Which single laboratory parameter is the earliest and most specific indicator of depleted iron stores, falling before hemoglobin drops in developing iron deficiency anemia?

• A Serum iron
• B Transferrin saturation
• C Red cell distribution width
• D Serum ferritin ✓

Explanation

Ferritin reflects the storage iron pool and is the first value to fall when stores are exhausted, preceding a drop in serum iron, transferrin saturation, and hemoglobin. Serum iron and transferrin saturation fluctuate diurnally and rise after meals, so they are neither early nor specific. RDW rises later once anisopoikilocytosis develops. The main caveat is that ferritin is an acute phase reactant, so levels up to about 100 ng/mL may still coexist with deficiency in inflammation.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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