Classification

Microcytic (MCV < 80 fL): often due to defective hemoglobin synthesis
- Defective globin chain:
  - Thalassemias
- Defective heme synthesis:
  - Anemia of chronic disease (late)
  - Iron deficiency (late) ≈ sideroblastic anemia
  - Lead poisoning
Normocytic (MCV 80-100 fL): categorized by whether there is appropriate bone marrow compensation (high reticulocyte count) or not (low reticulocyte count)
- Nonhemolytic (low reticulocyte index): Underproduction of RBCs.
  - Iron deficiency (early)
  - Anemia of chronic disease (early)
  - Aplastic anemia
  - Chronic kidney disease
  - Acute blood loss (hemorrhage)
- Hemolytic (high reticulocyte index): Increased destruction of RBCs.
  - Intrinsic
    - Membrane defects
      - Hereditary spherocytosis
      - Paroxysmal nocturnal hemoglobinuria
    - Enzyme deficiencies
      - G6PD deficiency
      - Pyruvate kinase deficiency
    - Hemoglobinopathies
      - Sickle cell anemia
      - HbC disease
  - Extrinsic
    - Autoimmune
    - Microangiopathic
    - Macroangiopathic
    - Infections
Macrocytic (MCV > 100 fL)
- Megaloblastic: Impaired DNA synthesis leads to delayed nuclear maturation relative to the cytoplasm.
  - DNA affected
    - Defective DNA synthesis
    - Defective DNA repair
      - Fanconi anemia c
- Nonmegaloblastic: Macrocytosis without impaired DNA synthesis. Hypersegmented neutrophils are absent.
  - Diamond-Blackfan anemia c
  - Liver disease
  - Chronic alcohol overuse

Clinical features

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