Causes of Reactive Leukocytosis
Reactive leukocytosis results from three major categories: infections (bacterial, viral, fungal), inflammatory/immunological insults (autoimmune diseases, drug reactions, allergic disorders), and physiological stressors (trauma, surgery, hemorrhage, physical/emotional stress).
Infectious Causes
Bacterial infections are the predominant infectious trigger, accounting for 30-57% of cases in patients with underlying conditions like cirrhosis, and represent the most common precipitating factor for acute-on-chronic liver failure 1. In immunocompromised patients receiving chemotherapy, invasive fungi and bacterial infections play substantial roles in driving leukocytosis 2.
Viral infections can trigger reactive leukocytosis, particularly in the context of reactivation syndromes 1. Hepatitis B reactivation is the main precipitating event in Asian populations, while superimposed hepatotropic viruses (especially HAV, HEV) can induce leukocytosis in Western countries 1.
Parasitic infections, particularly tissue-invasive helminths, commonly cause eosinophilic leukocytosis during their migratory phase 3, 4. Strongyloides stercoralis can cause hyperinfection syndrome with severe leukocytosis in immunosuppressed patients 3, 5.
Inflammatory and Autoimmune Causes
Adult-Onset Still's Disease produces marked neutrophilic leukocytosis, with 50% of patients having WBC >15,000/μL and 37% exceeding 20,000/μL 6, 7. This represents one of the most dramatic reactive leukocytoses seen in rheumatologic conditions 6.
Chronic inflammatory conditions cause persistent leukocytosis through ongoing bone marrow granulocyte hyperplasia 6, 7. Connective tissue diseases and autoimmune disorders are well-established causes 7.
Allergic and atopic disorders constitute approximately 80% of cases of secondary reactive eosinophilia, including asthma, food allergies, atopic dermatitis, and drug reactions 4.
Drug-Induced Leukocytosis
Corticosteroids are among the most common medications causing leukocytosis through direct bone marrow stimulation 8, 7. Lithium and beta-agonists also frequently elevate white blood cell counts 8, 7.
Drug hypersensitivity reactions from NSAIDs, beta-lactam antibiotics, and nitrofurantoin can cause reactive leukocytosis, typically with eosinophilia 7, 3, 4.
Physiological and Stress-Related Causes
Physical stress from seizures, anesthesia, overexertion, major trauma, or surgery triggers leukocytosis through catecholamine-mediated demargination of neutrophils 8, 9, 10. Emotional stress similarly elevates white blood cell counts 8.
Hemorrhage and hemodynamic derangement following procedures (GI bleeding, surgery, large-volume paracentesis without albumin) induce reactive leukocytosis 1.
Hepatotoxic Injury
Active alcohol intake or binge drinking is a major trigger in alcoholic cirrhosis patients, leading to more severe leukocytosis than other triggers 1. Drug-induced liver injury (DILI) also precipitates reactive leukocytosis 1.
Metabolic and Tissue Damage
Iron deficiency, splenectomy, metastatic cancer, and lymphoproliferative disorders can cause reactive thrombocytosis and leukocytosis 11, 7.
Extensive tissue damage from major trauma, cerebrovascular accidents, or ischemic injury drives prolonged leukocytosis through release of damage-associated molecular patterns (DAMPs), often manifesting as persistent inflammation-immunosuppression and catabolism syndrome (PICS) 9. These patients develop mean peak WBC of 26,400 ± 8,800 with bandemia of 18.4% ± 13.8%, persisting for 14.5 ± 10.6 days 9.
Critical Distinguishing Features from Primary (Malignant) Leukocytosis
Absence of clonal markers (JAK2 V617F, BCR-ABL1, MPL mutations) distinguishes reactive from primary leukocytosis 11, 7. Normal peripheral blood smear without blasts, dysplasia, or left shift beyond bands suggests reactive etiology 8, 10, 12.
Clinical context is paramount: reactive leukocytosis typically occurs with identifiable precipitating events (infection, trauma, inflammation), whereas primary leukocytosis often presents incidentally without clear triggers 8, 10, 13.