When you have a bone mineral density test,
it compares your bone density to a "young normal" healthy 30-year-old adult with peak bone density (also called peak bone mass). Peak bone density is the point at which a person has the greatest amount of bone that she or he will ever have.

You will get the result of your BMD test in a special number called a T-score. It stands for "standard deviations" or "SD." It indicates how much your bone density is above or below normal.

Healthcare providers use the T-score to diagnose osteoporosis. If more than one bone is tested, they use the lowest T-score to make a diagnosis of osteoporosis. The World Health Organization (WHO) has defined the T-scores and what they mean.

What your T-score means:

• A T-score between +1 and -1 is normal bone density. Examples are 0.8, 0.2 and -0.5.
• A T-score between -1 and -2.5 indicates low bone density or osteopenia. Examples are T-scores of -1.2, -1.6 and -2.1.
• A T-score of -2.5 or lower is a diagnosis of osteoporosis. Examples are T-scores of -2.8, -3.3 and -3.9.

The lower a person's T-score, the lower the bone density. A T-score of -1.0 is lower than a T-score of 0.5; a T-score of -2.0 is lower than a T-score of -1.5; and a T-score of -3.5 is lower than a T-score of -3.0.

For most BMD tests, 1 SD difference in a T-score equals a 10-15 percent decrease in bone density. For example, a person with a T-score of -2.5 has a 10-15 percent lower BMD than a person with a T-score of -1.5.

Your BMD test result also includes a Z-score that compares your bone density to what is normal in someone your age and body size. Healthcare providers do not use Z-scores to diagnose osteoporosis in postmenopausal women and men age 50 or older. Among older adults low bone mineral density is common, so Z-scores can be misleading. An older person might have a "normal" Z-score but still be at high risk for breaking a bone.

Most experts recommend using Z-scores rather than T-scores for younger men, premenopausal women and children. However, healthcare providers often use
T-scores for perimenopausal women. A Z-score above -2.0 is normal according to the International Society for Clinical Densitometry (ISCD). A diagnosis of osteoporosis in younger men, premenopausal women and children should not be based on a BMD test result alone. NOF does not recommend routine BMD testing in children, adolescents, healthy young men or premenopausal women. (source National Osteoporosis Foundation, 2008)

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