Neutrophils are essential for fighting infection. Neutropenia, or low levels of neutrophils, is a common disorder usually caused by chemotherapy, adverse drug reactions or autoimmune disorders. Severe neutropenia, or agranulocytosis, is defined as an absolute neutrophil count less than 500 per μL (0.5 × 10⁹ per L).

Munshi and Montgomery used information obtained through a MEDLINE search for their review. Management and evaluation of neutropenia should depend on the severity of the neutropenia and on the presence of symptoms suggesting infection. Patients with severe neutropenia with fever or signs of infection and those with evidence of malignant disease should be hospitalized. The cause of neutropenia should be determined, potential sources of infection evaluated and antibiotic therapy started. The diagnostic evaluation includes history of prior infection, a review of prior blood counts and exploration for evidence of infection or rheumatologic conditions.

Patients with mild neutropenia and no evidence of infection may be monitored with blood cell counts for six to eight weeks. Neutropenia in children is likely to be benign and often resolves without intervention. Laboratory tests include microscopic examination of a peripheral blood smear, appropriate cultures and specific chemical, bacteriologic and viral studies, and serologic testing for collagen vascular disease. Bone marrow aspiration is needed when the cause of neutropenia is not clear, when the neutropenia does not resolve or if there is evidence of multiple cytopenias.

The types of neutropenia are listed in the accompanying table. Neutropenia is more likely to be acquired than congenital, with the most common cause being a drug reaction that causes direct marrow suppression or immune destruction of the neutrophil or myeloid precursors. Viral infection is another common cause of neutropenia, related to bone marrow suppression or peripheral destruction. Acute bacterial infections, notably salmonellosis, brucellosis, pertussis, tuberculosis and rickettsial infections, can cause neutropenia. Collagen vascular disease can feature neutropenia. Benign hematopoietic disease, including vitamin B₁₂ and folate deficiencies, can cause neutropenia, anemia and thrombocytopenia. Malignant hematopoietic diseases such as leukemia and multiple myeloma may cause neutropenia by suppressing normal myelopoiesis.

Treatment of neutropenia may be appropriate if the condition does not resolve spontaneously or after an intervention such as discontinuation of an offending drug. In neutropenia resulting from chemotherapy or radiation, cytokines such as granulocyte colony–stimulating factor can increase the neutrophil count although their use has not been shown to substantially decrease infection mortality. Therapy should be initiated in patients who are at increased risk of infection. Allogeneic bone marrow transplantation may be effective in patients with bone marrow disease.