Bone Mineral Density

DEXA Scan

Results

A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray, computed tomography (CT) scan, or ultrasound. Results are usually available in 2 to 3 days.

Results of bone mineral density tests can be reported in several ways.

T-score

Your T-score is your BMD compared to the average score of a healthy 30-year-old. It is expressed as a standard deviation (SD), which is a statistical measure of how closely each person in a group is to the average (mean) of the group. The average BMD is determined by measuring the bone density of a large group of healthy 30-year-olds (young adult reference range). BMD values are then reported as a standard deviation from the mean of this reference group. Almost all 30-year-old people have a BMD value within 2 standard deviations of this mean.

• A negative (–) value indicates that you have thinner bones (lower bone density) than an average 30-year-old. The more negative the number is, the less bone density you have compared with an average 30-year-old.
• A positive (+) value indicates that your bones are thicker and stronger than an average 30-year-old.

The following table contains the World Health Organization's definitions of osteoporosis based on bone mineral density T-scores.

Bone mineral density

	T-score
Normal:	Less than 1 standard deviation (SD) below the young adult reference range (more than –1)
Low bone mass (osteopenia):	1 to 2.5 SDs below the young adult reference range (–1 to –2.5)
Osteoporosis:	2.5 or more SDs below the young adult reference range (–2.5 or less)
Severe osteoporosis:	2.5 or more SDs below the young adult reference range (–2.5 or less) and the person has had one or more broken bones

 

If your bone mineral density test result is low:

• You may have osteoporosis. Doctors usually use the lowest T-score to diagnose osteoporosis. For example, if your T-score at your spine is –3 and your T-score at your hip is –2, the spine T-score would be used to diagnosis osteoporosis.
• You have a higher-than-average chance of breaking a bone. The more negative your T-score, the greater your chances of breaking a bone during a fall or from a minor injury. Every change of 1 SD means a twofold increase in the risk of fracture at that site. For example, if you have a T-score of –1, your chances of having a broken bone are 2 times greater than if your T-score was 0.

Low BMD values may be caused by other problems, including:

• Cancer, such as multiple myeloma.
• Cushing's syndrome, hyperthyroidism, or hyperparathyroidism.
• Diseases of the spine, such as ankylosing spondylitis.
• Premature menopause.
• Vitamin D deficiency such as rickets.

Z-score

Your BMD value may also be compared to other people of your age, sex, and race. This is called your Z-score. It is given in standard deviations (SD) from the average value for your age group.

• A negative (–) value means that your bones are thinner (lower bone density) and weaker than most people in your age group. The more negative the number is, the less bone density you have compared with others in your age group.
• A positive (+) value means that your bones are thicker and stronger than most people in your age group.

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Author:	Jan Nissl, RN, BS	Last Updated: October 4, 2006
Medical Review:	Joy Melnikow, MD, MPH - Family Medicine Carla J. Herman, MD, MPH - Internal Medicine
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