What are the criteria for diagnosing pneumonia?

Diagnostic Criteria for Pneumonia

Pneumonia diagnosis requires the combination of respiratory symptoms (cough or difficulty breathing) plus radiographic evidence of a new infiltrate on chest imaging, as clinical features alone lack sufficient specificity to confirm the diagnosis. 1, 2, 3

Core Diagnostic Algorithm

Step 1: Clinical Assessment

Begin by identifying respiratory symptoms and signs:

• Required respiratory symptoms: Cough or difficulty breathing (either reported or observed) 1, 2
• Supportive clinical signs (though non-specific): Fever, tachypnea, chest indrawing, hypoxemia (SpO₂ <93%), or signs of respiratory distress 1, 2
• Important caveat: Fever, tachycardia, and leukocytosis are common but have low specificity and can be caused by any inflammatory state 2, 4

Clinical diagnosis alone is insufficient. Physical examination findings like crackles or bronchial breath sounds are important but less sensitive and specific than imaging 2, 4. In elderly patients, typical clinical features may be absent or atypical 2.

Step 2: Radiographic Confirmation (Mandatory)

Chest radiography is required to confirm pneumonia diagnosis in all cases except uncomplicated outpatients where empirical treatment may be initiated based on clinical suspicion alone. 1, 3

• Standard approach: Posteroanterior and lateral chest radiographs should be obtained to document infiltrates 3
• Key finding: A new or progressive infiltrate within 48 hours of presentation 2
• Sensitivity limitation: Chest radiography has only 46-77% sensitivity and may be negative early in disease or in elderly patients 3, 5
• If initial radiograph is negative but clinical suspicion remains high: Treat presumptively with antibiotics and repeat imaging in 24-48 hours 2

Alternative imaging modalities:

• Lung ultrasound: Shows superior sensitivity and specificity compared to chest radiography, with no radiation exposure and portability advantages 3
• CT with IV contrast: Gold standard for evaluating complications (necrotizing pneumonia, lung abscess, empyema) 3

Step 3: Microbiological Testing (Context-Dependent)

Outpatients: Routine microbiological testing is not recommended for uncomplicated community-acquired pneumonia 3, 6

Hospitalized patients: Testing is optional unless specific high-risk features are present 1

Mandatory testing indications (obtain before antibiotics):

• ICU admission or severe pneumonia 1, 6
• Failure of outpatient antibiotic therapy 6
• Cavitary infiltrates 6
• Severe immunosuppression 6
• Active alcohol abuse 6
• Suspected resistant organisms 1

Recommended tests for severe cases:

• Blood cultures (sensitivity <25% but important for bacteremia detection) 1, 6
• Expectorated sputum for Gram stain and culture (only if good-quality specimen: >25 PMNs and <10 squamous epithelial cells per low-power field) 1, 6
• Urinary antigen tests for Legionella pneumophila and Streptococcus pneumoniae 1, 6
• For intubated patients: Endotracheal aspirate 1, 6

Critical limitation: No organism is identified in 40-60% of cases despite comprehensive testing 6

Severity Assessment for Severe Pneumonia

Define pneumonia as severe if ANY of the following are present:

Major Criteria (either one requires ICU):

• Need for mechanical ventilation 1
• Septic shock 1

Minor Criteria (≥3 suggest ICU need):

• Respiratory rate ≥30 breaths/min 1
• PaO₂/FiO₂ ratio ≤250 1
• Multilobar infiltrates 1
• Confusion 1
• Blood urea nitrogen ≥20 mg/dL 1
• Leukopenia from infection 1
• Thrombocytopenia 1
• Hypothermia 1
• Hypotension requiring aggressive fluid resuscitation 1

WHO General Danger Signs (for pediatric severe pneumonia):

• Inability to drink 1
• Vomiting everything 1
• Convulsions 1
• Lethargy or unconsciousness 1
• Severe malnutrition 1
• Hypoxemia (SpO₂ <93% or altitude-adjusted) 1

Common Pitfalls to Avoid

• Do not rely on clinical features alone: Up to one-third of patients are misdiagnosed when imaging is not obtained 5
• Do not use chest radiography to distinguish viral from bacterial pneumonia: This cannot be done reliably 3
• Do not culture inadequate sputum specimens: Screen microscopically first to avoid contamination with oral flora 6
• Do not delay antibiotics for culture results: Collect specimens before antibiotics when possible, but antibiotic administration reduces culture yield within 24 hours 6
• Do not treat colonization: Purulent tracheal secretions are common in ventilated patients and rarely indicate pneumonia 2
• Do not over-interpret biomarkers: White blood cell count, procalcitonin, and C-reactive protein provide little diagnostic benefit 5

Special Populations

Pediatric field trial definition (resource-limited settings):

1. Assess for cough or difficulty breathing 1
2. If present, evaluate for tachypnea (age-specific), chest indrawing, severe respiratory distress, or hypoxemia 1
3. Confirm with imaging showing opacification 1
4. Use adjudication panels for imaging interpretation 1

Ventilator-associated pneumonia: Clinical criteria have low accuracy in ARDS patients; the combination of infiltrates plus 2 of 3 clinical criteria has only 69% sensitivity and 75% specificity 2