A 38-year-old female presents with exertional dyspnea and dry cough for 3 months. Spirometry reveals FEV₁/FVC ratio of 0.82 with FEV₁ at 88% predicted and FVC at 72% predicted. Which investigation is most appropriate to establish the diagnosis?

Explanation

## Pattern Recognition and Diagnosis The spirometry shows a **restrictive pattern**: - FEV₁/FVC ratio **normal or elevated** (0.82, normal range 0.70–0.85) - **Reduced FVC** (72% predicted) - **Reduced FEV₁** (88% predicted, but proportionally less reduced than FVC) This pattern indicates **restrictive lung disease**, not obstructive disease. ### Why Lung Volumes and DLCO? **Key Point:** The next investigation of choice is **measurement of lung volumes (TLC, RV, VC) and DLCO** because: 1. **Confirms restrictive pattern**: TLC <80% predicted confirms restriction (spirometry alone cannot definitively exclude mild restriction) 2. **Identifies mechanism**: - Low TLC + low DLCO → parenchymal disease (ILD, sarcoidosis, pulmonary fibrosis) - Low TLC + normal DLCO → chest wall/neuromuscular disease 3. **Guides further imaging**: Abnormal DLCO prompts HRCT; normal DLCO may suggest extrapulmonary cause ### Restrictive vs. Obstructive Pattern Differentiation | Parameter | Obstructive | Restrictive | |-----------|-------------|-------------| | **FEV₁/FVC** | <0.70 | ≥0.70 | | **FEV₁** | ↓↓ | ↓ | | **FVC** | Normal or ↓ | ↓↓ | | **TLC** | Normal or ↑ | ↓ | | **RV** | ↑↑ | Normal or ↓ | | **Next test** | DLCO | **Lung volumes + DLCO** | **High-Yield:** Spirometry **cannot diagnose restriction** — it requires **lung volume measurement** (body plethysmography or helium dilution) to confirm TLC <80% predicted. **Clinical Pearl:** The combination of low TLC + low DLCO in a young woman with exertional dyspnea and cough strongly suggests **interstitial lung disease** (sarcoidosis, hypersensitivity pneumonitis, early pulmonary fibrosis), which would then be confirmed by HRCT. [cite:Harrison 21e Ch 246]