Bone Mineral Density Test
What is a bone mineral density test?
This test, commonly known as a BMD, is an X-ray-like screening of your bone to find out whether or not you have osteoporosis, a bone-weakening disease that affects primarily postmenopausal women and also strikes some elderly men.
The test can also indicate whether you're at risk for developing osteoporosis, and can be used to track bone density changes over time.
While traditional X-rays can show visible bone thinning, the newer BMD tests are much more accurate and can measure small percentages of lost bone density. About 20 years ago doctors used regular X-rays to look for osteoporosis, but they could pick up changes only after a 25 to 40 percent drop in bone density -- by which time the disease is already far advanced. But today, machines called densitometers, which expose you to only 1-50th the radiation you'd get in a chest X-ray, can detect as little as a 1 percent bone loss.
Who should have a BMD test?
The U.S. Preventive Task Force recommends that women who weigh less than 154 pounds should be screened for osteoporosis beginning at age 60. The task force also recommends that women who don't fit these descriptions should be screened regularly beginning at age 65. The National Osteoporosis Foundation recommends bone density testing if you're a woman over 65, if you've suffered a bone fracture after menopause, or if you've gone through menopause and have one or more risk factors for the disease. The latest medical guidelines also recommend the BMD test for younger postmenopausal women who have one or more risk factors (other than being white, postmenopausal, and female). Major risk factors for osteoporosis:
- Age (the disease is more common starting at age 35 and accelerating after age 50)
- Gender (women get osteoporosis more frequently than men, although 2 million men have the disease)
- Thin bone structure and low body weight (under 127 pounds)
- Early menopause
- Low calcium or vitamin D intake
- Poor health
- A history of osteoporosis in the family
Your doctor, however, may recommend that you get a BMD test at menopause so you can begin preventive treatment if necessary. Some physicians may also recommend testing for men under the age of 65 if they have fractured a bone, are sedentary, or take steroids. Men who have prostate cancer and receive a hormone treatment called androgen deprivation therapy are also at risk and should consider getting tested.
What kind of BMD tests are available?
A screening known as DXA is the standard, most accurate, and most commonly used test. (DXA is short for dual-energy x-ray absorptiometry). It measures bone mineral density in the sites most often fractured when someone has osteoporosis: the spine and hip. The scan is painless, takes only about ten minutes, and gives much less radiation exposure than a standard chest X-ray. This test, however, can be expensive (up to $150) and is not available everywhere. Medicare and some insurance companies cover the test for high-risk women over 65.
Other tests, known for short as pDXA, are mini-versions of the DXA that measure bone density in the forearm, finger, wrist, and sometimes the heel of the foot. Machines that perform this test are less accurate than the DXA, but can give a general idea of osteoporosis risk.
Another test called radiographic absorptiometry (RA) measures bone density in the hand, using a standard or computer-generated x-ray. Its accuracy is similar to the DXA. Still another screening tool is known as quantitative computed tomography. It can measure bone loss at several sites in the body, but is usually used as an alternative to DXA to measure bone density in the spine.
One other test a quantitative ultrasound uses sound waves to measure bone density.
How are the results measured, and what do they mean?
Most of the tests produce what is called a "T-score" for your bones: if you score above -1, you're considered normal (your bone density is equivalent to that of a healthy 35-year-old); if you score -1 to -2.5, you may be at risk for the disease; if your score is lower than -2.5, you're diagnosed with osteoporosis. The National Osteoporosis Foundation (NOF) released new clinical guidelines in 2008 that offer guidance on when to treat people with low bone mass. Their new guidelines use the World Health Organizations Absolute Fracture Risk, which considers bone mineral density and nine other risk factors to determine a persons likelihood of fracturing a bone in the next 10 years.
What can you do to maintain good bone health?
Calcium and vitamin D deficiency are widespread, particularly in adults 50 and older. These nutrients are also essential in avoiding osteoporosis and maintaining good bone health. Calcium preserves bone strength and is vital in preventing osteoporosis-related fractures, and vitamin D helps absorb the calcium you consume.
In 2007, the National Osteoporosis Foundation (NOF) updated its recommendations for calcium and vitamin D intake. The NOF recommends that adults under the age of 50 get 1,000 mg of calcium and 400 to 800 IU of vitamin D3 (the best form of vitamin D for bone health) daily. For adults over the age of 50, the NOF recommends 1,200 mg of calcium and 800 to 1,000 IU of vitamin D daily. You can find calcium in dark green, leafy vegetables, such as kale and spinach, in milk, dried beans, legumes, and supplements. Good sources of vitamin D include fortified milk, egg yolks, saltwater fish, liver, and supplements. Also, your skin produces vitamin D through exposure to sunlight.
Recent consensus guidelines published by the American College of Rheumatology and the American Geriatrics Society recommend regular exercise to increase balance, coordination, muscle strength, and bone density. Weight-bearing exercise is especially helpful in developing strong bones. When physical activity puts stress on bones, bone cells respond by making the bone denser. The guidelines also caution against excessive alcohol consumption and smoking. Good health, good nutrition, and regular exercise, the guidelines stress, create strong bones that can help prevent falls and fractures.
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