Clinical meaning
The NP systematically evaluates iron deficiency anemia (IDA) through a staged diagnostic approach integrating laboratory interpretation with clinical context. Iron deficiency progresses through three biochemical stages before overt anemia develops. Stage 1 (iron depletion): bone marrow iron stores are depleted, serum ferritin falls below 30 ng/mL, but hemoglobin remains normal and the patient is asymptomatic. Stage 2 (iron-deficient erythropoiesis): serum iron decreases, transferrin (TIBC) increases as the liver upregulates transferrin synthesis to scavenge scarce iron, transferrin saturation falls below 20%, and soluble transferrin receptor (sTfR) rises as erythroid precursors upregulate receptor expression to compete for limited iron; the reticulocyte hemoglobin content (CHr or Ret-He) drops below 28 pg, providing the earliest indicator of iron-restricted erythropoiesis. Stage 3 (iron deficiency anemia): hemoglobin falls below normal, MCV decreases below 80 fL (microcytosis), MCH and MCHC decrease (hypochromia), RDW increases above 14.5% (anisocytosis reflecting a mixed population of normocytic and microcytic cells), and the peripheral smear shows microcytic hypochromic red cells with pencil cells, target cells, and increased central pallor. The NP must differentiate IDA from other causes of microcytic anemia: anemia of chronic disease/inflammation (ACD) is characterized by low serum iron but also LOW TIBC (unlike IDA where TIBC is elevated) and normal-to-elevated ferritin (as an acute phase reactant); thalassemia trait shows microcytosis with a normal-to-elevated RBC count and normal iron studies (Mentzer index MCV/RBC less than 13 suggests thalassemia); sideroblastic anemia shows ringed sideroblasts on bone marrow biopsy with elevated serum iron and ferritin. Ferritin interpretation requires clinical context: as an acute phase reactant, ferritin may be falsely normal or elevated in concurrent infection, inflammation, malignancy, or liver disease, masking iron deficiency. A ferritin level less than 30 ng/mL is diagnostic in uncomplicated patients, but in the setting of chronic inflammation, a ferritin less than 100 ng/mL with transferrin saturation less than 20% is suggestive of functional iron deficiency.