How is bronchitis diagnosed?

How to Diagnose Bronchitis

Bronchitis diagnosis depends critically on whether you are evaluating acute bronchitis (cough <3 weeks) or chronic bronchitis (productive cough ≥3 months per year for ≥2 consecutive years), as these represent fundamentally different disease processes with distinct diagnostic approaches.

Diagnosing Acute Bronchitis

Primary Diagnostic Criteria

Acute bronchitis is a clinical diagnosis made when a patient presents with predominant cough lasting less than 3 weeks, with or without sputum production, after excluding pneumonia, common cold, acute asthma, and COPD exacerbation. 1, 2

Step 1: Rule Out Pneumonia

Assess four vital clinical findings to determine if chest radiography is needed: 1, 2

• Heart rate >100 beats/min
• Respiratory rate >24 breaths/min
• Oral temperature >38°C (100.4°F)
• Focal chest examination abnormalities (rales, egophony, increased tactile fremitus)

If any of these four findings are present, obtain a chest X-ray to exclude pneumonia. 1, 2 In otherwise healthy adults under 70 years without comorbidities, if all four findings are absent, pneumonia is unlikely and chest radiography is not required. 2

Step 2: Exclude Asthma

This is critical because approximately 33% of patients initially labeled with acute bronchitis actually have undiagnosed asthma. 1, 2 Suspect asthma when: 1, 2

• Patient reports recurrent similar episodes (65% of patients with ≥2 "bronchitis" episodes in the past 5 years have mild asthma)
• Wheezing is prominent
• Symptoms worsen at night, with cold air exposure, or with exercise
• Severe airflow obstruction is present on examination

Important caveat: Transient bronchial hyperresponsiveness occurs commonly in acute bronchitis and typically resolves within 2-3 weeks (occasionally up to 2 months). Do not diagnose asthma during the acute phase unless severe airflow obstruction is present. 1, 2

Step 3: Distinguish from Common Cold and COPD Exacerbation

• Common cold: Upper respiratory symptoms (rhinorrhea, nasal congestion, sore throat) predominate, with cough as a secondary feature. 1, 2
• Acute bronchitis: Cough is the predominant symptom, even if an upper respiratory prodrome is present. 1, 2
• COPD exacerbation: Patients with known COPD who develop acute cough have a COPD exacerbation, not acute bronchitis, and require different management. 2

Critical Pitfall: Purulent Sputum

Green or yellow sputum occurs in 89-95% of viral bronchitis cases and does NOT indicate bacterial infection or need for antibiotics. 2 The discoloration reflects inflammatory cells and sloughed epithelium, not bacterial presence. 1, 2 This is one of the most common diagnostic errors leading to inappropriate antibiotic prescribing.

Diagnostic Testing

Routine laboratory testing (viral cultures, serology, sputum cultures, inflammatory markers) is NOT indicated for diagnosing acute bronchitis. 2 Chest imaging is reserved only for cases where pneumonia cannot be excluded clinically. 2

Special Consideration: Pertussis

If cough persists >2 weeks with paroxysmal coughing, post-tussive vomiting, inspiratory "whoop," or known pertussis exposure, consider pertussis testing. 3, 4

Diagnosing Chronic Bronchitis

Primary Diagnostic Criteria

Adults with chronic cough and sputum expectoration occurring on most days for at least 3 months per year and for at least 2 consecutive years should be diagnosed with chronic bronchitis when other respiratory or cardiac causes of chronic productive cough are ruled out. 1

Essential History Components

Obtain a complete exposure history regarding: 1

• Cigarette, cigar, and pipe smoking (responsible for 85-90% of chronic bronchitis cases)
• Passive smoke exposure
• Occupational and environmental hazards in home and workplace

Distinguishing Chronic Bronchitis from Asthma

This distinction is critical because treatment differs fundamentally. 5

Chronic bronchitis features: 5

• Heavy smoking history (typically >20 pack-years)
• Chronic productive cough worse in morning
• Gradually progressive breathlessness over many years
• Minimal bronchodilator reversibility (<12% and <200 mL FEV₁ improvement)

Asthma features: 5

• Recurrent episodic symptoms rather than constant symptoms
• Significant bronchodilator reversibility (≥12% and ≥200 mL FEV₁ improvement)
• Variable symptoms with fluctuating intensity
• Often history of atopy

Perform spirometry with bronchodilator response testing to objectively distinguish these conditions. 5 If FEV₁ is ≥70% predicted but clinical suspicion for asthma remains high, perform methacholine challenge testing. 5

When Cough Persists Beyond 3 Weeks

If cough exceeds 3 weeks duration, acute bronchitis is no longer the diagnosis. 2 Systematically evaluate for: 2, 6

• Upper airway cough syndrome (postnasal drip, rhinosinusitis)
• Asthma or cough-variant asthma
• Gastroesophageal reflux disease
• Pertussis
• Post-infectious cough (only valid diagnosis for 3-8 weeks post-infection)

Obtain chest radiography and spirometry at this point to evaluate for structural abnormalities and obstructive lung disease. 6

Common Diagnostic Pitfalls to Avoid

• Do not diagnose chronic bronchitis based on self-report alone: 88.4% of patients with self-reported or physician-confirmed chronic bronchitis do not meet standard diagnostic criteria. 1
• Do not assume purulent sputum indicates bacterial infection in acute bronchitis. 1, 2
• Do not miss underlying asthma in patients with recurrent "bronchitis" episodes. 1, 2
• Do not label persistent cough at 3 months as "post-infectious cough"—this requires evaluation for alternative diagnoses. 6