A 38-year-old man with known asthma presents with acute dyspnea and wheezing despite regular inhaled corticosteroid use. He has been using his salbutamol inhaler 4–5 times daily for the past 2 weeks. On examination, peak expiratory flow (PEF) is 65% of predicted. To assess the reversibility of airway obstruction and guide bronchodilator therapy, which investigation is most appropriate?

Explanation

## Investigation of Choice for Assessing Bronchodilator Responsiveness ### Clinical Context The patient has acute dyspnea with increased salbutamol use and reduced PEF, suggesting inadequate control. To confirm reversible airway obstruction and guide bronchodilator escalation, **spirometry with bronchodilator response testing** is the gold standard. ### Why Spirometry with Bronchodilator Response? **Key Point:** Spirometry with bronchodilator response (FEV₁ improvement ≥12% and ≥200 mL after short-acting beta-2 agonist) is the diagnostic gold standard for reversible airway obstruction in asthma. **High-Yield:** - Measures baseline FEV₁, FVC, and FEV₁/FVC ratio - Repeat measurement 15 minutes after inhaled salbutamol (400 μg) - Positive response = ≥12% improvement in FEV₁ AND absolute increase ≥200 mL - Confirms asthma diagnosis and predicts response to bronchodilators ### Clinical Pearl In acute asthma exacerbation, spirometry may be difficult to perform, but once stabilized, it is essential to: 1. Confirm reversibility (supports asthma over COPD) 2. Assess baseline lung function 3. Guide step-up in therapy (inhaled corticosteroid dose, addition of LABA) ### Comparison with Other Investigations | Investigation | Indication | Limitation in This Case | |---|---|---| | **Spirometry + bronchodilator** | Confirm reversible obstruction, guide therapy | **Gold standard — BEST CHOICE** | | HRCT chest | Detect structural lung disease, bronchiectasis, ILD | Not indicated for acute asthma; no reversibility assessment | | Methacholine challenge | Diagnose asthma in non-obstructed patients | Contraindicated in acute obstruction (FEV₁ <70%); not for acute exacerbation | | ABG | Assess oxygenation and CO₂ retention in severe exacerbation | Does not assess reversibility; used for severity, not diagnosis | **Tip:** Remember — methacholine challenge is a **provocative test** (reproduces obstruction) and is contraindicated when FEV₁ is already <70% or in acute exacerbation. Bronchodilator response is a **therapeutic test** (reverses obstruction) and is safe and diagnostic. [cite:KD Tripathi 8e Ch 27]