How do you differentiate between pneumonia and pneumonitis?

Differentiating Pneumonia from Pneumonitis

Pneumonitis is non-infectious lung inflammation caused by drugs, radiation, or hypersensitivity reactions, while pneumonia is an infectious process requiring antimicrobial therapy—the key distinction lies in identifying temporal relationships to exposures versus infectious symptoms with purulent sputum. 1

Core Pathophysiologic Distinction

Pneumonitis represents non-infectious lung inflammation triggered by drug reactions, radiation exposure, or hypersensitivity reactions to inhaled antigens, whereas pneumonia is an infectious alveolar process. 1 This fundamental difference drives all subsequent diagnostic and therapeutic decisions.

Clinical Presentation Differences

Pneumonitis Features:

• Dyspnea and dry cough predominate, with fever typically absent or low-grade 1
• Critical temporal relationship to drug initiation, radiation therapy, or antigen exposure 1
• Subacute onset over days to weeks rather than acute presentation 1
• Absence of purulent sputum production 1

Pneumonia Features:

• Productive cough with purulent sputum is characteristic 1
• High fever (>38°C), chills, rigors, and myalgia are prominent 2, 1
• Pleuritic chest pain commonly present 2, 1
• Acute onset with rapid symptom progression 3
• Vital sign abnormalities: heart rate >100 beats/min, respiratory rate >24 breaths/min 2

Radiologic Pattern Recognition

Pneumonitis Imaging:

• Bilateral, non-segmental ground-glass opacities without lobar distribution 1
• Cryptogenic organizing pneumonia-like appearance with peripheral distribution 1
• Interstitial pneumonia pattern rather than alveolar consolidation 1

Pneumonia Imaging:

• Lobar or segmental consolidation with air bronchograms (96% specificity when present) 1, 4
• Air space process abutting a fissure 1, 4
• Unilateral or asymmetric distribution 4
• Rapid cavitation suggests specific bacterial pathogens 1
• Pleural effusions in 10-32% of cases 4

Important caveat: Initial chest radiographs show typical pneumonia in only 36% of cases, and normal imaging does not exclude pneumonia—repeat imaging at 24-48 hours if clinical suspicion remains high. 4

Diagnostic Algorithm

Step 1: Assess Clinical Context

• Identify recent exposures: new medications (especially chemotherapy, amiodarone, methotrexate), radiation therapy, or environmental antigens within preceding weeks 1
• Evaluate infectious symptoms: purulent sputum, high fever, rigors strongly favor pneumonia 1

Step 2: Vital Signs Assessment

Pneumonia is unlikely if ALL of the following are absent: 2

• Heart rate >100 beats/min
• Respiratory rate >24 breaths/min
• Temperature >38°C
• Focal consolidation, egophony, or fremitus on examination

Step 3: Laboratory Markers

• C-reactive protein >100 mg/L makes pneumonia probable; <20 mg/L with symptoms >24 hours makes pneumonia very unlikely 5
• White blood cell count and procalcitonin provide limited diagnostic value for distinguishing between the two 6
• Leukocytosis occurs in both conditions 1

Step 4: Imaging Interpretation

• Obtain chest radiograph (PA and lateral views) for all suspected cases 2, 4
• Bilateral ground-glass opacities without segmental distribution → consider pneumonitis 1
• Lobar consolidation with air bronchograms → pneumonia 1, 4
• Consider CT scan if chest radiograph negative but clinical suspicion high 2
• Lung ultrasound has 93-96% sensitivity and specificity for pneumonia 4

Critical Pitfall to Avoid

Never assume all pulmonary infiltrates with fever are infectious—fever, leukocytosis, and infiltrates occur in both pneumonitis and pneumonia. 1 However, if pneumonia cannot be excluded, do not delay antibiotics, as delayed antimicrobial therapy increases mortality. 1 The safer approach is to initiate empiric antibiotics while pursuing the diagnosis, then discontinue if pneumonitis is confirmed.

Treatment Implications

For Pneumonitis:

• Primary intervention is drug cessation or antigen avoidance 1
• Supportive care with oxygen as needed 1
• Corticosteroids for moderate-to-severe cases 1
• Antimicrobials are not indicated 1

For Pneumonia:

• Antimicrobial therapy is the cornerstone and must be started immediately 1
• Empiric antibiotics based on local resistance patterns 1
• Blood cultures and respiratory cultures before antibiotics, but do not delay treatment 1
• Thoracentesis for pleural effusions ≥10 mm on lateral decubitus film 1

Special Populations

In elderly patients, maintain high suspicion for pneumonia as this population presents with fewer respiratory and non-respiratory symptoms, making clinical diagnosis more challenging. 2, 5 The threshold for obtaining imaging and initiating empiric therapy should be lower in this group.