CBC Findings in Legionella Pneumonia
A complete blood count should be performed in all patients with suspected Legionella pneumonia, with the most characteristic findings being leukocytosis (WBC >14,000 cells/mm³), a left shift (band neutrophils ≥6% or >1,500/mm³), and thrombocytopenia. 1, 2
Standard CBC Findings
White Blood Cell Count
- Leukocytosis is common, with WBC counts typically exceeding 14,000 cells/mm³, warranting careful assessment for bacterial infection 1
- A left shift is frequently present, defined as band neutrophils ≥6% or total band neutrophil count >1,500/mm³ 1
- These findings should prompt aggressive diagnostic workup for Legionella, particularly in patients with severe pneumonia requiring ICU admission 1
Platelet Count
- Thrombocytopenia can occur and may be severe, though this is a less common manifestation 2, 3
- Low platelet counts (decreased from baseline) serve as an independent predictor of Legionella pneumonia in multivariate analysis 3
- Rarely, Legionella can cause immune thrombocytopenic purpura (ITP) through complement-mediated platelet destruction or antibody cross-reactivity 2
Clinical Context for CBC Interpretation
When to Obtain CBC
- All patients with suspected infection in any setting should have a complete blood count with differential performed 1
- All hospitalized pneumonia patients should have CBC as part of their initial assessment 1
- The presence of elevated WBC or left shift, even without fever, warrants careful bacterial infection assessment 1
Associated Laboratory Abnormalities
While not part of the CBC itself, Legionella pneumonia characteristically presents with:
- Hyponatremia (low serum sodium), which is highly suggestive 1, 4, 5, 6, 3
- Elevated lactate dehydrogenase (LDH >700 U/mL) 1, 5, 3
- Elevated C-reactive protein 3
Diagnostic Algorithm
High-Risk Scenarios Requiring Testing
Testing for Legionella should be pursued when CBC shows leukocytosis/left shift in combination with:
- Severe pneumonia requiring ICU admission 1, 4
- Failure to respond to β-lactam therapy 1
- Immunosuppressed patients (transplant recipients, hematologic malignancies, systemic steroids) 4, 7, 5
- Recent travel or spa/hot tub exposure 1, 5
- Nosocomial pneumonia (symptoms ≥10 days after hospitalization) 4
Diagnostic Score Utility
A clinical scoring system incorporating CBC findings has high diagnostic accuracy (AUC 0.86) 3:
- High fever
- Absence of sputum production
- Hyponatremia
- Elevated LDH
- Elevated CRP
- Low platelet count 3
Patients with ≥4 points have 66% probability of Legionella pneumonia, while those with 0-1 points have only 3% probability 3
Critical Pitfalls
- CBC findings alone cannot distinguish Legionella from other bacterial pneumonias—specific testing (urinary antigen or culture) is required for definitive diagnosis 1, 4, 5
- Normal CBC does not exclude Legionella pneumonia—clinical and epidemiologic features must guide testing decisions 1
- Thrombocytopenia, when severe, requires urgent recognition as it may indicate immune-mediated complications requiring additional treatment beyond antibiotics 2
- Only 26% of eligible patients are actually tested for Legionella despite guidelines, and testing is particularly underutilized in high-risk groups (only 30% with hyponatremia, 27% in ICU) 8