Osteoporosis
and Related Bone Diseases
Bone Mass Measurement: What the Numbers Mean
Question: I am confused about my bone density test results – what do the different numbers mean?
Answer: A bone mineral density test (BMD), a non-invasive and painless test, is the best way to determine your bone health. BMD tests can identify osteoporosis, determine your risk for fractures and monitor your response to an osteoporosis treatment. Different BMD tests may measure your hip, spine, wrist, finger, shin bone or heel.
Your BMD is compared to two norms, “young normal” and “age-matched.” Young normal, known as your T-score, compares your BMD to optimal or peak density of a 30-year old healthy adult and determines your fracture risk, which increases as BMD falls below young-normal levels. Age-matched, known as your Z-score, compares your BMD to what is expected in someone your age and body size. Among older adults, however, low BMD is common, so comparison with age-matched norms can be misleading.
The difference between your BMD and that of a healthy young adult is referred to as a standard deviation (SD). As outlined in the World Health Organization’s diagnostic categories, individuals whose 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 10-12 percent decrease in bone density.
The NOF Physician’s Guide to Prevention and Treatment of Osteoporosis suggests BMD testing for the following postmenopausal women:
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Those with one or more additional risk factors for osteoporotic fracture (besides menopause);
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Those who have had a fracture (broken bone) to determine if osteoporosis is the underlying cause;
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Those who are ages 65 and older, regardless of other risk factors;
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Those who are considering therapy for osteoporosis, if BMD testing will facilitate the decision; and
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Those who have been on hormone replacement therapy for prolonged periods.
The information provided by a BMD test can help your doctor decide which prevention or treatment options are right for you.
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World Health Organization Definitions of Osteoporosis Based on Bone Density Levels |
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Normal. |
Bone density is within 1 SD (+1 or -1) of the young adult mean. |
Low Bone Mass. |
Bone density is 1 to 2.5 SD below the young adult mean (-1 to -2.5 SD). |
Osteoporosis |
Bone density is 2.5 SD or more below the young adult mean (> -2.5 SD). |
Severe (established) osteoporosis. |
Bone density is more than 2.5 SD below the young adult mean and there has been one or more osteoporotic fractures. |
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