Differentiating Asthma from Pneumonia: Clinical Presentation and Findings
Direct Clinical Algorithm for Differentiation
Suspect pneumonia when acute cough is present PLUS any one of the following: new focal chest signs on examination, dyspnea or tachypnea, pulse rate >100 beats/min, or fever lasting >4 days. 1, 2
Step 1: Apply the Four-Point Pneumonia Rule-Out Criteria
If ALL four of the following are absent, pneumonia is sufficiently unlikely that chest radiography can be omitted: 2, 3
- Heart rate >100 beats/min 2
- Respiratory rate >24 breaths/min 2
- Oral temperature >38°C 2
- Focal chest examination findings (consolidation, egophony, fremitus, dull percussion note, or pleural rub) 1, 2
Step 2: Use C-Reactive Protein to Refine Diagnosis
When pneumonia is suspected after clinical assessment: 1, 2, 3
- CRP <20 mg/L with symptoms >24 hours makes pneumonia highly unlikely 1, 2
- CRP >100 mg/L makes pneumonia likely 1, 2
- Proceed to chest X-ray if doubt persists after CRP testing 1, 3
Step 3: Identify Asthma/COPD Exacerbation Features
Consider lung function testing when ≥2 of the following are present: 1, 3, 4
- Wheezing on examination 1, 3
- Prolonged expiration 1, 3
- Smoking history 1, 3
- History of allergy 1, 3
- Previous consultations for wheezing or cough 1, 3
Key Clinical Distinctions
Pneumonia Presentation
- Focal chest signs are the most discriminating feature: when present, 39% have pneumonia versus 5-10% baseline risk 1
- Fever persisting >4 days is highly suggestive 1
- Dyspnea and tachypnea are prominent 1
- Absence of upper respiratory symptoms supports pneumonia over viral illness 1
Asthma Exacerbation Presentation
- Wheezing and prolonged expiration predominate 1, 3
- Absence of focal chest signs 1, 3
- Fever typically <4 days if present 3
- Increased dyspnea, cough, and wheeze without focal findings 3
Critical Pitfalls to Avoid
Approximately 40% of patients with acute bronchitis develop transient bronchial hyperresponsiveness that mimics asthma, making differentiation challenging in the acute setting 2
Asthma patients can develop pneumonia, particularly those on high-dose inhaled corticosteroids (≥1,000 μg), who have a 2.04-fold increased risk of pneumonia compared to asthmatics not using inhaled corticosteroids 5. This risk is dose-dependent and more pronounced with fluticasone propionate than budesonide 6.
Do not assume all pulmonary infiltrates with fever are infectious, as fever, leukocytosis, and infiltrates occur in both infectious pneumonia and non-infectious pneumonitis 2
High-Risk Populations Requiring Lower Threshold for Imaging
In patients ≥65 years, consider chest X-ray even with fewer clinical findings if any of the following are present: 1, 2, 3
- COPD, diabetes, or heart failure 1, 3
- Previous hospitalization in past year 1, 3
- Oral glucocorticoid use 1, 3
- Recent antibiotic use 1, 3
- General malaise or confusion 1, 3
For patients <65 years, diabetes and a clinical diagnosis of pneumonia are the primary risk factors for complications 1, 3
Diagnostic Testing Recommendations
Chest radiography (PA and lateral views) is the gold standard for pneumonia diagnosis and should be performed when clinical suspicion persists after CRP testing 1, 2, 3
Microbiological tests (cultures, gram stains) are NOT recommended in primary care for routine lower respiratory tract infection evaluation 1, 2
Biomarkers to assess bacterial pathogens are not recommended in primary care settings 1, 2
Clinical Context: Asthma and Pneumonia Overlap
Asthma patients hospitalized with COVID-19 pneumonia did not present with asthma exacerbations, and eosinopenia (median 0 cells/mm³) was a typical feature, contrasting with the usual eosinophilia seen in asthma 7. This highlights that pneumonia in asthmatics may suppress typical asthma inflammatory markers.
Asthma itself confers a 3.35-fold increased risk of hospitalization with pneumonia compared to non-asthmatics, independent of inhaled corticosteroid use 6