Summary

Neutrophilia is defined as an elevated circulating neutrophil count (>7.7×10^9/L) in adults with a normal total WBC count of <11×10^9/L. [1] View image The terms hyperleukocytosis and leukaemoid reaction are reserved for total leukocyte counts of >50×10^9/L, with leukaemoid reaction defined as having a non-malignant aetiology. An elevated total WBC count (>11×10^9/L) and an absolute neutrophil count >7.7×10^9/L is defined as a neutrophilic leukocytosis. [1] The absolute neutrophil count can be estimated by multiplying the total WBC count by the percentage of polymorphonuclear cells and band forms (immature neutrophils have a band-shaped nucleus). The terms granulocytosis and neutrophilia are often used interchangeably, although granulocytosis also includes elevations in eosinophils and basophils.

Neutrophilia results from an increased production of neutrophils, demargination (process of neutrophils entering the peripheral circulation from areas of intravascular marginated polymorphonuclear cell pools), or decreased egress (outwards migration) of neutrophils from peripheral circulation to the tissues.

Epidemiology

Generalising which patients are most at risk for neutrophilia is limited, as multiple factors contribute to the development of neutrophilia. Evaluation of the opposite condition, neutropenia (an absolute neutrophil count <1.5×10^9/L), has demonstrated reproducible ethnic differences. [2] Relative to white people, black people have a lower total WBC count (mean difference 0.89×10^9/L) and lower neutrophil count (mean difference 0.83×10^9/L). In contrast, Mexican Americans have been shown to have a higher leukocyte count (mean difference 0.16×10^9/L) and higher neutrophil count (mean difference 0.11×10^9/L) relative to white people. [2] Lifestyle factors also influence the risk of neutrophilia, including exercise levels, stress, and smoking status.

Pathophysiology

The peripheral circulation of neutrophils, which is included in the total leukocyte count, consists mostly of mature neutrophils in transit from the bone marrow to peripheral tissues. This process is short, lasting about 3 to 6 hours, and depends on signaling through the chemokine receptor CXCR4 on the surface of neutrophils. Only 5% of the total number of neutrophils is present in the circulation at any one time, as the neutrophil lifespan includes 9 days within the bone marrow, and 1 to 4 days in peripheral tissues, with only 3 to 6 hours spent in the circulation.

Bone marrow production of cells within the myeloid lineage is stimulated by interleukin 3 and granulocyte macrophage colony-stimulating factor. Granulocyte colony-stimulating factor, however, is responsible for catalysing differentiation of early myeloid cells into mature neutrophils. Multiple signals induce neutrophil migration to the peripheral circulation from the marrow, including corticosteroids, endotoxins, complement-derived leukocyte-mobilising factor, C5a chemoattractant, TNF-alpha, and androgens. When triggered, release of neutrophils from bone marrow storage triples circulating neutrophils within 4 hours. In addition to neutrophils contained within the bone marrow, circulation, and peripheral tissues, a moderate population adheres to the vascular endothelium and enters the circulation on demargination. Release of catecholamines, including epinephrine (adrenaline), results in almost immediate demargination and subsequent doubling of the peripheral neutrophil count.