A bone mineral density (BMD) test is used to provide a snapshot of bone health. The test can identify osteoporosis, determine risk for fractures and measure typical responses to osteoporosis treatment. The most widely recognized BMD test is called a central dual-energy x-ray absorptiometry, or central DXA test. It is a painless test (similar to having an x-ray) that can measure bone density at the hip and spine.
Although not 100% accurate, the BMD test measures bone mineral density and compares it to that of an established norm or standard to give a score. Commonly, the BMD test results are compared to the ideal or peak bone mineral density of a healthy 30-year-old adult and a T-score is assigned. A score of 0 means the BMD is equal to the norm for a healthy young adult. Differences between the 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 the BMD and the higher the risk of fracture.
World Health Organization Definitions Based on Bone Density Levels
Level – Definition
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.
From the table above, a T-score between +1 and −1 is considered normal or healthy. A T-score between −1 and −2.5 indicates low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of −2.5 or lower indicates osteoporosis. The greater the negative number, the more severe the osteoporosis.
The number of standard deviations away from the mean is represented in most reports as a Z score. A Z score of -1 means that the subject is 1 SD below the mean, and +2, 2 SD above the mean. The Z score may be expressed relative to normal people of the same age, or compared with young normals. It is the latter comparison (frequently designated the T score) which relates more closely to fracture risk. In other words, when comparisons are made with people of the same age as the subject, then z-scores are used. While on the other hand, when comparisons are made with a typical young adult, then t-scores are used. Often, many clinical and non-clinical decisions (such as Medicare rebate eligibility) are related to both T & Z scores.
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