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Interpretation of a single measurement of bone mass, like evaluation of a child's height, must take age into account. Like cholesterol concentrations, bone mass is associated with risk even when values are in the "normal" range. Considering this will avoid those "below the fracture threshold" being frightened or those who have a "normal" bone mass being falsely reassured. The absolute risk of a fracture doubles with each decade after the age of 50, and, at any age, the relative risk of fracture increase 1.5-fold to twofold for each standard deviation decrease in bone mass (roughly 10%). 1
Interpreting a change in bone mass requires caution. Problems arise from the imprecision of instruments, the increasing risks with multiple measurements of rejecting the null hypothesis when it is true (type I error), assumptions that bone density is a volumetric density, suggestions that treatment causes linear increases in bone mass, and assumptions that the pattern of bone loss is reversible.
Dual energy x ray absorptiometry provides the most precise measurements of bone mass. Even with this new technique, however, the size of clinically important changes in bone mass is less than the measurement error. A walk around the room causes the measurement to change by up to 6% (at the hip), which corresponds to six years of bone lost at the usual rate. Thus, for individual patients, clinically important changes in bone mass may take several years to detect. Only severe loss can be measured with two determinations. Alterations in machine function must be anticipated and careful quality control applied. 2
In clinical studies the newer technology has improved the power to detect changes in bone mass, and...