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(A-Z listing includes diseases, conditions, tests and procedures)
 

Bone Densitometry

(DEXA, Bone Density Test, Bone Mineral Density [BMD] Test, Bone Mineral Content, Bone Absorptiometry)

What is a bone density test?

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 is also used to determine your future fracture risk. The testing procedure 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 is not as precise as the nonportable methods because only one 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 be beneficial.

Bone density test results

Illustration of T-score and what it means
Click Image to Enlarge

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 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 healthcare provider may recommend other types of tests, such as blood tests, which may be used to find 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.

Why might I need a bone density test?

A bone density test is mainly done to look for osteoporosis (thin, weak bones) and osteopenia (decreased bone mass) so that these problems can be treated as soon as possible. Early treatment helps to prevent bone fractures. The complications of broken bones related to osteoporosis are often severe, particularly in the elderly. The earlier osteoporosis can be diagnosed, the sooner treatment can be started to improve the condition and/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 and indications for densitometry testing. Some common risk factors for osteoporosis include:

  • Post-menopausal women not taking estrogen

  • Advancing age, women over 65 and men over 70

  • Smoking

  • Family history of hip fracture

  • Using steroids long-term or certain other medicines

  • Certain diseases, including rheumatoid arthritis, type 1 diabetes mellitus, liver disease, kidney disease, hyperthyroidism, or hyperparathyroidism

  • Excessive alcohol consumption

  • Low BMI (body mass index)

What are the risks of a bone density test?

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 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 tell your healthcare provider. Risks associated with radiation exposure may be related to the cumulative number of X-ray exams and/or treatments over a long period.

If you are pregnant or think that you may be, you should tell your healthcare 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 healthcare provider before the procedure.

Certain factors or conditions may interfere with a bone density test. These include the following:

  • Metal jewelry or other metal objects

  • Body piercing

  • 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

How do I prepare for a DEXA scan?

PRECAUTIONS: Notify the technologist if you are pregnant or suspect you may be pregnant. Ensure that you have not undergone a barium or nuclear medicine study within 72 hours of the DEXA exam.

CLOTHING: In some cases, you may stay dressed but will be asked to remove all metallic objects, such as belt buckles, zippers, coins, keys and jewelry. In other cases, you will be given a gown to wear so that no buttons, zippers or hooks will interfere with the imaging process.

EAT/DRINK: Do not take calcium supplements the day of the exam.

Based on your medical condition, your doctor may request other specific preparation.

What happen during a DEXA scan?

Bone densitometry testing may be performed 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, bone densitometry follows this process:

  1. In some cases, you may stay dressed but will be asked to remove all metallic objects, such as belt buckles, zippers, coins, keys, and jewelry. In other cases, you will be given a gown to wear so that no buttons, zippers, or hooks will interfere with the imaging process.

  2. You will be positioned on an table, lying flat on your back. Your legs will be supported on a padded box which serves to flatten the pelvis and lumbar spine.

  3. Under the table, a photon generator will pass slowly beneath you, while an X-ray detector camera will pass above the table parallel to the photon generator beneath, projecting images of the lumbar and hip bones onto a computer monitor.

  4. After the scan of the lumbar and hip bones is complete, your foot will be inserted into a brace that moves your nondominant hip (the side you use the least) into an internally-rotated position, and the previous imaging procedure is repeated.

  5. The next imaging procedure may involve the radius, one of the two bones of the lower arm. The nondominant arm (the arm you use the least) is usually examined, unless there is a history of a fracture of that arm.

  6. 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.

While the bone densitometry procedure itself causes no pain, the manipulation of the body part being examined may cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure such as surgery. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.

What happens after a DEXA scan?

There is no special type of care following bone densitometry testing. You may resume your usual diet and activities, unless your doctor advises you differently. Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.

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