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A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray or computed tomography (CT) scan. 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 take steps to increase your bone strength and reduce your chances of having a fracture. Some ways to increase bone density and strength include combining calcium and vitamin D supplements with weight-bearing exercise (such as walking), weight training (such as lifting weights or using weight machines), and using medicines.
There are several different ways to measure BMD.
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, DEXA is recommended to confirm the results. Ultrasound uses sound waves to measure BMD, 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.
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A bone mineral density (BMD) test is suggested for:
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.
A bone mineral density (BMD) scan is usually done in the special radiology department or clinic by a technologist. Peripheral dual-energy X-ray absorptiometry (P-DEXA) 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 DEXA 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-DEXA) 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 BMD. It is best to test the same bones and to use the same measurement technique and BMD equipment each time.
A bone mineral 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 mineral density (BMD) scan, you are exposed to a very low dose of radiation. A BMD scan is not recommended for pregnant women because of the radiation exposure to the unborn baby.
A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray or computed tomography (CT) scan. Results are usually available in 2 to 3 days.
Results of bone mineral density tests can be reported in several ways.
Your T-score is your BMD 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 BMD is determined by measuring the bone density of a large group of healthy 30-year-olds (young adult reference range). BMD values are then reported as a standard deviation from the mean of this reference group. Almost all 30-year-old people have a BMD value within 2 standard deviations of this mean.
The following table contains the World Health Organization's definitions of osteoporosis based on bone mineral density T-scores.
Less than 1 standard deviation (SD) below the young adult reference range (more than –1)
1 to 2.5 SDs below the young adult reference range (–1 to –2.5)
More than 2.5 SDs below the young adult reference range (–2.5 or less)
If your bone mineral density test result is low:
Low BMD values may be caused by other problems, including:
Your BMD value may also be compared to other people of your age, sex, and race. This is called your Z-score. It is given in standard deviations (SD) from the average value for your age group.
Reasons you may not be able to have the bone mineral density (BMD) test or why the results may not be helpful include:
CitationsU.S. Preventive Services Task Force (2011). Screening for Osteoporosis: Recommendation Statement. Available online: http://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.Other Works ConsultedChernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.Liu H, et al. (2008). Screening for osteoporosis in men: A systematic review for an American College of Physicians guideline. Annals of Internal Medicine, 148(9): 685–701.Nayak, S, et al. (2006). Meta-analysis: Accuracy of quantitative ultrasound for identifying patients with osteoporosis. Annals of Internal Medicine, 144 (11): 832–841.Qaseem A, et al. (2008). Screening for osteoporosis in men: A clinical practice guideline from the American College of Physicians. Annals of Internal Medicine, 148(9): 680–684. Also available online: http://www.acponline.org/clinical_information/guidelines/guidelines.U.S. Preventive Services Task Force (2011). Screening for Osteoporosis: Recommendation Statement. Available online: http://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.
Last Revised: August 30, 2012
Author: Healthwise Staff
Medical Review: Anne C. Poinier, MD - Internal Medicine & Carla J. Herman, MD, MPH - Geriatric Medicine
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