How do you differentiate between acute bronchitis and asthma in a patient with respiratory symptoms?

Differentiating Acute Bronchitis from Asthma

In patients presenting with respiratory symptoms, differentiate acute bronchitis from asthma by identifying whether cough is a new, self-limited symptom lasting less than 3 weeks (acute bronchitis) versus recurrent episodes of dyspnea, wheeze triggered by allergens, or symptoms persisting beyond 3 weeks (asthma). 1, 2, 3

Key Diagnostic Features

Acute Bronchitis Presentation

• Sudden onset of cough as the predominant symptom, with or without sputum production, lasting less than 3 weeks 2, 3
• Cough typically resolves within 2-3 weeks without specific treatment 4
• May have transient wheezing or airflow obstruction that resolves within 6 weeks in approximately 40% of patients 1
• No history of recurrent similar episodes 1

Asthma Presentation

• Recurrent attacks of respiratory symptoms rather than isolated episodes 1, 5
• Episodes of dyspnea or attacks of shortness of breath over the past year 5
• Symptoms triggered by specific allergens or irritants 5
• Wheezing as a prominent feature 1, 5
• Symptoms persisting beyond 3 weeks suggest asthma rather than acute bronchitis 1

Critical Differentiating Clinical Features

A substantial 37% of patients initially diagnosed with acute bronchitis actually have underlying asthma, making this differentiation clinically essential 5. The American College of Chest Physicians emphasizes that in patients with recurrent attacks of presumed acute bronchitis, asthma—not acute bronchitis—may be the actual cause 1.

High-Yield Predictors of Asthma (Not Acute Bronchitis)

• Female sex 5
• Current wheeze on history 5
• Recurrent episodes of dyspnea over the past year 5
• Allergen-triggered symptoms 5
• Prolonged expiration on examination 1
• History of allergy 1

When to Perform Lung Function Testing

Consider spirometry or peak flow testing if at least two of the following are present 2:

• Wheezing
• Prolonged expiration
• Smoking history
• Symptoms of allergy

Up to 45% of patients with acute cough lasting more than 2 weeks actually have asthma or COPD 1. Reversibility of FEV1 greater than 15% has been demonstrated in 17% of patients with presumed acute bronchitis, suggesting underlying reactive airway disease 1.

Essential First Step: Rule Out Pneumonia

Before diagnosing either condition, exclude pneumonia if ANY of the following are present 2, 3:

• Heart rate >100 beats/min
• Respiratory rate >24 breaths/min
• Oral temperature >38°C
• Focal chest examination findings (consolidation, egophony, fremitus)

If all four criteria are absent and the patient is under 60 years old, pneumonia is sufficiently unlikely that chest radiography can be omitted 3.

Common Diagnostic Pitfalls

Purulent Sputum Does Not Indicate Bacterial Infection

The presence of green or yellow sputum occurs in 89-95% of viral bronchitis cases due to inflammatory cells and does not differentiate bacterial from viral infection 6, 7. Do not use sputum color to guide diagnosis or antibiotic decisions 3.

Transient Bronchial Hyperresponsiveness Can Occur in Acute Bronchitis

Approximately 40% of previously healthy individuals with acute viral respiratory infections develop transient airflow obstruction and bronchial hyperresponsiveness that resolves within 6 weeks 1. This does not necessarily indicate asthma unless symptoms recur or persist beyond this timeframe 1.

Recurrent "Acute Bronchitis" Is Usually Asthma

Patients with multiple episodes of presumed acute bronchitis over time likely have undiagnosed asthma with recurrent exacerbations rather than repeated infections 1.

Diagnostic Algorithm

1. Assess vital signs and perform chest examination to exclude pneumonia 2, 3
2. Obtain detailed history focusing on:
   • Duration of current symptoms (< or > 3 weeks) 2, 3
   • Previous similar episodes (single vs. recurrent) 1, 5
   • Allergen triggers 5
   • Attacks of dyspnea in past year 5
3. Examine for wheezing and prolonged expiration 1, 5
4. Consider lung function testing if ≥2 risk factors present (wheezing, prolonged expiration, smoking, allergy symptoms) 2
5. Diagnose acute bronchitis if: sudden onset cough <3 weeks, no recurrent episodes, no allergen triggers, and symptoms expected to resolve 2, 3
6. Diagnose asthma if: recurrent episodes, allergen triggers, attacks of dyspnea, symptoms >3 weeks, or reversible airflow obstruction on testing 1, 5

Management Implications

This distinction matters because beta-agonists and steroids are beneficial for asthma exacerbations but not indicated for routine acute bronchitis 1. Antibiotics are not indicated for either condition unless pertussis is suspected 3, 4.