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Bone mineral density may be quoted as the number of standard deviations(SD) from the young adult mean. This is known as the T score.
The World Health Organisation define normal bone density to be within 1 SD of the young adult mean i.e. T score > -1. Osteopenia is defined by a T score between -1 and -2.5. Osteoporosis is defined as a T score less than -2.5.
It should be noted that the WHO definition of osteoporosis encompasses a large proportion of the elderly population and as mentioned elsewhere it is not the only consideration in determining treatment.