Bone Mineral Density
Results of bone mineral density tests can be reported in
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 means 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
- A positive (+) value means 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 density2
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
More than 2.5 SDs below the young adult reference range
(–2.5 or less)
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
- 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,
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
- 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.