A 52-year-old man with a 30 pack-year smoking history presents with progressive dyspnea. Spirometry shows FEV₁/FVC ratio of 0.68 (68%), FEV₁ 55% predicted, and a normal TLC. Which single spirometric finding best distinguishes his obstructive airway disease from a restrictive pattern?

## Distinguishing Obstructive from Restrictive Patterns **Key Point:** The hallmark difference between obstructive and restrictive spirometry lies in TLC and the FEV₁/FVC ratio. ### Spirometric Patterns Comparison | Feature | Obstructive | Restrictive | |---------|------------|-------------| | FEV₁/FVC ratio | **Reduced (<0.70)** | **Normal or elevated (>0.80)** | | TLC | **Normal or increased** | **Reduced (<80% predicted)** | | FVC | Normal or reduced | **Reduced** | | FEV₁ | Reduced | Reduced | | RV/TLC ratio | Elevated (>35–40%) | Normal (<35%) | **High-Yield:** In obstructive disease (COPD, asthma, bronchiectasis), air trapping causes TLC to remain **normal or high** because the patient cannot empty the lungs fully. In restrictive disease (pulmonary fibrosis, chest wall disease, neuromuscular weakness), the lungs themselves are stiff or small, so TLC is **reduced**. ### Why TLC is the Discriminator 1. Both patterns show reduced FEV₁ and reduced FEV₁/FVC ratio — but obstructive disease has a **disproportionately reduced FEV₁** relative to FVC due to airway collapse during expiration. 2. A **normal or elevated TLC** in the presence of a low FEV₁/FVC ratio is pathognomonic for obstruction (air trapping). 3. Restrictive disease reduces all lung volumes proportionally, including TLC. **Clinical Pearl:** If you see FEV₁/FVC <0.70 AND TLC >80% predicted, you are dealing with obstruction. If FEV₁/FVC <0.70 AND TLC <80% predicted, consider a mixed pattern or measure TLC to confirm. **Mnemonic:** **OAT** — **O**bstructive → **A**ir trapping → **T**LC elevated.