Bone Density Tests: What the Numbers Mean
If you’re considering taking a bone density test, or have taken one but find the result puzzling, here are some helpful explanations from the NIH Osteoporosis and Related Diseases National Resource Center, part of the National Institutes of Health:
A bone mineral density (BMD) test is can provide a snapshot of your bone health. The test can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. The most widely recognized BMD test is called a central dual-energy x-ray absorptiometry, or central DXA test. It is painless—a bit like having an x-ray. The test can measure bone density at your hip and spine.
Another procedure, a peripheral bone density test, measure bone density in the lower arm, wrist, finger or heel. These tests are often used for screening purposes and can help identify people who might benefit from additional bone density testing.
What Does the Test Do?
A BMD test measures your bone mineral density and compares it to that of an established norm or standard to give you a score. Although no bone density test is 100-percent accurate, the BMD test is an important predictor of whether a person will have a fracture in the future.
Most commonly, your BMD test results are compared to the ideal or peak bone mineral density of a healthy 30-year-old adult, and you are given a T-score. A score of 0 means your BMD is equal to the norm for a healthy young adult. Differences between your BMD and that of the healthy young adult norm are measured in units called standard deviations (SDs). The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.
A T-score between +1 and −1 is considered normal or healthy. A T-score between −1 and −2.5 indicates that you have low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of −2.5 or lower indicates that you have osteoporosis. The greater the negative number, the more severe the osteoporosis.
Low Bone Mass Versus Osteoporosis
The information provided by a BMD test can help your doctor decide which prevention or treatment options are right for you.
If you have low bone mass that is not low enough to be diagnosed as osteoporosis, this is sometimes referred to as osteopenia. Low bone mass can be caused by many factors such as:
The development of less-than-optimal peak bone mass in your youth
A medical condition or medication to treat such a condition that negatively affects bone
Abnormally accelerated bone loss.
Although not everyone who has low bone mass will develop osteoporosis, everyone with low bone mass is at higher risk for the disease and the resulting fractures.