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Chapter 126 Leukocytosis

Leukocytosis is an elevation in the total leukocyte, or white blood cell (WBC), count that is 2 standard deviations above the mean count for a particular age (Chapter 708). The various causes of leukocytosis are categorized by the class of leukocyte that is elevated and whether the process is acute, chronic, or lifelong. To evaluate the patient with leukocytosis, it is critical to determine which class of WBCs is elevated, and also the duration and extent of the leukocytosis. Each blood count should be evaluated with regard to the absolute number of cells/µL and the normal range for the patient’s age.

A WBC count exceeding 50,000/µL is termed a leukemoid reaction because of the similarity to features of leukemia. Leukemoid reactions are usually neutrophilic and are most frequently associated with septicemia and severe bacterial infections, including shigellosis, salmonellosis, and meningococcemia. Infection in children with leukocyte adhesion deficiency results in WBC counts approaching or exceeding 100,000/µL.

A >5% proportion of immature neutrophil cells is termed a left shift and indicates rapid release of cells from the bone marrow. This release may result in increased circulating band forms, which usually constitute 1-5% of circulating neutrophilic cells, or metamyelocytes and myelocytes, which are not usually found in the peripheral circulation. Higher degrees of left shift with more immature neutrophil precursors are indicative of serious bacterial infections but may also be encountered with trauma, burns, surgery, acute hemolysis, or hemorrhage.


Neutrophilia is an increase in the total number of blood neutrophils, which for older children and adults is >8,000/µL. During the 1st day of life, the upper limit of the normal neutrophil count ranges from 7,000 to 12,000/µL. In the 1st mo of life, the neutrophil count ranges from 1,800 to 5,400/µL, and by 1 yr of age, the range is 1,500-8,500/µL.

An increase in circulating neutrophils is a result of a disturbance of the normal equilibrium involving bone marrow neutrophil production, movement out of the marrow compartments into the circulation, and neutrophil destruction. Neutrophilia may arise either alone or in combination with enhanced mobilization into the circulating pool from either the bone marrow storage compartment or the peripheral blood marginating pool, by impaired neutrophil egress into tissues, or by expansion of the circulating neutrophil pool secondary to increased granulocytopoiesis. Myelocytes are not released to the blood except under extreme circumstances.