What are the diagnostic criteria for acute bronchitis in an adult presenting with a new cough of less than three weeks following an upper‑respiratory viral prodrome?

Diagnostic Criteria for Acute Bronchitis

Acute bronchitis is a clinical diagnosis made when an adult presents with acute cough lasting less than 3 weeks, with or without sputum production, after you have excluded pneumonia, the common cold, acute asthma, and COPD exacerbation. 1

Exclusion of Pneumonia is Mandatory

Before diagnosing acute bronchitis, you must rule out pneumonia by checking four vital signs and performing a focused chest examination 1:

• Heart rate >100 beats/min – if present, consider pneumonia 1
• Respiratory rate >24 breaths/min – if present, consider pneumonia 1
• Oral temperature >38°C (100.4°F) – if present, consider pneumonia 1
• Abnormal chest examination findings (focal rales, egophony, or tactile fremitus) – if present, consider pneumonia 1

If any one of these four findings is present, obtain a chest radiograph to rule out pneumonia rather than treating as simple bronchitis. 1 In otherwise healthy adults under 70 years without comorbidities, if all four are absent, pneumonia is unlikely and chest X-ray is not required. 1

Exclusion of Other Differential Diagnoses

Rule Out Asthma

• Approximately one-third of patients initially diagnosed with acute bronchitis actually have undiagnosed asthma. 1
• In retrospective studies, 65% of patients with at least two similar episodes of "acute bronchitis" in the past 5 years were found to have mild asthma. 1
• Suspect asthma if the patient has recurrent episodes, wheezing, or symptoms that worsen at night or with cold/exercise exposure. 1
• However, transient bronchial hyperresponsiveness is common in acute bronchitis and typically resolves within 2-3 weeks (occasionally up to 2 months), so do not diagnose asthma during the acute illness unless there is severe airflow obstruction. 1

Rule Out COPD Exacerbation

• Patients with known COPD presenting with acute cough should be considered to have an exacerbation of chronic bronchitis, not acute bronchitis. 1
• These patients require different management and are outside the scope of uncomplicated acute bronchitis. 1

Rule Out the Common Cold

• The common cold predominantly involves upper respiratory symptoms (rhinorrhea, nasal congestion, sore throat) with cough as a secondary feature. 1
• Acute bronchitis is diagnosed when cough is the predominant symptom with or without upper respiratory prodrome. 1

Positive Clinical Features of Acute Bronchitis

Once you have excluded the above conditions, acute bronchitis is characterized by 1:

• Acute cough (lasting <3 weeks) as the predominant symptom 1
• With or without sputum production 1
• Normal chest radiograph (if obtained) 1
• May have accompanying symptoms such as wheezing, dyspnea, or chest discomfort 1

Critical Pitfalls to Avoid

Purulent Sputum Does NOT Indicate Bacterial Infection

• Purulent (green or yellow) sputum occurs in 89-95% of viral bronchitis cases and does not indicate bacterial infection or need for antibiotics. 1, 2, 3
• Purulence reflects inflammatory cells and sloughed epithelial cells, not bacteria. 1

Cough Duration Does NOT Indicate Bacterial Infection

• Viral bronchitis cough typically lasts 10-14 days and may persist up to 3 weeks. 1, 2, 3, 4
• Cough duration alone does not justify antibiotic prescription. 1, 3

No Routine Laboratory Testing or Imaging

• Viral cultures, serologic assays, sputum cultures, serum inflammatory markers, and other laboratory tests are not routinely indicated in the diagnosis of acute bronchitis. 1
• Chest radiography is only indicated if pneumonia cannot be excluded clinically (see criteria above) or if cough persists ≥3 weeks without other known cause. 1, 5

When Cough Persists Beyond 3 Weeks

If cough persists for more than 3 weeks, acute bronchitis is no longer the diagnosis and you must consider 1:

• Post-infectious cough 1
• Upper airway cough syndrome (formerly post-nasal drip syndrome) 1
• Asthma or cough-variant asthma 1
• Gastroesophageal reflux disease 1
• Pertussis (especially if paroxysmal cough, post-tussive vomiting, or inspiratory whoop) 1

Special Consideration: Pertussis

• Suspect pertussis if cough persists >2 weeks with paroxysmal cough, post-tussive vomiting, inspiratory "whoop," or known pertussis exposure. 4
• If pertussis is confirmed or strongly suspected, prescribe a macrolide antibiotic (azithromycin or erythromycin) immediately and isolate the patient for 5 days from treatment start. 1

Summary Algorithm

1. Patient presents with acute cough (<3 weeks duration) 1
2. Check vital signs and perform chest examination to exclude pneumonia (HR, RR, temp, focal findings) 1
3. Exclude asthma (recurrent episodes, wheezing, nocturnal symptoms) 1
4. Exclude COPD exacerbation (known COPD history) 1
5. Exclude common cold (predominantly upper respiratory symptoms) 1
6. If all exclusions are met → diagnose acute bronchitis 1
7. Do not order routine labs, cultures, or imaging unless pneumonia cannot be excluded 1
8. Educate patient that cough will last 10-14 days (up to 3 weeks) 1, 2, 3, 4
9. Do not prescribe antibiotics (they reduce cough by only 0.5 days while causing adverse effects) 1, 3, 4
10. Reassess if fever persists >3 days or cough persists >3 weeks 1