Bone Mass Measurement: What the Numbers Mean | NIH Osteoporosis and Related Bone Diseases National Resource Center
A bone mineral density (BMD) test is can provide a snapshot of your bone health.
The test can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment.
The most commonly used BMD test is called a central dual-energy x-ray absorptiometry, or central DXA test. It is painless – a bit having an x-ray. The test can measure bone density at your hip and lumbar spine.
Peripheral bone density tests measure bone density in the lower arm, wrist, finger or heel. These tests are often used for screening purposes and can help identify people who might benefit from follow-up bone density testing at the hip and lumbar spine.
What does the test do?
A BMD test measures your bone mineral density and compares it to that of an established norm or standard to give you a score. Although no bone density test is 100-percent accurate, the BMD test is an important predictor of whether a person will have a fracture in the future.
Most commonly, your BMD test results are compared to the bone mineral density of a healthy young adult, and you are given a T-score. A score of 0 means your BMD is equal to the norm for a healthy young adult.
Differences between your BMD and that of the healthy young adult norm are measured in units called standard deviations (SDs).
The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.
As shown in the table below, a T-score between +1 and −1 is considered normal or healthy. A T-score between −1 and −2.5 indicates that you have low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of −2.5 or lower indicates that you have osteoporosis. The greater the negative number, the more severe the osteoporosis.
|Normal||Bone density is within 1 SD (+1 or −1) of the young adult mean.|
|Low bone mass||Bone density is between 1 and 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 or lower).|
|Severe (established) osteoporosis||Bone density is more than 2.5 SD below the young adult mean, and there have been one or more osteoporotic fractures.|
Low bone mass versus osteoporosis
The information provided by a BMD test can help your doctor decide which prevention or treatment options are right for you.
If you have low bone mass that is not low enough to be diagnosed as osteoporosis, this is sometimes referred to as osteopenia. Low bone mass can be caused by many factors such as:
- Low body weight.
- A medical condition or medication to treat such a condition that negatively affects bone.
Although not everyone who has low bone mass will develop osteoporosis, low bone mass is an important risk factor for osteoporosis fractures.
As a person with low bone mass, you can take steps to help slow down your bone loss and prevent osteoporosis in your future. Your doctor will want you to develop – or keep – healthy habits such as eating foods rich in calcium and vitamin D and doing weight-bearing exercise such as walking, jogging, or dancing. In some cases, your doctor may recommend medication to prevent osteoporosis.
Osteoporosis: If you are diagnosed with osteoporosis, these healthy habits will help, but your doctor will probably also recommend that you take medication.
Several effective medications are available to slow – or even reverse – bone loss.
If you do take medication to treat osteoporosis, your doctor can advise you concerning the need for future BMD tests to check your progress.
Who should get a bone density test?
The U.S. Preventive Services Task Force recommends that all women age 65 and older should have a bone density test. Postmenopausal women who are younger than age 65 and at increased risk of osteoporosis – as determined by a formal clinical risk assessment tool – should also have a BMD test.
Due to a lack of available evidence, the Task Force did not make recommendations regarding osteoporosis screening in men.
For your information
This publication contains information about medications used to treat the health condition discussed here. When this publication was developed, we included the most up-to-date (accurate) information available. Occasionally, new information on medication is released.
For updates and for any questions about any medications you are taking, please contact
U.S. Food and Drug AdministrationToll Free: 888-INFO-FDA (888-463-6332)