Which protein stores iron and reflects marrow iron stores in serum tests?

• A. Transferrin
• B. Ferritin
• C. Hematocrit
• D. Myoglobin

Answer: (B)

Ferritin is the iron storage protein that holds iron inside cells as a ferritin-iron complex, mainly in the liver, spleen, and bone marrow. Serum ferritin levels mirror total body iron stores, so measuring ferritin is the best way to assess how much iron the marrow has available to use. Clinically, ferritin is particularly sensitive for detecting iron deficiency: low ferritin indicates depleted iron stores even before red blood cell counts drop. Keep in mind ferritin can rise with inflammation or infection because it acts as an acute-phase reactant, so interpretation should consider inflammatory status. The other options don’t fit because transferrin is the transport protein conveying iron in the blood, hematocrit is the proportion of red blood cells in blood, and myoglobin stores oxygen in muscle rather than iron.