Bone mineral density (BMD) tests are performed in individuals who are at risk for osteoporosis or have experienced a suspicious fracture. These tests help doctors diagnose osteoporosis or osteopenia (low bone mass, a warning sign for osteoporosis). BMD tests can also 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, hip, and total body) 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, an older test, is used only rarely today.)
Dual-energy x-ray absorptiometry (DXA)
This technique is the "gold standard" for the measurement of BMD. As the name implies, 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 involves a low radiation exposure.
DXA is a valuable test for monitoring BMD changes over time as well as a person’s response to osteoporosis treatment. Although DXA is the best tool for measuring BMD, it doesn't provide accurate spinal measurements in people with a spinal deformity, arthritis in the lower spine, or a history of spinal surgery. Also, 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. QCT 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 simply uses different software than standard CT to analyze BMD. It takes about 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. (Single-photon absorptiometry, an older test, is rarely used.)
Peripheral techniques are valuable for osteoporosis screening; however, their lack of precision makes them less useful for following changes in BMD over time or for tracking response to therapy. In addition, peripheral techniques cannot 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. Research indicates that 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. In general, the results of RA testing provide an accurate measure of bone density. RA takes just one to three minutes to perform, and the test involves only limited exposure to radiation.
Peripheral quantitative computed tomography (pQCT)
pQCT 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)
This test is 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.