A 38-year-old man with newly diagnosed asthma presents with episodic dyspnea and wheezing. Clinical examination reveals bilateral end-expiratory wheeze. The physician suspects reversible airway obstruction and wants to confirm the diagnosis before initiating bronchodilator therapy. Which investigation is most appropriate to assess bronchodilator responsiveness?

## Investigation of Choice for Bronchodilator Responsiveness ### Spirometry with Bronchodilator Response Test **Key Point:** Spirometry with bronchodilator response testing is the gold standard investigation to confirm reversible airway obstruction and assess bronchodilator responsiveness in suspected asthma. **High-Yield:** The diagnostic criterion for positive bronchodilator response is an improvement of ≥12% and ≥200 mL in FEV₁ (forced expiratory volume in 1 second) after inhalation of a short-acting beta-2 agonist (SABA) like salbutamol. ### Mechanism of Assessment 1. **Baseline spirometry** — measure FEV₁, FVC (forced vital capacity), and FEV₁/FVC ratio 2. **SABA administration** — typically 400 mcg salbutamol via metered-dose inhaler (MDI) with spacer 3. **Post-bronchodilator spirometry** — repeat measurements after 15 minutes 4. **Calculate improvement** — percentage change in FEV₁ from baseline **Clinical Pearl:** A positive bronchodilator response strongly supports asthma diagnosis and confirms the potential benefit of bronchodilator therapy. This test helps differentiate asthma from COPD (where response is typically <12%). ### Why This Test? - **Objective measurement** of airway obstruction reversibility - **Guides therapeutic decisions** — confirms indication for bronchodilators - **Prognostic value** — positive response predicts good treatment response - **Non-invasive and safe** — can be repeated for monitoring **Mnemonic: FEV₁ RESPONSE** — FEV₁ improvement ≥12% AND ≥200 mL = positive bronchodilator response. [cite:Harrison 21e Ch 242]