The normal total white blood cell (WBC) count in adults varies from 4400 to 11,000 cells/microL (4.4 to 11.0 x 109/L), the majority of which (approximately 60 percent) are mature neutrophils.
Leukocytosis is defined as a total WBC more than two standard deviations above the mean, or a value of greater than 11,000/microL in adults. 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 this value. This becomes important when an otherwise normal patient with a modest increase in WBC count is being evaluated. Such patients may be included with those considered to have chronic idiopathic neutrophilia (see below).
Neutrophilia, the major subject of this topic review, is defined as an increase in the absolute neutrophil count. It is most often seen in the setting of an increased total WBC count (see 'Definitions' below).
The major causes of neutrophilia will be reviewed here. The mechanisms of neutrophilia and the approach to the patient with this condition are discussed separately. (See "Definition and mechanisms of leukocytosis and neutrophilia" and "Approach to the patient with neutrophilia".)
As stated above, leukocytosis is defined as a total white blood cell (WBC) count more than two standard deviations above the mean, or a value of greater than 11,000/microL in adults. While leukocytosis is most commonly due to an increase in the absolute number of mature neutrophils (neutrophilia), it can, on occasion, reflect a marked increase in the absolute numbers of lymphocytes, eosinophils, monocytes, or, more rarely, basophils.