Most cases of neutropenia in adults are acquired and are due to decreased granulocyte production or increased destruction. Neutropenia can be predictable and dose related, as in the case of cytotoxic chemotherapy, or an idiosyncratic reaction. The mechanisms of the neutropenia are varied and the propensity to infection depends upon the adequacy of bone marrow reserve.
Drug-induced neutropenia and agranulocytosis will be reviewed here. Others causes of acquired neutropenia, such as primary immune mechanisms, chemotherapy, and infections, as well as congenital neutropenia in children are discussed separately. (See "Immune neutropenia" and "Infectious causes of neutropenia" and "Congenital neutropenia" and "Overview of neutropenic fever syndromes".)
The overall approach to the patient with neutropenia is discussed separately. (See "Overview of neutropenia in children and adolescents" and "Approach to the adult with unexplained neutropenia".)
Neutropenia is defined as an absolute neutrophil count (ANC) <1500/microL. The ANC is numerically equal to the product of the white blood cell count (WBC) and the fraction of polymorphonuclear cells (PMNs) and band forms noted on the differential analysis:
ANC = WBC (cells/microL) x percent (PMNs + bands) ÷ 100