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Red Blood Cell Indices

  • Hematocrit (HCT): Volume percentage of RBCs in blood. See Table 12-1 for age-related normal levels

  • Mean corpuscular volume (MCV): Average RBC volume

  • Mean corpuscular hemoglobin (MCH): Average quantity of hemoglobin per RBC

  • Mean corpuscular hemoglobin concentration (MCHC): Grams of hemoglobin per 100 mL of packed RBCs (amount of hemoglobin per unit volume)

  • Red cell distribution width (RDW): Index of the variation in RBC size

  • Reticulocyte count: Percentage of young RBCs in the plasma

Table 12-1Normal Hemoglobin, Hematocrit, and Mean Corpuscular Volume Values by Age

Differential Diagnosis Based on MCV

Microcytic Anemia

  • Iron deficiency, thalassemia, anemia of inflammation, lead poisoning, sideroblastic anemia, copper deficiency

  • Iron deficiency versus thalassemia trait: RBC count and RDW are often normal in thalassemia trait; in iron deficiency, low RBC count and elevated RDW; Mentzer index (MCV/RBC) serves as a useful screen (<13 suggests thalassemia trait, >13 suggests iron deficiency).

Normocytic Anemia

  • In normocytic anemia, assess the reticulocyte count. The reticulocyte index accurately reflects erythropoiesis by adjusting for the degree of anemia.

  • Reticulocyte index (RI) = % Reticulocyte × Patient HCT/Normal HCT

  • RI is greater than 3% in compensated bleeding or hemolysis and less than 2% in anemia owing to decreased RBC production; 1% is normal marrow activity without anemia.

  • Reticulocytes low:

    • ✓ Indicates hypoproductive process

    • ✓ Pure RBC dysplasia (Diamond–Blackfan anemia)

    • ✓ Transient erythroblastopenia of childhood

    • ✓ Aplastic crisis (e.g., secondary to parvovirus B19 infection)

    • ✓ Renal disease; acute bleeding (without compensation)

    • ✓ Marrow infiltration (e.g., leukemia, solid tumor, hemophagocytosis, or myelofibrosis)

    • ✓ Marrow aplasia (aplastic anemia)

    • ✓ Hormone deficiencies (e.g., hypothyroidism or growth hormone deficiency)

    • ✓ Anemia of inflammation

  • Reticulocytes high:

    • ✓ Chronic blood loss with compensatory erythropoiesis

    • ✓ Extrinsic hemolysis: Antibody-mediated hemolysis (e.g., auto- or drug-induced), DIC, HUS, TTP, prosthetic heart valve, vitamin E deficiency, Wilson disease, liver disease, renal disease, burns

    • ✓ Intrinsic hemolysis: Membrane disorders (e.g., hereditary spherocytosis), enzyme deficiencies (e.g., G6PD), hemoglobin disorders (e.g., sickle cell disease)

Macrocytic Anemia

  • Folic acid deficiency, vitamin B12 deficiency, normal newborn (MCV, 100–125), trisomy 21, medications (e.g., valproate, trimethoprim–sulfamethoxazole, hydroxyurea, azathioprine), bone marrow ...

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