Hyperleukocytosis is a total white blood cell (WBC) count of ≥100,000/μL.

Risk Factors

  • Presentation with hyperleukocytosis depends on the type of leukemia.
  • Percent with hyperleukocytosis
    • Chronic myeloid leukemia (CML) especially in blast crisis ~100%
    • Acute myeloid leukemia (AML) especially in infants 5–25%
    • Acute lymphoblastic leukemia (ALL) especially with mediastinal mass 8–13%
  • Factors associated with more severe clinical course:
    • Coagulopathy
    • Metabolic derangements
    • Mortality associated more with clinical symptoms (CNS or respiratory) than with WBC
Children with trisomy 21 (Down syndrome) have an increased risk of developing transient leukemoid reactions and have an increased risk of developing acute leukemia, more commonly ALL.


The primary mechanism for symptoms is leukostasis due to increased viscosity and impaired blood flow. Contributory factors other than number of leukemic cells include their shape and size, which increases viscosity, as well as endothelial cell damage, which further impedes microcirculation.

  • WBCs lack the concave shape that enables reversible deformability characteristic of red blood cells (RBCs) allowing passage through the microvasculature. As compared to normal WBCs, blasts are much larger with less deformability.
  • Myeloblasts are twice the size of lymphoblasts, which are 25% larger than mature neutrophil granulocytes. Monoblasts are larger than myeloblasts. Aggregates of these large, undeformable cells cannot pass through capillaries.
  • In addition to increased cell-to-cell adhesions, leukemic blasts have increased adhesion to the damaged endothelium, which promote additional cell aggregates through endothelial toxin and cytokine release.
  • Leukemic cells have a hypersensitive response to cytokines, which may account for clinical leukostasis at lower peripheral blast counts.
  • Leukostasis impairs blood flow and exacerbates hypoxemia; as expected, leukemic blasts have increased oxygen consumption due to a high rate of cell division.

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