Leukocytosis

Leukocytosis is a raised white blood cell count (the leukocyte count) above the normal range. This increase in leukocytes (primarily neutrophils) is usually accompanied by a "left shift" in the ratio of immature to mature neutrophils. The increase in immature leukocytes increases due to proliferation and release of granulocyte and monocyte precursors in the bone marrow which is stimulated by several products of inflammation including C3a and G-CSF.

Although it may be a sign of illness, leukocytosis in-and-of itself is not a disorder, nor is it a disease. It is simply a laboratory finding.

A leukocyte count above 25 to 30 x 10⁹/L is termed a leukemoid reaction, which is the reaction of a healthy bone marrow to extreme stress, trauma, or infection. (It is different from leukemia and from leukoerythroblastosis, in which immature blood cells are present in peripheral blood.)

Leukocyte counts

The normal adult human leukocyte count in peripheral blood is 4.4-10.8 x 10⁹/L. A white blood count of 11.0 x 10⁹/L or more suggests leukocytosis.

Causes

Leukocytosis is very common in acutely ill patients. It occurs in response to a wide variety of conditions, including viral, bacterial, fungal, or parasitic infection, cancer, hemorrhage, and exposure to certain medications or chemicals including steroids. Leukocytosis can also be the first indication of neoplastic growth of leukocytes.

For lung diseases such as pneumonia and tuberculosis, WBC count is very important for the diagnosis of the disease, as leukocytosis is usually present.

The mechanism that causes leukocytosis can be of several forms: an increased release of leukocytes from bone marrow storage pools, decreased margination of leukocytes onto vessel walls, decreased extravasation of leukocytes from the vessels into tissues, or an increase in number of precursor cells in the marrow.

Classification

Leukocytosis can be subcategorized by the type of white blood cell that is increased in number. Leukocytosis in which neutrophil count is elevated is neutrophilia; leukocytosis in which lymphocyte count is elevated is lymphocytosis; leukocytosis in which monocyte count is elevated is monocytosis; and leukocytosis in which eosinophil count is elevated is eosinophilia.