What tests confirm pneumonia diagnosis?

Diagnostic Tests to Confirm Pneumonia

Chest radiography is the primary diagnostic test for confirming pneumonia, with CT scan providing higher sensitivity when radiographic findings are equivocal or negative despite high clinical suspicion. 1, 2

Clinical Assessment and Initial Testing

• Clinical suspicion should be raised when a patient presents with:

  • Acute cough plus one of: new focal chest signs, dyspnoea, tachypnoea, or fever >4 days 1
  • Absence of runny nose (suggests pneumonia rather than upper respiratory infection) 1
  • Crackles, diminished breath sounds, tachycardia (>100 beats/min), and fever (≥37.8°C) 1

• C-reactive protein (CRP) testing:

  • CRP <20 mg/L with symptoms >24 hours makes pneumonia highly unlikely
  • CRP >100 mg/L makes pneumonia likely
  • CRP >30 mg/L improves diagnostic classification when combined with clinical findings 1

Imaging Studies

Chest Radiography

• Standard first-line test for pneumonia diagnosis 1, 2
• Diagnostic findings include:
  • New or progressive infiltrate, consolidation, or cavitation 1
  • In patients without underlying pulmonary or cardiac disease, one definitive chest radiograph is acceptable 1
• Limitations:
  • May be normal in early pneumonia (typical appearances present in only 36% of initial CXRs) 1
  • Limited sensitivity for detecting early or subtle infiltrates, particularly in elderly patients 2, 3

Computed Tomography (CT)

• Superior to chest radiography for detecting:
  • Cavitary lesions
  • Pleural complications
  • Early or subtle infiltrates 2
• Particularly valuable when:
  • Chest X-ray findings are equivocal
  • High clinical suspicion despite negative chest X-ray 2, 3
• Not recommended for routine initial assessment due to cost and radiation exposure 2

Microbiological Testing

• Not routinely recommended in primary care 1
• Should be considered in:
  • Hospitalized patients
  • Patients with severe illness
  • Cases where results may change management 1

When Indicated, Microbiological Tests Include:

• Blood cultures: Low yield (7-11%) but should be collected before antibiotics in hospitalized patients 2
• Respiratory specimen collection:
  • Sputum samples for Gram stain, culture, and sensitivity testing
  • Bronchoscopy with protected specimen brush or bronchoalveolar lavage for severely ill patients 2
• Specific pathogen testing when clinically indicated:
  • Mycobacterial studies for TB risk factors
  • Legionella and pneumococcal urinary antigen tests 2

Diagnostic Categories

The Pneumonia in Stroke Consensus Group recommends categorizing pneumonia as:

1. Probable pneumonia: Clinical criteria met but without diagnostic changes on chest X-ray
2. Definite pneumonia: All clinical criteria met including diagnostic chest X-ray changes 1

Diagnostic Algorithm

1. Initial assessment: Evaluate for clinical features of pneumonia
2. CRP testing: If available, use cutoffs to guide further testing
3. Chest X-ray: Order if pneumonia is clinically suspected based on symptoms and CRP
4. CT scan: Consider if chest X-ray is negative but clinical suspicion remains high
5. Microbiological testing: Reserve for hospitalized or severely ill patients

Pitfalls to Avoid

• Overreliance on chest X-ray: Up to 64% of early pneumonias may have normal initial radiographs 1, 3
• Neglecting CRP values: CRP testing can significantly improve diagnostic accuracy when combined with clinical assessment 1
• Routine microbiological testing in primary care: Not cost-effective and rarely changes management 1
• Delaying antibiotic treatment while waiting for test results: Empiric therapy should be initiated promptly based on clinical presentation in severe cases 2
• Misinterpreting radiographic findings: Consider alternative diagnoses like pulmonary edema, malignancy, or pulmonary infarction when microbiological workup is negative 2

By following this evidence-based approach to pneumonia diagnosis, clinicians can accurately identify patients with pneumonia and initiate appropriate management to reduce morbidity and mortality.