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. By convention, leukocytosis to values in excess of 50,000 cells/microL, when due to causes other than leukemia, is termed a leukemoid reaction or hyperleukocytosis.
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 the two terms are somewhat different, since granulocytosis can also reflect leukocytosis due to increased numbers of eosinophils or basophils.
The absolute neutrophil count (ANC) is equal to the product of the white blood cell count (WBC) and the percentage of polymorphonuclear cells (PMNs) and band forms noted on the WBC differential, and is calculated as follows:
ANC (cells/microL) = WBC (cells/microL) x percent (PMNs + bands) ÷ 100
Neutrophilic leukocytosis is defined as a total WBC greater than 11,000/microL plus an ANC more than two standard deviations above the mean, or a value greater than 7700/microL in adults. (See "Definition and mechanisms of leukocytosis and neutrophilia".)