Leukocytosis and Leukopenia

Leukocytosis and Leukopenia is a topic covered in the Washington Manual of Medical Therapeutics.

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General Principles


  • Leukocytosis is an elevation in the absolute WBC count (>10,000 cells/μL).
  • Leukopenia is a reduction in the WBC count (<3500 cells/μL).


  • Leukocytosis
    • An elevated WBC can be the normal BM response to an infectious or inflammatory process, corticosteroids, β-agonist or lithium therapy, or splenectomy, and is usually associated with an absolute neutrophilia.
    • Occasionally, leukocytosis is due to a primary BM disorder with an increase in WBC production and/or delayed maturation. This may occur in the setting of hematologic malignancies such as leukemias or myeloproliferative neoplasms (MPNs) and can affect any cell in the leukocyte lineage.
    • A “leukemoid reaction” is defined as an excessive WBC response usually reserved for neutrophilia (>50,000/μL) due to a reactive cause.
    • Lymphocytosis is less commonly encountered and is typically associated with atypical infections (i.e., viral), medication use, or leukemia/lymphoma.
  • Leukopenia
    • Leukopenia can occur in response to infection (i.e., HIV), inflammation, primary BM disorders (i.e., malignancy), autoimmune disorders, medications, environmental exposure (i.e., heavy metals or radiation), and vitamin deficiencies.
    • Many cases are medication induced (i.e., chemotherapeutic or immunosuppressive drugs).
    • Idiopathic chronic benign neutropenia may be caused by an antineutrophil antibody or an inherited disorder.
    • Large granular lymphocytic leukemia can be a cause of neutropenia, especially in patients with rheumatoid arthritis.

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