Common sites for measurement of bone mineral density:
(1) femoral neck (best site for predicting hip fractures)
(2) vertebra (best site for predicting spinal fractures)
(3) radius
(4) calcaneus
Parameters:
(1) mean bone density for adults of the same age and gender in g per square cm
(2) standard deviation (SD) in the bone density for adults of the same age and gender
(3) patient's bone density in g per square cm
(4) cutoff Z score for this population associated with osteopenia
(5) cutoff Z score for this population associated with osteoporosis
Z score for the patient =
= number of standard deviations above or below the mean bone mineral density for adults of the same age and gender =
= ((patient bone mineral density in g/cm^2) - (mean bone mineral density for adults of the same age and gender)) / (standard deviation for adults of the same age and gender)
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Z score of patient
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Interpretation
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above the cutoff for osteopenia
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relatively normal bone density
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between the 2 cutoffs
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osteopenia
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below the cutoff for osteoporosis
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osteoporosis
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Comparison with the T-score:
(1) The T score involves a similar calculation but the reference population consists of healthy adults from 29 to 50 years of age.
(2) Unlike the Z score the reference mean and standard deviation are constant each year. This simplifies the analysis.
