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. …