What is the differential diagnosis and treatment approach for an adult patient with no significant past medical history presenting with either an acute or subacute cough?

Acute vs Subacute Cough: Definitions and Management Approach

Acute cough lasts less than 3 weeks, while subacute cough persists between 3 and 8 weeks—this temporal distinction is the critical first step in narrowing your differential diagnosis and guiding empiric treatment. 1

Duration-Based Classification

The American College of Chest Physicians established these time-based definitions to create a practical framework for diagnosis and treatment:

• Acute cough: <3 weeks duration 1
• Subacute cough: 3-8 weeks duration 1
• Chronic cough: >8 weeks duration 1

This classification system has been validated globally across multiple continents and consistently predicts the most likely etiologies. 1

Acute Cough (<3 Weeks)

Most Common Causes

Viral upper respiratory tract infection (common cold) is by far the most frequent cause of acute cough in otherwise healthy adults. 1, 2, 3

Other important considerations include:

• Acute bronchitis (viral lower respiratory tract infection, including influenza) 1
• Acute asthma exacerbation 1
• Acute exacerbation of chronic bronchitis 1
• Pneumonia (less common but potentially life-threatening) 2

Key Clinical Assessment Points

Look specifically for these red flags that suggest pneumonia rather than simple viral infection:

• Fever ≥38°C (100.4°F) 2
• Tachycardia (heart rate >100 bpm) 2
• Tachypnea or respiratory distress 2
• Abnormal lung findings on examination 2

Assess for risk factors that increase complication risk: comorbidities, frailty, impaired immunity, or reduced ability to clear secretions. 2

Evidence-Based Treatment Approach

For acute cough due to common cold, prescribe a first-generation antihistamine plus decongestant combination—this is the only treatment proven in double-blind placebo-controlled trials to decrease cough severity and hasten resolution. 1, 2, 3

Additional symptomatic measures:

• Adequate fluid intake (no more than 2 liters daily to avoid dehydration) 2
• Paracetamol (acetaminophen) for fever and achiness 2
• Honey for cough suppression (if culturally acceptable and patient >1 year old) 2
• Naproxen (NSAID) has been shown to favorably affect cough in controlled trials 1

Antibiotics are NOT indicated for most cases of acute cough, as the cause is viral. 1, 3 Reserve antibiotics for documented bacterial infections like bacterial sinusitis or early Bordetella pertussis. 1

When to Obtain Chest Radiograph

Order chest X-ray if pneumonia is suspected based on: tachypnea, tachycardia, dyspnea, abnormal lung findings, or hypoxemia. 4, 2

Subacute Cough (3-8 Weeks)

Most Common Causes

Postinfectious cough is the predominant cause of subacute cough, representing the lingering effects of a preceding respiratory infection. 1, 5, 6

The pathophysiology involves multiple mechanisms:

• Persistent postnasal drip 1
• Bronchial hyperresponsiveness (transient or asthma-related) 1
• Mucus hypersecretion or impaired mucociliary clearance 1
• Upper airway irritation 1

Other important considerations:

• Exacerbation of underlying asthma 1, 5
• Exacerbation of COPD 1
• Upper airway cough syndrome (UACS) 1, 5
• Bordetella pertussis infection (consider if paroxysmal cough, post-tussive vomiting, or inspiratory whoop present) 1

Critical Decision Point

The first step is determining whether the subacute cough followed an obvious preceding respiratory infection. 1, 5 This distinction fundamentally changes your management approach.

Treatment Algorithm for Postinfectious Subacute Cough

If cough persists beyond 8 weeks, stop treating it as postinfectious and evaluate as chronic cough. 1

For postinfectious cough lasting 3-8 weeks:

1. First-line: Inhaled ipratropium has fair evidence for attenuating postinfectious cough 1

2. If ipratropium fails and cough affects quality of life: Add inhaled corticosteroids 1

3. For severe paroxysms: Consider prednisone 30-40 mg daily for a short course (only after ruling out UACS, asthma, and GERD) 1

4. If other measures fail: Central-acting antitussives (codeine or dextromethorphan) 1

Antibiotics have NO role in postinfectious cough unless bacterial sinusitis or early pertussis is documented. 1

Treatment for Non-Postinfectious Subacute Cough

If the cough is NOT postinfectious, manage it using the chronic cough algorithm:

• First-generation antihistamine/decongestant for suspected UACS 5
• Inhaled bronchodilators and corticosteroids for suspected asthma 5
• Inhaled corticosteroids for suspected non-asthmatic eosinophilic bronchitis 5

Important Medication History

Discontinue ACE inhibitors regardless of temporal relationship—the original cause may have resolved and the drug could be perpetuating the cough. 1, 5 Cough typically resolves within days to 2 weeks of stopping the ACE inhibitor, though median time is 26 days. 1

Key Differences in Management Approach

The fundamental distinction between acute and subacute cough management:

Acute cough: Focus on symptomatic relief with antihistamine/decongestant combinations, as most cases are self-limited viral infections. 1, 2

Subacute cough: Determine if postinfectious, then use targeted therapy with ipratropium and inhaled corticosteroids rather than simple symptomatic measures. 1, 5

Common Pitfalls to Avoid

• Don't prescribe antibiotics for viral acute or postinfectious subacute cough—this promotes resistance without benefit. 1, 3
• Don't rely on cough characteristics (timing, quality) for diagnosis—they lack sensitivity and specificity. 1
• Don't continue treating as postinfectious if cough persists beyond 8 weeks—switch to chronic cough evaluation. 1
• Don't forget to ask about ACE inhibitor use—this is a reversible cause that must be addressed. 1, 5
• Don't miss pertussis—consider it when cough lasts ≥2 weeks with paroxysms, post-tussive vomiting, or inspiratory whoop. 1

When Multiple Causes Coexist

More than one cause of cough is frequently present, particularly in subacute and chronic cases. 1 Use sequential and additive therapy—maintain partially effective treatments while adding new interventions targeting additional suspected causes. 1