How to Diagnose Bronchitis
Bronchitis is diagnosed clinically based on history and physical examination—specifically, acute cough lasting less than 3 weeks with or without sputum production—but only after ruling out pneumonia, common cold, asthma, and COPD exacerbation. 1
Diagnostic Approach for Acute Bronchitis
Clinical Criteria
The diagnosis requires:
- Predominant symptom: Acute cough (with or without sputum production)
- Duration: Less than 3 weeks 1
- Exclusion criteria: No clinical or radiographic evidence of:
- Pneumonia
- Common cold (primarily upper respiratory symptoms)
- Acute asthma
- COPD exacerbation 1
Key Clinical Assessment
Rule out pneumonia first by checking for:
- Vital sign abnormalities: heart rate ≥100 bpm, respiratory rate ≥24 breaths/min, or temperature ≥38°C
- Focal consolidation on chest exam: rales, egophony, or fremitus 2
Important caveat: The presence or absence of purulent (green) sputum does NOT differentiate bacterial from viral infection and should NOT guide diagnosis or antibiotic decisions 2, 3. Purulence simply indicates inflammatory cells and can occur with viral infections.
When Diagnostic Testing is NOT Needed
For uncomplicated acute bronchitis, routine testing is not indicated 4, 5:
- No chest x-ray (unless pneumonia suspected)
- No sputum cultures
- No viral PCR testing
- No spirometry or peak flow
- No blood work (CRP, procalcitonin, CBC)
The 2020 CHEST guidelines explicitly recommend against routine investigation in immunocompetent adults with suspected acute bronchitis 4.
When to Consider Testing
Chest radiography is warranted if:
- Tachypnea, tachycardia, or dyspnea present
- Abnormal vital signs as noted above
- Focal lung findings on examination 2, 6
Consider pertussis if:
- Cough persists >2 weeks with paroxysmal cough, whooping, or post-tussive vomiting
- Recent pertussis exposure 6
Reassessment and targeted investigations if:
- Symptoms persist beyond 3 weeks (consider postinfectious cough, upper airway cough syndrome, asthma, GERD) 1
- Symptoms worsen despite initial management 4
Critical Diagnostic Pitfall
Asthma is frequently misdiagnosed as acute bronchitis: Approximately one-third of patients presenting with acute cough labeled as bronchitis actually have asthma 1. For patients with at least two similar episodes of "acute bronchitis" in the past 5 years, 65% have mild asthma 1, 4.
However, distinguishing acute bronchitis from asthma at initial presentation (<2-3 weeks) is difficult because many bronchitis patients have transient bronchial hyperresponsiveness 2. Reserve asthma evaluation for cough persisting beyond 3 weeks.
Diagnostic Approach for Chronic Bronchitis
Chronic bronchitis is defined by: Cough with sputum production on most days during 3 consecutive months for more than 2 successive years 7, 8.
Primary cause: Cigarette smoking (85-90% of cases) 8
Key distinction: This is a symptom-based diagnosis requiring longitudinal assessment, not a single-visit diagnosis 9.
Differential Considerations
When evaluating chronic cough with sputum:
- COPD (often coexists)
- Bronchiectasis
- Occupational exposures (15% of cases) 8
- Passive smoke exposure
Bottom Line Algorithm
- Patient presents with acute cough → Check duration
- If <3 weeks → Assess vital signs and lung exam
- If vital signs/exam normal → Diagnose acute bronchitis clinically; no testing needed 4
- If vital signs/exam abnormal → Consider chest x-ray to rule out pneumonia 2
- If cough >3 weeks → Reassess for alternative diagnoses (asthma, postinfectious cough, upper airway cough syndrome, GERD) 1
Patient education is essential: Emphasize that cough typically lasts 2-3 weeks, is usually viral, and antibiotics are not beneficial 5, 6, 3.