Neutrophilic Leukocytosis Causes and Clinical Significance

Infectious Causes

Bacterial infections are the most common cause of neutrophilic leukocytosis, particularly with a left shift, and include pathogens such as coagulase-negative staphylococci, Staphylococcus aureus, and Escherichia coli, with an elevated total band count (>1500 cells/mm³) having a high likelihood ratio for detecting documented bacterial infection 1, 2, 3, 4
Septicemia is often associated with high-grade leukocytosis and left shift, and leukocytosis has been associated with increased mortality among patients with bloodstream infections (WBC count >20,000 cells/mm³) 1

Systemic inflammatory disorders, including vasculitis and connective tissue diseases, can cause neutrophilic leukocytosis, with Adult-onset Still's disease (AOSD) resulting in striking neutrophilia, with 50% of patients having peripheral leukocyte counts >15,000 cells/L and 37% having WBC counts >20,000 cells/L 5
Malignancy, particularly solid tumors, can cause neutrophilic leukocytosis, especially when necrotic or causing obstruction 4, 6

Severity markers, such as an increase in the percentage of neutrophils (>90%) and band neutrophils (>16%), have likelihood ratios of 7.5 and 4.7, respectively, for bacterial infection 2
Reactive leukocytosis may occur in neutropenic patients when neutrophil counts recover from severe neutropenia (<500 cells/mm³), especially in the setting of infection 7, 8

When evaluating neutrophilia in a patient with no significant past medical history, the American College of Physicians recommends looking for infection first, examining for fever, localizing symptoms, and signs of bacterial infection, with a priority on identifying the underlying cause to provide prompt treatment 9