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Bone Density Test

How a Bone Density Test 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 Do the Results Mean?

As outlined in the World Health Organization's diagnostic categories, individuals whose T-score on a test for bone density 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 to 12 percent decrease in bone density.
 

Who Should Get a Bone Density Test?

A bone density test is recommended for those people:
 
  • With one or more additional risk factors for osteoporotic fracture (besides menopause)
  • Who have had a fracture (broken bone); this test can help determine if osteoporosis was the underlying cause
  • Who are age 65 and older
  • Who are considering therapy for osteoporosis (if BMD testing will help make the decision)
  • Who have been on hormone replacement therapy for a long time.
 
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