Pulmonary function tests (PFTs) measure lung volumes, airflow rates, and gas exchange capacity to characterize respiratory pathology. Spirometry, the most fundamental PFT, measures FVC (forced vital capacity — total air exhaled forcefully after full inspiration), FEV1 (volume exhaled in the first second), and the FEV1/FVC ratio. The FEV1/FVC ratio differentiates obstructive from restrictive patterns: a ratio < 0.70 (or below lower limit of normal) indicates obstruction (airflow limitation during exhalation — COPD, asthma), while a normal or elevated ratio with reduced FVC suggests restriction (reduced lung volume — pulmonary fibrosis, chest wall disease, neuromuscular weakness). Diffusing capacity (DLCO) measures the lung's ability to transfer gas across the alveolar-capillary membrane, reduced in emphysema (loss of alveolar surface area) and interstitial lung disease (thickened membrane) but preserved or increased in asthma.