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Osteoporosis Health Center

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Bone Density

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

Bone density 2



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)


More than 2.5 SDs below the young adult reference range (-2.5 or less)

If your bone 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 bone density values may be caused by other problems, including:


Your bone density 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.

What Affects the Test

Reasons you may not be able to have the bone density test or why the results may not be helpful include:

  • Inability to be correctly positioned during the test.
  • Having a broken bone in the past. This can cause falsely high bone density results.
  • Arthritis of your spine. In this case, the changes caused by arthritis in the spine may not make the spine the best place to measure for osteoporosis.
  • Metal implants from hip replacement surgery or hip fracture.
  • Having an X-ray test that uses barium within 10 days of the bone density test.

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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