Bone Density: An Overview
Bone density is a measurement of the mineral density in your bones. It is used to determine a person's risk for
osteoporosis.
How Bone Density Is Measured
A bone mineral density test (BMD), a non-invasive and painless test, is the best way to determine your bone density. A
bone density test can identify osteoporosis, determine your risk for fractures, and monitor your response to
osteoporosis treatment. Different bone density tests may measure your hip, spine, wrist, finger, shin bone, or heel.
How Bone Density Is Scored
Your bone density test is compared to two norms: "young normal" and "age-matched." Young normal, known as your T-score, compares your bone mineral density to optimal or peak density of a 30-year-old, healthy adult and determines your fracture risk, which increases as bone mineral density falls below young-normal levels. Age-matched, known as your Z-score, compares your bone mineral density to what is expected in someone your age and body size. Among older adults, however, low bone mineral density is common, so comparison with age-matched norms can be misleading.
The difference between your bone mineral density and that of a healthy, young adult is referred to as a standard deviation (SD).
What Does the Bone Density Score Mean?
As outlined in the World Health Organization's diagnostic categories, individuals whose bone density test T-score is within one standard deviation of the "norm" are considered to have normal bone density.
Scores below the "norm" are indicated in negative numbers. For example, a score from -1 to -2.5 SD below the norm indicates low bone mass, or
osteopenia, and a score of more than -2.5 SD below the norm is considered a diagnosis of osteoporosis. For most BMD tests, -1 SD equals a 10 percent to 12 percent decrease in bone density.
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