Bone mineral density
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A bone mineral density (BMD) test, also called a bone mass measurement, is used to measure bone density and determine fracture risk for osteoporosis. It may also be used to determine how effective an osteoporosis treatment is. The National Osteoporosis Foundation recommends BMD testing for the following individuals [1]:
- All women aged 65 and older regardless of risk factors
- Younger postmenopausal women with one or more risk factors (other than being white, postmenopausal and female).
- Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity).
- Estrogen deficient women at clinical risk for osteoporosis.
- Individuals with vertebral abnormalities.
- Individuals receiving, or planning to receive, long-term glucocorticoid (steroid) therapy.
- Individuals with primary hyperparathyroidism.
- Individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy.
While there are many different types of BMD tests, all are non-invasive. Most tests differ in which bones are measured to determine the BMD result.
These tests include:
- Dual Energy X-ray Absorptiometry (DXA or DEXA)
- Quantitative Computed Tomography (QCT)
- Qualitative Ultrasound (QUS)
The test works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine risk for fractures and the stage of osteoporosis in an individual.
Results are generally scored by two measures, the T-score and the Z-score. The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old, and the Z-score is a comparison of a patient's BMD to the average BMD of their, sex, and race. Scores indicate the amount ones bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.
Osteoporosis is defined as a T-score of -2.5, meaning a bone density that is two and a half standard deviations below the mean of a thirty year old woman. The T-score is used more commonly in clinical practice because it better predicts risk of future fracture.
Average bone mineral density = BMC / W [g/cm²]
- BMC = bone mineral content = g/cm
- W = width at the scanned line
