Leukemoid reaction

The term leukemoid reaction describes an increased white blood cell count (> 50,000 cells/μL), which is a physiological response to stress or infection (as opposed to a primary blood malignancy, such as leukemia). It often describes the presence of immature cells such as myeloblasts or red blood cells with nuclei in the peripheral blood. It may be lymphoid or myeloid. As noted above, a leukemoid reaction is typically a response to an underlying medical issue. Causes of leukemoid reactions include: Severe hemorrhage (retroperitoneal hemorrhage) Drugs Use of sulfa drugs Use of dapsone Use of glucocorticoids Use of G-CSF or related growth factors All-trans retinoic acid (ATRA) Ethylene glycol intoxication Infections Clostridium difficile Tuberculosis Pertussis Infectious mononucleosis (lymphocyte predominant) Visceral larva migrans (eosinophil predominant) Asplenia Diabetic ketoacidosis Organ necrosis Hepatic necrosis Ischemic colitis As a feature of trisomy 21 in infancy (incidence of ~10%) As a paraneoplastic phenomenon (rare) Conventionally, a leukocytosis exceeding 50,000 WBC/mm3 with a significant increase in early neutrophil precursors is referred to as a leukemoid reaction. The peripheral blood smear may show myelocytes, metamyelocytes, promyelocytes, and rarely myeloblasts; however, there is a mixture of early mature neutrophil precursors, in contrast to the immature forms typically seen in acute leukemia. Serum leukocyte alkaline phosphatase is normal or elevated in leukemoid reaction, but is depressed in chronic myelogenous leukemia. The bone marrow in a leukemoid reaction, if examined, may be hypercellular but is otherwise typically unremarkable. Leukemoid reactions are generally benign and are not dangerous in and of themselves, although they are often a response to a significant disease state (see Causes above). However, leukemoid reactions can resemble more serious conditions such as chronic myelogenous leukemia (CML), which can present with identical findings on the peripheral blood smear.