What causes an elevated white blood cell count?

Causes of Elevated White Blood Cell Count

Bacterial infection is the most common cause of elevated white blood cell count, characterized by neutrophil predominance and left shift, but other important causes include stress, medications (especially corticosteroids), inflammation, and hematologic malignancies. 1, 2, 3

Common Causes of Leukocytosis

Infectious Causes

• Bacterial infections - most common cause, typically with neutrophil predominance 1
  • Localized infections: pneumonia, urinary tract infections, skin/soft tissue infections, intra-abdominal infections 1
  • Systemic infections: sepsis, endocarditis 1
• Viral infections - often cause lymphocytosis, especially in children 2
• Parasitic infections - can cause eosinophilia 1

Non-Infectious Causes

• Physiologic stress responses 3
  • Physical stress (surgery, trauma, seizures, exercise)
  • Emotional stress
• Medications 2, 3, 4
  • Corticosteroids - can increase WBC count by up to 4.84 × 10⁹/L within 48 hours after high-dose administration 4
  • Lithium
  • Beta agonists
• Inflammatory conditions 2
  • Autoimmune diseases
  • Inflammatory bowel disease 1
  • Rheumatologic conditions 1
• Other non-malignant causes 2
  • Smoking
  • Obesity
  • Asplenia
  • Pericarditis 1

Hematologic Malignancies

• Acute leukemias - patients often ill at presentation 3
• Chronic leukemias - often diagnosed incidentally 3
• Myeloproliferative disorders 3

Diagnostic Clues from the WBC Count

WBC Differential Patterns

• Neutrophilia with left shift (increased immature neutrophils/bands) - strongly indicates bacterial infection 1
  • Band counts >1,500 cells/mm³ (likelihood ratio 14.5 for bacterial infection)
  • Neutrophils >90% (likelihood ratio 7.5)
  • Band neutrophils >16% (likelihood ratio 4.7) 1
• Lymphocytosis - common in viral infections, especially in children 2
• Eosinophilia - allergic reactions, parasitic infections 1, 3
• Basophilia - allergic reactions, certain myeloproliferative disorders 3

Severity Indicators

• WBC count >14,000 cells/mm³ - likelihood ratio 3.7 for bacterial infection 1
• WBC count >100,000 per mm³ - medical emergency due to risk of brain infarction and hemorrhage 3
• Concurrent abnormalities in red blood cell or platelet counts suggest primary bone marrow disorders 3

Clinical Approach to Elevated WBC Count

Key Clinical Features to Assess

• Presence of fever (>100°F/37.8°C) or increase of 2°F (1.1°C) over baseline 1
• Localized signs of infection 1
• Weight loss, bruising, fatigue (suggesting malignancy) 2
• Liver, spleen, or lymph node enlargement 3
• Recent medication changes, particularly corticosteroids 4
• Recent stressors (surgery, trauma, emotional stress) 3

Diagnostic Workup

• Complete blood count with differential 1
• Peripheral blood smear to assess cell types, maturity, and morphology 2
• Blood cultures if sepsis, endocarditis, or complicated urinary tract infection is suspected 1
• Targeted testing based on suspected source of infection 1

Special Considerations

• Age-specific normal ranges should be used when interpreting WBC counts 2
• Pregnancy-specific normal ranges should be used for pregnant patients 2
• In elderly patients, elevated WBC count has prognostic significance for long-term mortality 5
• WBC count can double within hours after certain stimuli due to bone marrow storage and marginated pools of neutrophils 2

Remember that while leukocytosis is most commonly due to infection or inflammation, persistent unexplained elevation, extremely high counts, or abnormalities in other cell lines warrant hematology consultation to rule out primary bone marrow disorders 3.