Clinical meaning
Neutrophils are the most abundant white blood cells (60-70% of circulating WBCs) and serve as the primary defense against bacterial and fungal infections. They are produced in the bone marrow from myeloid stem cells through granulopoiesis, taking approximately 14 days from stem cell to mature neutrophil release. The absolute neutrophil count (ANC) is calculated as: WBC × (% neutrophils + % bands). Neutropenia is defined as ANC <1500/mm³; severe neutropenia as ANC <500/mm³; profound neutropenia as ANC <100/mm³. The risk of life-threatening infection increases exponentially as ANC decreases below 500 and correlates with the duration of neutropenia. Without adequate neutrophils, bacterial infections spread unchecked—classic inflammatory signs (erythema, purulence, swelling) may be absent because these responses depend on neutrophil infiltration. Fever may be the only sign of serious infection. Causes of neutropenia include: chemotherapy (most common in oncology nursing), bone marrow failure syndromes, drug-induced (carbamazepine, methimazole, clozapine), autoimmune neutropenia, and severe sepsis with marrow exhaustion. Febrile neutropenia (temperature >100.4°F/38°C or >100.0°F/38.0°C sustained for 1 hour with ANC <500) is a medical emergency requiring empiric broad-spectrum antibiotics within 60 minutes.