Definition: severe elevation in WBC (> 50,000 cells/mm³) caused by reactive causes outside the bone marrow. It’s a normal physiologic response to infection or stress.

The diagnostic workup consists of the exclusion of chronic myelogenous leukemia (CML) and chronic neutrophilic leukemia (CNL) and the detection of an underlying cause.

The diagnosis characteristics are marked mature neutrophilia with a left shift, a high leukocyte alkaline phosphatase (LAP) levels and hypercellular bone marrow with intact maturation and morphology of all the elements.  Absence of or decreased LAP in a patient with leukocytosis suggests bone marrow disease as leukemia because overproduced WBCs are not functioning properly.

The major causes of leukemoid reactions are:

  • Severe infections such as Clostridium difficile colitis, disseminated tuberculosis, and severe shigellosis
  • Drugs: corticosteroids and minocycline
  • Malignancies: no malignant hematological disorders. Malignancy-associated leukemoid reaction are commonly observed in a variety of carcinomas, most notably lung and kidney cancer
  • Toxin: ethylene glycol
  • Severe hemorrhage: retroperitoneal hemorrhage
  • Acute hemolysis
  • Alcoholic steatohepatitis

Further reading:

  • Sakka V, Tsiodras S, Giamarellos-BourboulisEJ, Giamarellou H. An update on the etiology and diagnostic evaluation of a leukemoid reaction. Eur J Intern Med. 2006 Oct;17(6):394-8. (link)

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