Leukocytosis

Leukocytosis is a condition characterized by an elevated white blood cell count above the normal upper limit. In adults, the upper reference range is typically about 11 x 10^9 cells per liter (11,000/μL), though values may vary slightly by laboratory. Leukocytosis is not a diagnosis in itself but a laboratory finding that prompts evaluation of potential underlying causes.

Most leukocytosis reflects a reactive or physiologic response rather than a blood cancer. The predominant change is neutrophilia (increased neutrophils), which occurs in bacterial infections and inflammatory states. Lymphocytosis, eosinophilia, and other patterns occur with different diseases or exposures. A left shift, with increased immature neutrophils, often accompanies acute infection.

Common causes include acute or chronic infections (especially bacterial), inflammation or tissue necrosis, hemorrhage, stress, exercise, pregnancy, and adverse drug reactions (for example corticosteroids). Less commonly, leukocytosis results from myeloproliferative disorders such as chronic myeloid leukemia or other hematologic diseases, or from bone marrow stimulation by growth factors.

Evaluation typically begins with a complete blood count with differential and peripheral smear to assess the type of cells and the presence of a left shift. Additional tests may include inflammatory markers, cultures, imaging, and bone marrow examination when leukemia or a myeloproliferative disorder is suspected.

Management focuses on identifying and treating the underlying cause. In reactive leukocytosis, the count may normalize with resolution of the trigger. Distinguishing leukemoid reactions from malignancy is important, as is monitoring for complications. Persistent unexplained leukocytosis requires hematology consultation.