Leukocytosis is defined as a white blood cell count greater than 11,000/mcL in nonpregnant adults. It is a common finding, and the differential diagnosis can be broadly divided into primary malignant diseases and secondary causes that are expected physiologic responses of the bone marrow. Infections and chronic inflammatory conditions are common causes of secondary leukocytosis. A thorough history, physical examination, and peripheral blood smear are the basis of the initial evaluation. Constitutional symptoms along with an abnormal peripheral blood smear result indicate the need for evaluation for malignancy. Patients with chronic leukemias usually present with less severe symptoms than patients with acute leukemias. Symptoms generally are gradual in onset. Acute leukemias should be recognized quickly because they may be associated with life-threatening complications. Urgent referral to a hematology subspecialist is indicated in cases of suspected acute leukemia.
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