How do you differentiate between pneumonia and a viral infection in an adult patient presenting with respiratory symptoms?

Differentiating Pneumonia from Viral Upper Respiratory Infection

Use a combination of clinical features, C-reactive protein (CRP) testing, and chest X-ray when in doubt to distinguish bacterial pneumonia from viral respiratory infection, with CRP <20 mg/L making pneumonia unlikely and >100 mg/L making it likely. 1

Clinical Features That Suggest Bacterial Pneumonia

Look for these specific patterns to identify bacterial pneumonia:

• Persistent symptoms without improvement for at least 10 days suggest bacterial infection rather than viral illness 2
• Severe symptoms with high fever (≥39°C) and purulent nasal discharge for at least 3 consecutive days indicate bacterial infection 2
• "Double-worsening" pattern where symptoms initially improve then worsen within 10 days strongly suggests bacterial superinfection 2
• Vital sign abnormalities are prominent in pneumonia: heart rate >100 beats/min, respiratory rate >24 breaths/min, temperature >38°C, blood pressure <90/60 1, 3
• Pleuritic chest pain, productive cough with purulent sputum, chills, and myalgia characterize bacterial pneumonia more than viral infection 3

Clinical Features That Suggest Viral Infection

Viral respiratory infections typically present with:

• Upper respiratory symptoms (rhinorrhea, sore throat, nasal congestion) that are absent or minimal in bacterial pneumonia 1
• Fever may be absent or low-grade in viral infections, whereas high fever is more common in bacterial pneumonia 3
• Myalgia is particularly associated with influenza infection 4
• Accessory symptoms like anosmia or ageusia suggest viral etiology, particularly COVID-19 5

Use of C-Reactive Protein (CRP)

CRP is the most valuable biomarker for distinguishing bacterial from viral infection in primary care:

• CRP <20 mg/L makes pneumonia unlikely 1
• CRP >100 mg/L makes pneumonia likely 1
• CRP has excellent positive and negative predictive values with area under the ROC curve of 0.80 1
• When clinical doubt persists after CRP testing, obtain a chest X-ray to confirm or exclude pneumonia 1

Imaging Considerations

Radiographic patterns help differentiate bacterial from viral etiology:

• Bacterial pneumonia shows lobar or segmental consolidation with air bronchograms and air space process abutting a fissure 3
• Viral pneumonia demonstrates bilateral, non-segmental ground-glass opacities with bronchiolitis and bronchopneumonia patterns 6, 7
• The presence of lobar consolidation in viral infection usually suggests bacterial coinfection 6
• Do not obtain imaging for patients meeting diagnostic criteria for acute rhinosinusitis unless complications or alternative diagnoses are suspected 2

Microbiological Testing

Viral testing should be considered in specific circumstances:

• Rapid antigen detection for influenza provides results in 15-30 minutes but has limited sensitivity (50-70% in adults), so negative results do not exclude diagnosis 1, 2
• Viral NAAT panels are suggested for critically ill patients with suspected pneumonia or new upper respiratory symptoms 1
• Routine microbiological cultures and Gram stains are not recommended in primary care 1
• Blood cultures should be obtained in hospitalized patients before antibiotics but should not delay treatment >45 minutes in sepsis 1

Risk Stratification

Identify patients at elevated risk for complications who require closer monitoring:

• Age >65 years with COPD, diabetes, heart failure, previous hospitalization in past year, oral glucocorticoid use, or recent antibiotic use 1
• General malaise, confusion/diminished consciousness, pulse >100, temperature >38°C, respiratory rate >30, blood pressure <90/60 1
• Approximately 10% of immunocompetent adults hospitalized with CAP have viral infection, but bacterial superinfection is common (26-77% in different studies) 1, 2

Common Pitfalls to Avoid

Critical errors that lead to poor outcomes:

• Do not assume purulent nasal discharge indicates bacterial infection—it reflects inflammation, not necessarily bacterial etiology 8
• Do not prescribe antibiotics for viral infections—this contributes to antibiotic resistance without benefit 2, 8
• Do not delay antibiotics when bacterial pneumonia cannot be excluded—delayed appropriate antimicrobial therapy increases mortality 3
• Do not rely solely on clinical features—no clinical or radiographic criteria reliably distinguish viral from bacterial infection without additional testing 1
• The most common bacterial superinfection following viral infection is Streptococcus pneumoniae, followed by Staphylococcus aureus 1, 2

Treatment Implications

Once the distinction is made, treatment differs fundamentally:

• Bacterial pneumonia requires immediate empiric antibiotics based on local resistance patterns, with amoxicillin or amoxicillin-clavulanate as first-line 2, 3
• Viral infections require only symptomatic treatment with analgesics, topical intranasal steroids, and/or nasal saline irrigation 2, 8
• Influenza and VZV/HSV pneumonia have specific antiviral treatments (neuraminidase inhibitors and acyclovir, respectively) 1
• Most other viral pneumonias have no established antiviral therapy 1