What to Know About Bone Density Tests
The National Osteoporosis Foundation recommends that all women age 65 and older get screened for osteoporosis with a bone mineral density (BMD) test. Postmenopausal women who are under age 65 but have additional risk factors for osteoporosis (for example, corticosteroid use or a family history of osteoporosis) or who have recently had a fracture should also be screened.
Widespread screening of pre- and perimenopausal women is not recommended, but a bone density test might be appropriate for some women. An individualized approach based on advancing age and risk factor profile is best.
For instance, testing may be warranted for a perimenopausal white woman in her early 50s with other significant risk factors, such as smoking and low body weight.
The National Osteoporosis Foundation and the Endocrine Society recommend that all men age 70 and older undergo BMD testing. Men who are between the ages of 50 and 70 should be tested for osteoporosis if they are at high risk.
This includes men who:
• Take a hormone-blocking drug for prostate cancer such as leuprolide (Lupron)
• Drink too much alcohol (three or more drinks per day)
• Are long-term steroid users
• Have a chronic disease that affects the lungs, kidneys, stomach or intestines, or alters hormone levels
• Have a low testosterone level
• Don’t consume enough calcium or vitamin D
• Don’t get enough exercise
• Have a family history of osteoporosis
The doctor also may use an Internet-based tool called FRAX, which stands for fracture risk assessment, to give an assessment of fracture risk based on your individual profile. If FRAX calculates that your 10-year risk of a major osteoporotic fracture is less than 10 percent, you are considered to be at low risk; if it is between 10 percent and 20 percent, you are at medium risk; and greater than 20 percent is considered high risk.
During an osteoporosis examination, the doctor will take an inventory of risk factors, including family history, history of fractures, menstrual history, dietary history, medications, habits such as cigarette smoking and alcohol consumption, and a review of past illnesses.
The physical examination includes an assessment of spinal tenderness and curvature, height measurement, and a search for signs of other medical conditions that may contribute to osteoporosis.
Blood and urine tests may be required to identify conditions that can lead to osteoporosis. Blood tests include those that measure levels of calcium, thyroid-stimulating hormone, parathyroid hormone, vitamin D, and the enzyme alkaline phosphatase, as well as kidney and liver function. Calcium and cortisol may be measured in the urine.
When active bone resorption occurs in people with osteoporosis or Paget’s disease, their blood and urine show increased amounts of collagen byproducts (pyridinium cross-links, N-telopeptides and C-telopeptides) created by the breakdown of bone. Blood and urine tests for these substances have been approved by the Food and Drug Administration (FDA) and are commercially available, but their usefulness is not universally accepted.
Although tests for these byproducts are not required for a diagnosis of osteoporosis, they are useful in assessing a person’s response to therapy, because they can gauge response after just a few months of treatment. It takes longer to assess response to therapy using X-ray tests (such as dual-energy X-ray absorptiometry, or DXA), which generally are not repeated until a full year of treatment has been completed.
Types of bone density tests
BMD testing is performed in individuals who are at risk for osteoporosis or have experienced a suspicious fracture or a fragility fracture. These tests help doctors diagnose osteoporosis or osteopenia (low bone mass, a warning sign for osteoporosis). BMD test results are also used to predict a person’s risk of experiencing a fracture, monitor progression of bone loss over time, and observe how well osteoporosis treatments are working.
To measure BMD, your doctor can use a variety of techniques.
Generally, these fall into two categories:
Tests that measure BMD in the central areas of the body (that is, the spine and hip) and those that measure BMD in peripheral areas (for example, the wrist, finger, shinbone, kneecap and heel).
All of the BMD tests are painless and safe. While the central techniques are considered the most precise, they are more expensive than the peripheral tests. In addition, the equipment used for the test is not portable. The type of test your doctor performs may reflect a balance between your needs and the availability of BMD equipment in your area.
