A. Normal bone density is defined as a bone mineral density (BMD) value within one standard deviation of the mean value in young adults of the same sex and race.
B. Osteopenia is defined as a BMD between 1 and 2.5 standard deviations below the mean.
C. Osteoporosis is defined as a value more than 2.5 standard deviations below the mean; this level is the fracture threshold. These values are referred to as T-scores (number of standard deviations above or below the mean value).
D. Dual x-ray absorptiometry. In dual x-ray absorptiometry (DXA), two photons are emitted from an x-ray tube. DXA is the most commonly used method for measuring bone density because it gives very precise measurements with minimal radiation. DXA measurements of the spine and hip are recommended.
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