What Blood Tests Diagnose Iron Deficiency?

Iron deficiency is diagnosed using a combination of blood tests that together assess iron stores, transport, and the effect on red blood cells.

Best Single Test: Ferritin

• Below 12 ng/mL = depleted iron stores (definitive)
• Below 20 ng/mL = iron deficiency likely
• 20–50 ng/mL = borderline; consider symptoms and risk factors
• Ferritin is also an inflammatory marker — check CRP if inflammation suspected

Iron Panel (Complete Assessment)

• Serum iron: low in deficiency
• TIBC: high in deficiency (body makes more transport protein)
• Transferrin saturation: below 20% in deficiency
• Together: low ferritin + high TIBC + low saturation = iron deficiency confirmed

CBC Changes in Iron Deficiency

• Low hemoglobin (anemia develops after stores are depleted)
• Low MCV — small red blood cells (microcytosis)
• Low MCH and MCHC — less hemoglobin per cell
• High RDW — size variation as new small cells mix with old normal cells
• Low ferritin + high RDW + low MCV = classic iron deficiency pattern

Distinguishing from Thalassemia

• Both have low MCV
• Iron deficiency: high RDW, low ferritin
• Thalassemia: normal RDW, normal/high ferritin
• Mentzer index (MCV/RBC) below 13 suggests thalassemia