Distribution of ferritin in subjects with normal sTfR and those with elevated sTfR. The ferritin concentrations of iron-replete and iron-deplete (diminished or exhausted iron stores) subjects (sTfR <2.75 mg/L) are compared with the corresponding values of subjects with IDE, ie, depletion of the functional iron compartment (sTfR >2.75 mg/L). The solid, horizontal line represents the ferritin concentration of 22 μg/L, which we suggest to be used as a cutoff value for clinically relevant depletion of the storage iron compartment.