After starting treatment, you may undergo BMD testing at one year to monitor your response to therapy. After this point, retesting every two to three years is recommended. Your doctor may want you to be retested more often if you’re switching from one treatment to another or you continue to experience rapid bone loss.
Central BMD tests include dual-energy X-ray absorptiometry and quantitative computed tomography. (Dual-photon absorptiometry is an older bone mineral density test that is used only rarely today.)
• Dual-energy X-ray absorptiometry (DXA). This technique is the “gold standard” for the measurement of BMD. It involves the use of two X-ray beams aimed at the bone. A computer analyzes the amount of energy that passes through the bone and uses the measurement to calculate BMD. The test takes about 20 minutes and exposure to radiation is low.
DXA measurement of the hip and spine is the technology now used to establish or confirm a diagnosis of osteoporosis and predict future fracture risk. It is also a valuable test for monitoring BMD changes over time as well as an individual’s response to osteoporosis treatment.
Although DXA is the best tool for measuring BMD, it does not provide accurate spinal measurements in people who have a spinal deformity, arthritis in the lower spine or a history of spinal surgery.
Furthermore, hip measurements are not accurate in a hip that has been replaced or contains metal hardware. BMD results from DXA are given as a T-score, which reflects how much the BMD is above or below normal. In general, a T-score of –1.0 or higher is considered to be normal, and a T-score of –2.5 or lower indicates osteoporosis. A person with a score between –1.0 and –2.5 may eventually develop osteoporosis or be at risk for a future fracture. A score in this range is classified as osteopenia.
• Quantitative computed tomography (QCT). This procedure is not well studied or standardized, but it is sometimes performed instead of DXA to analyze bone density in the spine. It is similar to a regular computed tomography (CT) scan, in which X-rays generate a series of cross-sectional pictures that create a three-dimensional image.
QCT uses different software than standard CT to analyze BMD. The QCT procedure takes approximately 10 to 15 minutes to perform, exposes the body to more radiation than other central or peripheral bone measurement methods (including DXA), and generally costs more than DXA.
Peripheral techniques used to measure BMD include quantitative ultrasound, peripheral dual-energy X-ray absorptiometry, radiographic absorptiometry, peripheral quantitative computed tomography and single-energy X-ray absorptiometry.
Quantitative ultrasound and peripheral dual-energy X-ray absorptiometry are the most widely used. Peripheral techniques are valuable for osteoporosis screening; their lack of precision, however, makes them less useful for following changes in BMD over time or for tracking response to therapy. Nor can they measure hip or spine BMD—the sites where fractures have the most serious consequences.
• Quantitative ultrasound (QUS). This technique involves passing high-frequency sound waves through bone to measure its density in peripheral sites like the shinbone, heel and kneecap. There is no exposure to radiation, and screening takes less than five minutes. QUS may be as accurate as DXA in predicting the risk of fracture and diagnosing osteoporosis.
• Peripheral dual-energy X-ray absorptiometry (pDXA). This test is similar to a central DXA test except that it uses a portable machine and measures BMD in the finger, wrist or heel. It takes less than five minutes to perform a pDXA test, and exposure to radiation is low.
• Radiographic absorptiometry (RA). This test uses standard X-ray technology to determine BMD in the hand. The X-ray is taken with a “reference wedge” placed near the hand; this wedge has a known density with which the density of bone in the hand is then compared. RA provides a generally accurate measure of bone density. It takes just minutes to perform, and involves only limited exposure to radiation.
• Peripheral quantitative computed tomography (pQCT). This method is similar to the central QCT scan, except that it is used to measure BMD in the wrist. The scan takes about 10 minutes, and the results are generally accurate. Peripheral QCT is less expensive than central QCT, and exposure to radiation is lower.
• Single-energy X-ray absorptiometry (SXA). A test commonly used to determine BMD in the heel or wrist. The procedure is similar in principle to DXA except that it uses one X-ray beam instead of two. SXA takes about 10 to 15 minutes, and exposure to radiation is minimal.