This topic will review the mechanisms causing an increased total white blood cell count due to an increased number of circulating neutrophils (ie, neutrophilic leukocytosis). The causes of leukocytosis (ie, neutrophilic, lymphocytic, monocytic, eosinophilic, and basophilic) and an approach to the patient with neutrophilia are discussed separately. (See "Causes of neutrophilia" and "Approach to the patient with eosinophilia" and "Approach to the patient with neutrophilia".)
The normal total white blood cell (WBC) count in adults varies from 4400 to 11,000 cells/microL, the majority of normal leukocytes (approximately 60 percent) are mature neutrophils .
Leukocytosis — Leukocytosis is defined as a total white blood cell (WBC) count more than two standard deviations above the mean, or a value greater than 11,000/microL in adults. Leukocytosis to values in excess of 50,000/microL, when due to causes other than leukemia, has been called a leukemoid reaction. However, this term conveys no useful information concerning etiology, and is mainly of historic interest. (See 'Leukemoid reaction' below.)
Since the limits of normal include two standard deviations above the mean, 2.5 percent of the normal population will have a total WBC count above 11,000/microL. This becomes important when an otherwise normal patient with a modest increase in WBC count is being evaluated.
While leukocytosis is most commonly due to an increase in the absolute number of mature neutrophils (neutrophilia), it can also reflect a marked increase in the absolute numbers of lymphocytes, eosinophils, monocytes, or, more rarely, basophils. Granulocytosis is generally used interchangeably with neutrophilia, although they are somewhat different. Granulocytosis can reflect leukocytosis due to increased numbers of neutrophils, eosinophils, or basophils, whereas neutrophilia only reflects increased numbers of neutrophils.