Bone Mineral Density

Test Overview

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 such calcitonin (Miacalcin), alendronate (Fosamax), or risedronate (Actonel). After menopause, women can use hormone therapy and raloxifene (Evista) to increase bone density.

There are several different ways to measure BMD.

  • Dual-energy X-ray absorptiometry (DEXA). This is the most accurate way to measure BMD. It uses two different X-ray beams to estimate bone density in your spine and hip. Strong, dense bones allow less of the X-ray beam to pass through them. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to each other. DEXA can measure as little as 2% of bone loss per year. It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to measure heel and forearm bone density, but SXA is not used as often as DEXA. See a picture of a DEXA X-ray of the hips or a DEXA X-ray of the spine.
  • Peripheral dual-energy X-ray absorptiometry (P-DEXA). P-DEXA is a type of DEXA test. It measures the density of bones in the arms or legs, such as the wrist—it cannot measure the density of the bones most likely to break, such as the hip and spine. P-DEXA machines are portable units that can be used in a doctor's office. P-DEXA also uses very low doses of radiation, and the results are ready faster than standard DEXA measurements. P-DEXA is not as useful as DEXA for finding out how well medicine used to treat osteoporosis is working.
  • Dual photon absorptiometry (DPA). This test uses a radioactive substance to measure bone density. It can measure BMD in your hip and spine. DPA also uses very low doses of radiation but has a slower scan time than the other methods.
  • Quantitative computed tomography (QCT). This is a type of CT scan that measures the density of a bone in the spine (vertebra). A form of QCT called peripheral QCT (pQCT) measures the density of bones in your arms or legs, usually your wrist. QCT is not usually used because it is expensive, uses higher radiation doses, and is less accurate than DEXA, P-DEXA, or DPA.

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 cannot 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 you have a high chance of getting osteoporosis. For more information, see:

Click here to view a Decision Point. Should I have a dual-energy X-ray absorptiometry (DEXA) test to diagnose osteoporosis?

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Decision Points focus on key medical care decisions that are important to many health problems. Decision Points focus on key medical care decisions that are important to many health problems.
  Osteoporosis: Should I have a dual X-ray absorptiometry (DEXA) test?

Why It Is Done

A bone mineral density (BMD) test is suggested for:

  • Postmenopausal women age 60 or older who have risk factors for developing osteoporosis.
  • All women who are age 65 or older.
  • Men older than age 70, or with other risk factors for osteoporosis.
  • Men and women who have hyperparathyroidism.
  • Men and women who have been taking corticosteroids, such as prednisone, for a long time.
  • Follow-up of how well treatment for osteoporosis is working for men and women being treated for 2 years or longer.

How To Prepare

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.

How It Is Done

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.

How It Feels

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.

Risks

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.

Results

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.

T-score

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.

  • A negative (–) value indicates that you have thinner bones (lower bone density) than an average 30-year-old. The more negative the number is, the less bone density you have compared with an average 30-year-old.
  • A positive (+) value indicates that your bones are thicker and stronger than an average 30-year-old.

The following table contains the World Health Organization's definitions of osteoporosis based on bone mineral density T-scores.

Bone mineral density
 

T-score

Normal:

Less than 1 standard deviation (SD) below the young adult reference range (more than –1)

Low bone mass (osteopenia):

1 to 2.5 SDs below the young adult reference range (–1 to –2.5)

Osteoporosis:

2.5 or more SDs below the young adult reference range (–2.5 or less)

 

If your bone mineral density test result is low:

  • You may have osteoporosis. Doctors usually use the lowest T-score to diagnose osteoporosis. For example, if your T-score at your spine is –3 and your T-score at your hip is –2, the spine T-score would be used to diagnosis osteoporosis.
  • You have a higher-than-average chance of breaking a bone. The more negative your T-score, the greater your chances of breaking a bone during a fall or from a minor injury. Every change of 1 SD means a twofold increase in the risk of fracture at that site. For example, if you have a T-score of –1, your chances of having a broken bone are 2 times greater than if your T-score was 0.

Low BMD values may be caused by other problems, including:

Z-score

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.

  • A negative (–) value means that your bones are thinner (lower bone density) and weaker than most people in your age group. The more negative the number is, the less bone density you have compared with others in your age group.
  • A positive (+) value means that your bones are thicker and stronger than most people in your age group.

What Affects the Test

Reasons you may not be able to have the bone mineral density (BMD) test or why the results may not be helpful include:

  • You cannot be correctly positioned during the test.
  • You have had a broken bone in the past. This can cause falsely high BMD results.
  • You have arthritis of your spine. In this case, the changes caused by arthritis in the spine may not make the spine the best place to measure for osteoporosis.
  • You have metal implants from hip replacement surgery or hip fracture.
  • You have had an X-ray test that uses barium within 10 days of the BMD test.

What To Think About

  • Experts disagree about which bones are best to use for BMD measurements. Bones in the lower spine and hip are tested most often. These bones generally have the most bone loss and are more likely to fracture. Sometimes bones in the wrist are measured. Ultrasound screening is done on the bone in the heel.
  • A BMD measurement should be done only when the information provided by the test will affect treatment decisions. BMD does not need to be measured more often than every 2 years to find out how well treatment is working.
  • Using DEXA to measure bone mineral density is replacing older methods, such as dual photon absorptiometry (DPA).
  • Regular X-rays cannot detect mild bone loss. A bone must lose at least a quarter of its weight before a regular X-ray can detect the problem.
  • If your bone density is lower than normal, you can increase bone density and strength by taking calcium and vitamin D supplements, exercising, lifting weights or using weight machines, and taking some medicines. For more information about how you can increase your bone strength and density, see the topic Osteoporosis.
  • Measuring BMD is recommended for women age 65 and older, and for women ages 60 to 65 who have risk factors for osteoporosis.
  • In the United States, legislation (called the Bone Mass Measurement Coverage Standardization Act) requires Medicare to pay for bone mineral density testing for people who have Medicare benefits and are at risk for losing bone mass. This includes:
    • Women who have gone through menopause and are at high risk for a bone fracture.
    • People who have increased bone loss (osteopenia) or have broken a bone because they have osteoporosis.
    • People using long-term doses of corticosteroids.
    • People using medicine to treat osteoporosis for 2 years or longer.
    • People who have hyperparathyroidism.

References

Other Works Consulted

  • Chernecky 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.
  • MacLean C, et al. (2008). Systematic review: Comparative effectiveness of treatments to prevent fractures in men and women with low bone density or osteoporosis. Annals of Internal Medicine, 148(3): 197–213.
  • 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.
  • U.S. Preventive Services Task Force (2002). Screening for osteoporosis in postmenopausal women: Recommendations and rationale. Annals of Internal Medicine, 137(6): 526–528.

Credits

Author Shannon Erstad, MBA/MPH
Editor Maria Essig
Editor Kathleen M. Ariss, MS
Associate Editor Tracy Landauer
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer Carla J. Herman - Geriatric Medicine
Last Updated September 23, 2008

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