What is the workup for Community-Acquired Pneumonia (CAP)?

Diagnostic Workup for Community-Acquired Pneumonia

The workup for community-acquired pneumonia (CAP) should be stratified based on severity and treatment setting, with all hospitalized patients requiring chest radiography, basic laboratory tests, and appropriate microbiological investigations, while outpatients with mild disease need minimal testing.

Initial Assessment and Severity Stratification

The first step in CAP workup is determining disease severity, which guides both diagnostic testing and site of care:

• Severity assessment tools:

  • CURB-65 (Confusion, Urea, Respiratory rate, Blood pressure, age ≥65) 1
  • Pneumonia Severity Index (PSI) 1
  • ATS/IDSA severe CAP criteria 1

• Criteria suggesting severe CAP (requiring ICU consideration):

  • Respiratory failure (Pa,O2/FI,O2 <250)
  • Systolic blood pressure <90 mmHg
  • Multilobar involvement on imaging
  • Need for mechanical ventilation or vasopressors 1

Diagnostic Workup by Treatment Setting

1. Outpatient Management (Mild CAP)

• Chest radiography:

  • Not routinely required for most outpatients 1
  • Consider if diagnosis is uncertain or complications suspected

• Laboratory tests:

  • Generally not required 1
  • Pulse oximetry recommended to assess oxygenation 1

• Microbiological investigations:

  • Not recommended routinely 1
  • Consider sputum examination if patient fails empiric therapy 1
  • Consider TB testing if persistent productive cough with constitutional symptoms 1

2. Inpatient Management (Non-Severe CAP)

• Chest radiography:

  • Required for all hospitalized patients 1, 2
  • Preferably posteroanterior and lateral views 2

• Laboratory tests:

  • Complete blood count 1
  • Urea, electrolytes, and liver function tests 1
  • C-reactive protein (CRP) when available 1
  • Oxygenation assessment (arterial blood gas or pulse oximetry) 1

• Microbiological investigations:

  • Blood cultures (before antibiotics if possible) 1
  • Sputum Gram stain and culture for patients who:
    • Can produce purulent samples
    • Have not received prior antibiotics
    • Specimens should be transported rapidly to lab 1
  • Urinary antigen tests:
    • Consider for Legionella if epidemiologically indicated (outbreaks, travel) 1
    • Consider for pneumococcal antigen in selected cases 1

3. Severe CAP (ICU Patients)

• Imaging:

  • Chest radiography required 1
  • Consider CT scan with IV contrast if necrotizing pneumonia suspected or patient not responding to treatment 2

• Laboratory tests:

  • All tests recommended for hospitalized patients 1
  • More frequent monitoring of vital signs and laboratory parameters 1

• Microbiological investigations:

  • Blood cultures (mandatory) 1
  • Sputum Gram stain and culture 1
  • Legionella urinary antigen testing 1
  • Pneumococcal urinary antigen testing 1
  • Lower respiratory tract samples in intubated patients:
    • Endotracheal aspirates
    • Bronchoalveolar lavage (BAL)
    • Protected specimen brush (PSB) 2
  • Thoracentesis if significant pleural effusion present 1, 2
  • Consider testing for viral pathogens including influenza and COVID-19 3

Special Considerations

• Risk factors for resistant pathogens (MRSA or Pseudomonas):

  • More extensive microbiological workup recommended 1
  • Both blood and sputum cultures mandatory 1

• Immunocompromised patients:

  • Consider broader microbiological testing including fungal studies 2
  • Lower threshold for bronchoscopy 2

• Non-responding pneumonia:

  • Consider CT scan if not improving 2
  • More extensive microbiological workup including bronchoscopy 1, 2

Common Pitfalls to Avoid

• Delaying antibiotic administration while waiting for test results 2
• Failing to obtain appropriate cultures before starting antibiotics 1
• Not considering atypical pathogens in appropriate clinical scenarios 1
• Inadequate follow-up imaging in patients not responding to therapy 2
• Overlooking the need for oxygen assessment in all hospitalized patients 1

By following this structured approach to CAP workup based on severity, clinicians can ensure appropriate diagnostic testing while avoiding unnecessary investigations in patients with mild disease.