Bone mineral density

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 : 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.
 * All women aged 65 and older regardless of risk factors
 * Younger postmenopausal women with one or more risk factors.
 * 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.

These tests include:
 * Dual Energy X-ray Absorptiometry (DXA or DEXA)
 * Quantitative Computed Tomography (QCT)
 * Qualitative Ultrasound (QUS)
 * Single Photon Absorptiometry (SPA)
 * Dual Photon Absorptiometry (DPA)
 * Digital X-ray Radiogrammetry (DXR)
 * Single Energy X-ray Absorptiometry (SEXA)

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.

Average bone mineral density = BMC / W [g/cm²]
 * BMC = bone mineral content = g/cm
 * W = width at the scanned line

Interpretation
Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.

T-score
The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old of the same sex and ethnicity. This value is used in post-menopausal women and men over aged 50 because it better predicts risk of future fracture. The criteria of the World Health Organization are :
 * Normal is a T-score of -1.0 or higher
 * Osteopenia is defined as less than -1.0 and greater than -2.5
 * Osteoporosis is defined as -2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a thirty year old woman.

Z-score
The Z-score is the number of standard deviations a patient's BMD differ from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children.