When you're young, your bones are dense and packed with calcium and other minerals that make bones strong. As you grow older, the bones lose minerals and become lighter and more porous (full of holes). Some loss of bone density is a normal part of aging. But if it is extreme, the bones become weak and prone to break. This is a condition called osteoporosis.
Based on your age and other factors, your doctor may want to do a bone mineral density (BMD) test to determine your risk for osteoporosis.
Why would I need a bone mineral density test?
A BMD test is done to check your bone health. The test can be used to:
- Diagnose osteoporosis
- Predict how likely you are to break a bone in the future
- Decide if you need treatment for osteoporosis
The U.S. Preventive Services Task Force recommends that:
- Women age 65 and older should have routine bone mineral density screening for osteoporosis.
- Women should start routine screening at age 60 if they have factors that raise their risk of bone loss. A low body weight is the single best predictor of low bone density.
Your doctor may suggest testing if you are a younger woman in menopause or an older man and you have if you have serious risk factors for bone loss.
How is bone density measured?
Dual-energy x-ray absorptiometry (DEXA) is the best and most common bone density test. It is similar to an x-ray, but it exposes you to a much smaller amount of radiation. DEXA is very accurate. It can measure bone at the lumbar spine and the hip. A DEXA measurement of the hip bone (femoral neck) is the best predictor of future hip fractures.
Bone density measurements of the heel, wrist, hand or forearm may be taken using other tests, such as:
- Peripheral dual-energy x-ray absorptiometry (pDEXA)
- Quantitative CT scan (QCT)
- Quantitative ultrasound (QUS)
Standard x-rays are not used to test bone density. They can be used to detect fractures, but they are not sensitive enough to spot early bone loss.
How do I prepare for the test?
A bone mineral density test is simple and pain-free, and it requires no real preparation. It will take about 15 minutes to perform. You will need to stay still during the test.
Tell the technician if you have had a recent nuclear medicine scan or a barium enema. The radiation from those tests could affect your bone density test results.
BMD testing is not advised if you are pregnant. The radiation dose is very small, but it may not be safe for a developing fetus.
What do the results mean?
A bone mineral density test gives results as number values called a T-score and a Z-score.
- The T-score compares your bone density to that of a healthy young adult of your sex. This score is the one usually used to diagnose osteoporosis. A negative score indicates bone loss. A T-score of minus 1 to minus 2.5 shows low bone mineral density (called osteopenia). A T-score of minus 2.5 or greater is osteoporosis. The higher the negative number, the greater your risk of fracture.
- The Z-score compares your bone density to that of other people of your age, sex and ethnic group. A negative Z-score means your bones are thinner and weaker than those of people similar to you, and a positive number means your bones are stronger. But the Z-score may not be very useful in older adults, because some bone loss is normal at this age. You could have a positive Z-score and still be at risk for fractures.
Your doctor will use your scores to decide if you need medication to treat or prevent osteoporosis. Whether you need treatment or not, you can take steps to lower your risk of fractures. These include not smoking, getting regular weight-bearing exercise, getting enough calcium and vitamin D, and limiting alcohol and caffeine.
