the point when 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 help.
A bone density test determines the bone mineral density (BMD). Your BMD is compared to 2 norms—healthy young adults (your T-score) and age-matched adults (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 mean the bone is stronger than normal. Negative T-scores mean the bone is weaker than normal.
Here is what the T-score levels mean:
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. So 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 1 or more past fractures due to osteoporosis.
Your BMD is then compared with 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.
Along with bone densitometry testing, your healthcare provider may advise other types of tests, such as blood tests. These may be used to see if you have kidney disease, check your parathyroid gland, evaluate the effects of cortisone therapy, or assess the levels of certain minerals in the body, such as calcium.
A bone density test is mainly done to look for osteoporosis (thin, weak bones) and osteopenia (decreased bone mass). These problems should be treated as soon as possible. Early treatment helps prevent bone fractures. The complications of broken bones due to osteoporosis are often severe, particularly in older adults. The earlier osteoporosis can be diagnosed, the sooner treatment can be started. Doing so can improve the condition or keep it from getting worse.
A bone density testing may be used to:
Confirm a diagnosis of osteoporosis if you have already had a bone fracture
Predict your chances of fracturing a bone in the future
Determine your rate of bone loss
See if treatment is working
There are many risk factors for osteoporosis. Some common risk factors are:
Post-menopausal women not taking estrogen
Advancing age, women older than age 65 and men older than age 70
Drinking too much alcohol
Family history of hip fracture or osteoporosis
Using steroids long-term or certain other medicines
Certain diseases, including rheumatoid arthritis, type 1 diabetes mellitus, liver disease, kidney disease, hyperthyroidism, inflammatory bowel disorders, or hyperparathyroidism
Low BMI (body mass index)
Bone densitometry testing uses a low dose of radiation. You may want to ask your healthcare provider about the amount of radiation used during the procedure and the risks related to your situation. It's 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 tell your healthcare provider. Risks from radiation exposure may be related to the cumulative number of X-ray exams or treatments over a long period.
If you are pregnant or think that you may be, tell your healthcare provider. Radiation exposure during pregnancy can lead to birth defects.
There may be other risks depending on your specific health problem. Talk about any concerns with your healthcare provider before the procedure.
Certain factors or conditions may interfere with a bone density test. These include:
Metal jewelry or other metal objects from a past surgery such as hip replacements or spine surgery
A barium X-ray within 10 to 14 days of testing
Calcified arthritic sclerosis of the posterior vertebrae
Calcified abdominal aortic aneurysm
Healed bone fractures
Metallic clips from previous stomach surgery
Your healthcare provider will explain the procedure to you and ask if you have any questions.
You will be asked to sign a consent form that gives your permission to do the test. Read the form carefully and ask questions if anything is not clear.
Generally, no preparation is needed, such as not eating or not taking medicine. You may be told to stop taking calcium supplements 24 to 48 hours before your bone density test.
Tell the technologist if you are pregnant or think you may be.
Based on your medical condition, your healthcare provider may give you other instructions on what to do before bone density testing.
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 healthcare provider'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 eyeglasses. 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 hipbones onto a computer screen. You will be asked to stay very still and may be asked to hold your breath for a short time to get a very clear picture.
The radius, a bone in your forearm, or your heel may be scanned depending on your healthcare provider's request. 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 healthcare provider.
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 reduce any discomfort or pain.
There is no special type of care needed after a bone density test. You may go back to your usual diet and activities, unless your healthcare provider advises otherwise.
Before you agree to the test or the procedure, make sure you know:
The name of the test or procedure
The reason you are having the test or procedure
What results to expect and what they mean
The risks and benefits of the test or procedure
What the possible side effects or complications are
When and where you are to have the test or procedure
Who will do the test or procedure and what that person’s qualifications are
What would happen if you did not have the test or procedure
Any alternative tests or procedures to think about
When and how you will get the results
Who to call after the test or procedure if you have questions or problems
How much you will have to pay for the test or procedure