Does Legionella infection typically present with an elevated White Blood Cell (WBC) count?

Legionella Infection and White Blood Cell Count

Legionella pneumonia typically presents with an elevated white blood cell (WBC) count, though this is not a universal finding and should not be used as the sole diagnostic criterion. The leukocyte response in Legionella infection is characterized by neutrophilia with a left shift, but laboratory findings must be interpreted in the context of clinical presentation.

Laboratory Findings in Legionella Infection

• WBC Count Pattern:

  • Neutrophilia is common, with elevated neutrophil counts being a significant finding 1
  • A recent study (2023) found differences in neutrophil count, lymphocyte count, and neutrophil-to-lymphocyte ratio between survivors and non-survivors of Legionella infection 2
  • Left shift (increased percentage of band neutrophils >6% or elevated absolute band neutrophil count >1500 cells/mm³) may be present 1

• Diagnostic Value:

  • WBC >14,000 cells/mm³ has a likelihood ratio of 3.7 for bacterial infection 1
  • Band counts >1,500 cells/mm³ has a likelihood ratio of 14.5 for bacterial infection 1
  • Neutrophils >90% has a likelihood ratio of 7.5 for bacterial infection 1

Clinical Correlation

Legionnaires' disease cannot be distinguished clinically or radiographically from pneumonia caused by other agents 3. The diagnosis requires specific testing:

• Preferred diagnostic tests are the urinary antigen assay and culture of respiratory secretions on selective media 3
• Testing for Legionella species is appropriate for any patient hospitalized with enigmatic pneumonia 3
• Blood cultures may be positive in approximately 38% of cases, though growth may not trigger automated detection systems 4

Important Considerations

1. Diagnostic Limitations:

   • No single laboratory test for Legionella detects all cases 3
   • Treatment should be initiated or continued in appropriate clinical and epidemiological settings even with negative test results 3

2. Risk Factors for Poor Outcomes:

   • Immunosuppression, advanced age, end-stage renal disease, cancer 3
   • Nosocomial acquisition (40% mortality vs. 20% for community-acquired) 3
   • Older age, cardiovascular diseases, late diagnosis, and neurological symptoms 2

3. Treatment Considerations:

   • Preferred treatment for hospitalized patients is azithromycin or a fluoroquinolone 3
   • For outpatients: erythromycin, doxycycline, azithromycin, clarithromycin, or a fluoroquinolone 3
   • Treatment should be initiated as rapidly as possible 3

Common Pitfalls

• Relying solely on WBC count for diagnosis of Legionella infection
• Failing to consider Legionella in patients with pneumonia who fail to respond to β-lactam antibiotics
• Not obtaining appropriate diagnostic tests (urinary antigen and culture) when Legionella is suspected
• Delaying treatment while awaiting test results in clinically suspicious cases

Remember that Legionella testing should be performed for patients with enigmatic pneumonia requiring ICU care, in the presence of an epidemic, or if there is failure to respond to β-lactam antibiotics 3.