Leukocytosis

Leukocytosis denotes an age-appropriate rise in the white blood cell (WBC) count. Typically, a WBC count surpassing 11,000 cells/µL in adults is deemed leukocytosis, and a WBC count >100,000 cells/µL is termed hyperleukocytosis. However, the threshold varies with age and pregnancy; for instance, 30,000 cells/µL in adults would be abnormal but within the normal range for a newborn.  Leukocytosis is a common finding with a broad differential and is typically classified further based on the type of WBC contributing to the elevation. Increased quantities of lymphocytes, neutrophils, eosinophils, basophils, monocytes, or immature cells called blasts can all contribute. Infection, inflammation, allergic reactions, malignancy, and hereditary disorders are all common causes of leukocytosis. Identifying changes in the distribution of WBCs can help identify the underlying cause. For example, eosinophilia may indicate allergic or parasitic illnesses, while lymphocytosis may be present in patients with viral syndromes, autoimmune diseases, and hyperthyroidism.  Further evaluation depends on the clinical presentation, duration of symptoms, changes in lab values over time, the differential, and the complete blood count (CBC) information. Some patients may require a bone marrow biopsy, flow cytometry, and molecular and genetic testing when evaluating for malignant conditions. Clinicians guide treatment decisions based on the underlying pathology. While many patients require no treatment, extreme cases like hyperleukocytosis and leukostasis are a medical emergency and require immediate treatment.