What labs should be ordered for a patient with leukocytosis (elevated white blood cell count)?

Laboratory Evaluation for Elevated White Blood Cell Count

For patients with leukocytosis, a complete blood count with differential and C-reactive protein should be ordered as the initial diagnostic workup, followed by targeted testing based on clinical presentation. 1

Initial Laboratory Evaluation

• Complete Blood Count (CBC) with Differential

  • Provides information on total WBC count and specific cell types
  • Helps identify left shift (increased immature neutrophils/bands)
  • Evaluates for concurrent abnormalities in red blood cells or platelets 1, 2

• C-reactive protein (CRP)

  • Helps distinguish inflammatory from non-inflammatory causes 1

• Peripheral Blood Smear

  • Reveals types and maturity of white blood cells
  • Shows uniformity of cells and toxic granulations
  • Critical for distinguishing benign from malignant causes 2, 3

Additional Testing Based on Differential Findings

For Neutrophilia (Elevated Neutrophils)

• Blood cultures (at least two sets from separate peripheral sites)

  • Essential when sepsis, endocarditis, complicated pyelonephritis, or urosepsis is suspected
  • Each set should include one aerobic and one anaerobic bottle (10 ml per bottle) 1

• Site-specific cultures based on suspected infection source:

  • Urine culture for suspected UTI
  • Sputum culture for respiratory symptoms
  • Wound cultures for skin/soft tissue infections 1

For Eosinophilia

• Stool examination for ova and parasites
• Serum IgE levels for allergic conditions
• Specific allergen testing if allergic reaction suspected 1, 2

For Lymphocytosis

• Viral studies (particularly in children)
• Flow cytometry if lymphoproliferative disorder suspected 4

Special Considerations

When to Suspect Malignancy

• WBC count >100,000/mm³ (medical emergency due to risk of brain infarction and hemorrhage)
• Concurrent abnormalities in RBC or platelet counts
• Constitutional symptoms (fever, weight loss, night sweats)
• Hepatosplenomegaly or lymphadenopathy
• Abnormal cells on peripheral smear 3, 5

Severity Assessment

The combination of left shift and elevated WBC count provides strong evidence for bacterial infection, with the following likelihood ratios:

• WBC >14,000 cells/mm³: LR 3.7
• Band counts >1,500 cells/mm³: LR 14.5
• Neutrophils >90%: LR 7.5
• Band neutrophils >16%: LR 4.7 1

Common Pitfalls to Avoid

1. Relying solely on WBC count without clinical context

   • Laboratory values should always be interpreted alongside clinical signs and symptoms 1

2. Failing to use age and pregnancy-specific normal ranges

   • Normal WBC ranges vary by age and physiological state 2

3. Overlooking non-infectious causes of leukocytosis

   • Stress (physical or emotional)
   • Medications (corticosteroids, lithium, beta-agonists)
   • Chronic inflammatory conditions
   • Smoking and obesity 2, 3

4. Premature antibiotic initiation

   • Obtain appropriate cultures before starting antibiotics unless patient is critically ill 1

5. Missing life-threatening leukocytosis

   • WBC counts >100,000/mm³ require immediate hematology consultation 3

By following this systematic approach to laboratory evaluation of leukocytosis, clinicians can efficiently identify the underlying cause and initiate appropriate management.