A 32-year-old woman with menorrhagia is evaluated for fatigue. CBC shows Hb 9.8 g/dL, MCV 74 fL, and increased red cell distribution width. To confirm that iron deficiency, rather than another microcytic state, is responsible for her anemia, which single test result is most specific?
- A Low mean corpuscular volume
- B Reduced serum iron concentration
- C Serum ferritin less than 15 ng/mL ✓
- D Elevated total iron binding capacity
Correct answer: C. Serum ferritin less than 15 ng/mL
Explanation
Microcytosis, raised RDW, low serum iron, and high TIBC are seen in several microcytic anemias and fluctuate with inflammation and diurnal variation. Ferritin reflects storage iron and a level below 15 ng/mL is highly specific for iron deficiency. Its main limitation is that it rises as an acute phase reactant, so in coexisting inflammation a normal ferritin does not exclude the diagnosis, but a frankly low value essentially clinches it.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.