What are the causes of leukocytosis?

Causes of Leukocytosis

Leukocytosis is most frequently caused by bacterial infections, followed by physical/emotional stress, medications (especially corticosteroids, lithium, and beta-agonists), and chronic inflammatory conditions, while primary bone marrow malignancies are less common but require urgent exclusion. 1

Infectious Causes

Bacterial infections are the leading cause of leukocytosis and should be systematically evaluated first:

• Respiratory tract infections, urinary tract infections, skin/soft tissue infections, and gastrointestinal infections are the most common bacterial sources. 1
• A left shift (≥16% band neutrophils) has a likelihood ratio of 4.7 for bacterial infection, even when total WBC is normal. 2, 1
• An absolute band count ≥1,500 cells/mm³ carries the highest predictive value with a likelihood ratio of 14.5 for documented bacterial infection. 2, 1
• Neutrophil percentage >90% has a likelihood ratio of 7.5 for bacterial infection. 2, 1
• In cirrhotic patients with ascites, spontaneous bacterial peritonitis frequently presents with altered WBC counts. 1
• In inflammatory bowel disease patients, Clostridium difficile and Cytomegalovirus should be excluded as triggers. 1

Critical pitfall: Absence of fever does NOT exclude serious bacterial infection; leukocytosis can be the sole clue, especially in older adults. 2

Physiological and Stress-Related Causes

Non-pathologic elevations occur through catecholamine and cortisol release:

• Emotional stress triggers leukocytosis through catecholamine and cortisol release. 1
• Acute exercise causes immediate WBC elevation, particularly affecting granulocytes and NK cells. 1
• Physical stress from seizures, anesthesia, overexertion, surgery, or trauma can double the peripheral WBC count within hours due to demargination from bone marrow storage pools. 3, 4

Medication-Induced Leukocytosis

Several drugs predictably elevate WBC counts:

• Corticosteroids are the most common medication cause. 1, 3
• Lithium therapy consistently causes leukocytosis; WBC <4,000/mm³ would be unusual in lithium-treated patients. 1
• Beta-agonists and epinephrine cause neutrophilia through adrenergic mechanisms. 1, 3

Chronic Inflammatory and Autoimmune Conditions

Persistent inflammation drives sustained leukocytosis:

• Adult-onset Still's disease presents with marked leukocytosis: 50% of patients have WBC >15×10⁹/L and 37% have >20×10⁹/L, resulting from bone marrow granulocytic hyperplasia. 1
• Inflammatory bowel disease activity correlates with leukocytosis and thrombocytosis. 1
• Kawasaki disease in the acute phase shows granulocyte-predominant leukocytosis; leukopenia or lymphocyte predominance should prompt alternative diagnoses. 1
• Chronic inflammatory conditions of various types can sustain elevated WBC counts. 1, 3

Hematologic Malignancies (Medical Emergency)

Primary bone marrow disorders require urgent recognition:

Acute Leukemias

• Detection of blast cells in peripheral blood signals acute leukemia and warrants urgent hematology consultation. 2
• Patients present with severe constitutional symptoms and require immediate cytoreductive therapy. 1
• Acute leukemias are life-threatening and demand rapid subspecialist referral. 5

Chronic Myeloid Leukemia (CML)

• Marked leukocytosis with basophilia, eosinophilia, and left-shifted myeloid maturation (increased myelocytes and promyelocytes) is characteristic. 2
• Immature granulocytes (metamyelocytes, myelocytes, promyelocytes, occasional blasts) together with elevated basophils or eosinophils strongly favor CML over reactive processes. 2
• Splenomegaly occurs in slightly more than 50% of patients at diagnosis. 2

Hyperleukocytosis (WBC >100,000/µL)

• This constitutes a medical emergency due to risks of leukostasis, cerebral infarction, hemorrhage, disseminated intravascular coagulation, and tumor-lysis syndrome. 6, 2, 3
• Immediate management includes aggressive IV hydration, uric-acid-lowering therapy (allopurinol or rasburicase), cytoreduction with hydroxyurea (50-60 mg/kg/day until WBC <10-20×10⁹/L), and consideration of leukapheresis. 6, 2
• Excessive red blood cell transfusions should be avoided until WBC is reduced, as they increase blood viscosity. 6

Chronic Leukemias and Myeloproliferative Disorders

• Patients often present asymptomatically with incidental laboratory findings, in contrast to acute leukemias. 3, 5
• Symptoms develop gradually when present. 5

Other Causes

Additional etiologies to consider:

• Allergic reactions and parasitic infections cause eosinophilia-driven leukocytosis. 3, 4
• Asplenia, smoking, and obesity are associated with chronic leukocytosis. 4
• Neuroleptic malignant syndrome presents with leukocytosis in the range of 15,000-30,000 cells/µL, accompanied by hyperthermia, rigidity, and altered mental status. 1

Red Flags Requiring Hematology Referral

Immediate subspecialist consultation is indicated when:

• Blast cells are present on peripheral smear. 2
• WBC >100,000/µL (medical emergency). 6, 2, 3
• Concurrent abnormalities in red blood cells or platelets. 3
• Constitutional symptoms: fever, weight loss, bruising, fatigue. 4, 5
• Splenomegaly or lymphadenopathy on examination. 1, 3
• Immunosuppression with unexplained leukocytosis. 3

Critical diagnostic principle: Manual differential count is essential—automated analyzers miss left shift, immature forms, and dysplastic features that are crucial for diagnosis. 2, 7