Is this patient likely to have pneumonia (pna)?

Diagnosing Pneumonia: Clinical and Radiographic Criteria

Pneumonia cannot be diagnosed by imaging alone—you must have both a radiographic infiltrate AND at least two clinical features of infection (fever, leukocytosis, new/worsened cough, dyspnea, or purulent secretions) to establish the diagnosis. 1

Essential Diagnostic Criteria

The diagnosis requires a two-step approach:

Step 1: Clinical Assessment

• Look for at least two of the following signs or symptoms 2:
  • Temperature >38°C or ≤36°C
  • Leukocyte count <4,000/μL or >10,000/μL
  • New or increased cough
  • Dyspnea or breathlessness
  • Purulent sputum production
  • Pleuritic chest pain 3

Step 2: Radiographic Confirmation

• Obtain posteroanterior and lateral chest radiographs showing air space density or infiltrate consistent with pneumonia 3
• The radiograph must demonstrate a new or progressive infiltrate without an alternative explanation 1

Critical Diagnostic Pitfalls

Using only one clinical criterion increases sensitivity but significantly decreases specificity, leading to overtreatment with antibiotics 1. Conversely, requiring all criteria is too insensitive and will miss true cases 1.

Special populations require heightened suspicion 3:

• Elderly patients may present with confusion, failure to thrive, or falls rather than typical respiratory symptoms
• Fever may be absent, but tachypnea is usually present
• Patients with dementia, stroke, delirium, alcoholism, or impaired consciousness have high pretest probability despite negative physical examination 3

When Initial Chest Radiograph is Negative or Equivocal

If clinical suspicion remains high despite negative/equivocal chest radiograph 3:

CT chest is indicated for:

• Patients who cannot reliably follow up
• Advanced age with significant comorbidities
• Any patient where delay in diagnosis could be life-threatening 3
• CT detects pneumonia in 27-33% of patients with negative chest radiographs when clinical suspicion is high 3, 1
• CT also excluded pneumonia in 29.8% of patients with opacities on chest radiograph, preventing unnecessary antibiotic use 3, 1

Ultrasound may be considered:

• Demonstrates 81.4% sensitivity versus 64.3% for chest radiograph 3, 1
• Particularly useful when radiographs are negative or equivocal 1

Alternative Diagnoses to Consider

Before diagnosing pneumonia, exclude 1:

• Pulmonary edema in patients with cardiomegaly or heart failure
• Aspiration without infection—not all aspiration events require antibiotics
• Malignancy, particularly in older smokers with persistent infiltrates 3

Microbiological Testing

Test all patients for COVID-19 and influenza when these viruses are circulating in the community, as results affect treatment and infection prevention strategies 2.

For hospitalized patients, obtain 3:

• Blood cultures before antibiotics
• Sputum Gram stain and culture if productive cough
• Consider testing for unusual pathogens based on epidemiologic clues (travel history, animal exposures, occupational risks) 3

Note: Only 38% of hospitalized CAP patients have a pathogen identified; of those, up to 40% are viral and approximately 15% are Streptococcus pneumoniae 2.

Severity Assessment

Once pneumonia is diagnosed, assess for ICU admission criteria 3:

• Presence of at least 3 minor criteria warrants ICU consideration
• Major criteria include respiratory failure requiring mechanical ventilation or septic shock requiring vasopressors