Leukocytosis may also occur as a result of physical and emotional stress. This is a transient process that is not related to marrow production or the release of band cells or other immature cells. Causes of stress leukocytosis include overexertion, seizures, anxiety, anesthesia and epinephrine administration. Stress leukocytosis reverses within hours of elimination of the inciting factor.
Other causes of leukocytosis include medications, splenectomy, hemolytic anemia and malignancy. Medications commonly associated with leukocytosis include corticosteroids, lithium and beta agonists. Splenectomy causes a transient leukocytosis that lasts for weeks to months. In hemolytic anemia, nonspecific increases in leukocyte production and release occur in association with increased red blood cell production; marrow growth factors are likely contributors. Malignancy is another recognized cause of
leukocytosis (and, occasionally, thrombocytosis); the tumor nonspecifically stimulates the marrow to produce leukocytosis.
An excessive white blood cell response (i.e., more than 50,000 white blood cells per cm3 [50 X 109 per L]) associated with a cause outside the bone marrow is termed a "leukemoid reaction." Even this exaggerated white blood cell count is usually caused by relatively benign processes (i.e., infection or inflammation). An underlying malignancy is the most serious but least common cause of a leukemoid reaction.
As mentioned previously, an increase in neutrophils is the most common cause of an elevated white blood cell count, but other subpopulations of cells (eosinophils, basophils, lymphocytes and monocytes) can also give rise to increased leukocyte numbers
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