Bone mineral density testDefinition:
A bone mineral density (BMD) test measures how much calcium and other types of minerals are present in a section of your bone. Your health care provider uses this test, along with other risk factors, to predict your risk of bone fractures in the future. Bone fracture risk is highest in people with osteoporosis .
BMD test; Bone density test; Bone densitometry; DEXA scan; DXA; Dual-energy x-ray absorptiometry
How the test is performed:
Several different kinds of machines can do BMD testing. The most common method, called a DEXA scan, uses low-dose x-rays (about 1/10th the radiation dose of a chest x-ray ).
While you are lying on a cushioned table, a scanner passes over your body. Typically, the machine takes x-rays of your lower spine and hip. This is called a central DEXA scan. In most cases you won't need to undress.
There are smaller machines that just measure the bone density in your wrist, fingers, leg, or heel. You may find these in pharmacies, shopping centers, and health fairs. However, a central DEXA scan that measures bone density in your lower spine or hip is the best test to predict your risk of fractures.
How to prepare for the test:
Remove any jewelry before the BMD test. Inform your health care provider if you may be pregnant.
How the test will feel:
The scan is painless, although you will need to remain still during the test.
Why the test is performed:
Guidelines recommend screening for those who are thought to be at increased risk for osteoporosis:
- Women over age 65 and men over age 70
- Women under age 65 and men ages 50 - 70 who have risk factors such as:
- A fracture in any man or woman over age 50
- Chronic rheumatoid arthritis , chronic kidney disease , eating disorders
- Early menopause (either from natural causes or surgery)
- History of hormone treatment for prostate cancer or breast cancer
- Significant loss of height (See: Compression fractures of the back )
- Strong family history of osteoporosis
- Taking corticosteroid medications (prednisone, methylprednisolone) every day for more than 3 months
- Three or more drinks of alcohol per day on most days
If you are being treated for osteoporosis, BMD testing can help your health care provider monitor your response to the treatment.
The results are used as part of the FRAX scoring system, which considers bone density test results with other fracture risk factors. From this score, you and your doctor can determine whether you might need medication to treat the osteoporosis.
The results of your test are usually reported as a "T score" and "Z score."
- The T score compares your bone density with that of healthy young women.
- The Z score compares your bone density with that of other people of your age, gender, and race.
In either score, a negative number means you have thinner bones than the standard. The more negative the number, the higher your risk of a bone fracture.
A T score is within the normal range if it is -1.0 or above.
What abnormal results mean:
Bone mineral density testing does not diagnose fractures. However, along with other risk factors you may have, it helps predict your risk of having a bone fracture in the future. Your doctor will help you understand the results.
- A T score between -1 and -2.5 indicates the beginning of bone loss (osteopenia).
- A T score below -2.5 indicates osteoporosis.
What the risks are:
BMD testing involves exposure to a low level of radiation. Most experts feel that the risk is very low compared with the benefits of identifying osteoporosis before you break a bone.
Simple bone density scans using portable machines may be available as part of health fairs or screenings. These portable scanners may check the density of your wrist or heel. However, keep in mind that hip and spine scans are more reliable.
Lim LS, Hoeksema LJ, Sherin K; ACPM Prevention Practice Committee. Screening for osteoporosis in the adult U.S. population: ACPM position statement on preventive practice. Am J Prev Med. 2009;36:366-375.
National Osteoporosis Foundation. Clinician's Guide to Prevention and Treatment of Osteoporosis. Washington, D.C.: National Osteoporosis Foundation; 2008.