A 35-year-old woman with a history of allergic rhinitis and recurrent episodes of wheezing presents with intermittent dyspnea and chest tightness. Initial spirometry shows FEV₁/FVC ratio of 0.68 with FEV₁ at 78% predicted. The patient denies current respiratory symptoms. To differentiate asthma from COPD and assess the reversibility of airflow obstruction, which is the most appropriate next investigation?

Explanation

## Assessing Reversibility of Airflow Obstruction: Bronchodilator Response Testing **Key Point:** Bronchodilator response testing (spirometry post-SABA) is the investigation of choice to assess reversibility and differentiate asthma from COPD. ### Bronchodilator Response Criteria **Significant Reversibility (Asthma Pattern):** - Increase in FEV₁ ≥ 12% AND ≥ 200 mL post-SABA - Suggests asthma or asthma-COPD overlap **Minimal Reversibility (COPD Pattern):** - Increase in FEV₁ < 12% or < 200 mL post-SABA - Confirms fixed airflow obstruction ### Why Bronchodilator Response is the Best Next Test | Feature | Bronchodilator Response | MCT | FeNO | Exercise Test | |---------|------------------------|-----|------|---------------| | **Differentiates asthma from COPD** | ✓ (Gold standard) | ✗ (Non-specific) | ✓ (Supportive) | ✗ (Non-specific) | | **Assesses reversibility** | ✓ | ✗ | ✗ | ✗ | | **Diagnostic for asthma** | ✓ (if positive) | ✓ (if positive) | ✗ (Supportive only) | ✗ | | **Safe in acute symptoms** | ✓ | ✗ (Risk of bronchospasm) | ✓ | ✗ | | **First-line test** | ✓ | ✗ (Second-line) | ✓ (Adjunctive) | ✗ | **High-Yield:** In a patient with borderline FEV₁/FVC (0.68) and clinical features suggestive of asthma (allergic rhinitis, intermittent symptoms), a positive bronchodilator response (≥12% and ≥200 mL improvement) confirms asthma and guides anti-inflammatory therapy. **Clinical Pearl:** The absence of current symptoms does NOT exclude asthma. Asthma is characterized by variable airflow obstruction; spirometry performed during symptom-free periods may show normal or near-normal values. Bronchodilator response testing can unmask reversibility even when baseline FEV₁ is relatively preserved. ### Mechanism of Bronchodilator Response Beta-2 agonists cause: 1. Smooth muscle relaxation in airways 2. Relief of bronchospasm (reversible component) 3. Increased airway caliber → improved FEV₁ In asthma, the airflow obstruction is primarily due to reversible bronchoconstriction, smooth muscle hypertrophy, and inflammation. In COPD, emphysematous destruction and fixed airway narrowing limit reversibility. [cite:Harrison 21e Ch 296; GINA 2023 Global Strategy for Asthma Management and Prevention]