Ferritin is recognized as the most sensitive and specific marker of iron storage, and low ferritin alone is diagnostic of IDA in the general population, i.e. uncomplicated cases of IDA. The measurement of iron is a poor biomarker for IDA as it is susceptible to preanalytical factors such as diurnal variation, diet, and exercise, and ultimately does not represent iron storage. In patients with complicating comorbidities (e.g. infection, autoimmune disease, kidney disease, or cancer), ferritin is an acute phase reactant and may be falsely elevated. In this setting, ordering a fasting transferrin saturation is useful to help diagnose iron deficiency together with the ferritin result.
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WHO Guideline on use of ferritin concentrations to assess iron status in individuals and populations.