What labs should be ordered for a patient with suspected pneumonia?

Laboratory Tests for Suspected Pneumonia

For patients with suspected pneumonia, a chest radiograph is the cornerstone of diagnosis, along with blood cultures, complete blood count with differential, and pulse oximetry as essential laboratory tests. 1

Initial Laboratory Assessment

Essential Tests for All Patients with Suspected Pneumonia:

• Chest radiography: Required to confirm the diagnosis, assess severity, detect complications, and establish a baseline for evaluating treatment response 2, 1

  • Posteroanterior and lateral views preferred for non-intubated patients
  • Helps identify multilobar involvement, pleural effusions, or cavitation

• Complete blood count (CBC) with differential 2

  • Elevated WBC count (≥14,000 cells/mm³) or left shift (≥16% bands or ≥1,500 band neutrophils/mm³) supports bacterial infection 2
  • Manual differential preferred to assess bands and immature forms

• Blood cultures (2 sets) 2

  • Collect before antibiotic administration when possible
  • Recommended for all hospitalized patients

• Pulse oximetry 2, 1

  • Essential to document hypoxemia (O₂ saturation <90-92%)
  • Helps assess severity and need for supplemental oxygen or hospitalization

Additional Tests Based on Clinical Presentation:

• Sputum Gram stain and culture 2

  • For patients who can produce purulent sputum
  • Collect before antibiotics when possible (sensitivity decreases after antibiotic administration) 3
  • Quality specimen should have <25 squamous epithelial cells per low-power field 2, 4
  • Sensitivity of 57-82% for detecting common pathogens when properly collected 3, 5

• Basic metabolic panel 2

  • Serum creatinine, BUN, glucose, electrolytes
  • Helps assess organ function and guide therapy

Additional Testing for Severe Pneumonia

For patients with severe pneumonia, add:

• Arterial blood gas analysis 2

  • For patients with respiratory distress or significant hypoxemia

• Urinary antigen tests 2, 1

  • Legionella pneumophila
  • Streptococcus pneumoniae

• Pleural fluid analysis (if significant effusion present) 1

  • Gram stain and culture
  • Cell count and differential
  • Biochemistry (pH, protein, LDH, glucose)

Testing Based on Specific Clinical Scenarios

• CT scan with IV contrast 1

  • For patients with cavitary lesions on chest X-ray
  • When complications are suspected
  • For elderly patients with atypical presentations and negative chest X-ray

• HIV testing 2

  • For patients aged 15-54 years

• Tests for specific pathogens based on epidemiological factors:

  • Acid-fast bacilli (AFB) smear and culture for TB suspicion
  • Fungal stain and culture for endemic fungi
  • Viral testing during appropriate seasons

Common Pitfalls to Avoid

1. Relying solely on clinical findings without radiographic confirmation

   • Clinical assessment alone frequently overestimates pneumonia probability 2

2. Poor quality sputum specimens

   • Ensure proper collection of deep cough specimens
   • Transport rapidly to laboratory (within 1-2 hours) 2

3. Delaying antibiotic therapy while waiting for test results

   • Collect specimens promptly but initiate empiric therapy without delay 1

4. Overlooking severity assessment

   • Use validated tools like CURB-65 or Pneumonia Severity Index to guide disposition decisions 1

5. Failing to reassess after initial testing

   • Regular reassessment is essential to adjust management appropriately 2

By following this systematic approach to laboratory testing for suspected pneumonia, clinicians can establish the diagnosis, assess severity, identify causative pathogens when possible, and guide appropriate antimicrobial therapy to improve patient outcomes.