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What Is a Bone Density Test

A bone mineral density (BMD) test is the best way to determine 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 dual-energy x-ray absorptiometry, or DXA test. It is painless—a bit like having an x ray. The test can measure bone density at your hip and spine.

A DXA 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 DXA test is the single most important predictor of whether a person will have a fracture in the future.

Most commonly, your DXA 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.

Sometimes your bone mineral density is compared to that of a typical individual whose age is matched to yours. This comparison gives you a Z-score. Because a low BMD level is common among older adults, comparisons with the BMD of a typical individual whose age is matched to yours can be misleading. Therefore, the diagnosis of osteoporosis or low bone mass is based on your T-score. However, a Z-score can be useful for determining whether an underlying disease or condition is causing bone loss.