## Distinguishing COPD from Asthma: The Reversibility Criterion ### Key Pathophysiological Difference **Key Point:** The hallmark distinction between COPD and asthma lies in the *degree of reversibility* of airflow obstruction on spirometry after bronchodilator administration. ### Reversibility Testing: The Gold Standard Discriminator According to GOLD and ATS/ERS guidelines, reversibility is assessed as: - **COPD:** Improvement in FEV₁ **<12% and <200 mL** after short-acting beta-2 agonist (SABA) - **Asthma:** Improvement in FEV₁ **≥12% and ≥200 mL** after SABA This single test separates the two diseases more reliably than any clinical feature. ### Why Reversibility Matters | Feature | COPD | Asthma | |---------|------|--------| | **Reversibility (post-bronchodilator)** | <12% and <200 mL | ≥12% and ≥200 mL | | **Pathology** | Fixed airway remodeling, emphysema | Smooth muscle hyperresponsiveness | | **Inflammation** | Neutrophilic | Eosinophilic | | **Response to steroids** | Modest | Excellent | ### Clinical Pearl **Clinical Pearl:** A patient with COPD may show *some* bronchodilator response (e.g., 8% improvement), but it falls short of the ≥12% + ≥200 mL threshold that defines asthma. This is why reversibility testing is **mandatory** in all patients with chronic airflow obstruction. ### High-Yield Mnemonic **Mnemonic:** **"COPD ≤12-200, Asthma ≥12-200"** — Remember the thresholds as the diagnostic cutoff. [cite:GOLD 2023 Strategy Document, Harrison 21e Ch 258] 
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