Spirometry measures lung volumes and airflow to diagnose and classify respiratory disease. Key measurements: FEV1 (forced expiratory volume in 1 second — the volume of air exhaled in the first second of a forced expiration after maximal inspiration), FVC (forced vital capacity — the total volume of air exhaled during a complete forced expiration), and the FEV1/FVC ratio. Normal FEV1/FVC ratio is ≥0.70 (or above the lower limit of normal [LLN] adjusted for age, sex, height, and race using GLI-2012 reference equations). Obstructive pattern: FEV1/FVC <0.70 (or <LLN); air trapping due to airway narrowing; FEV1 is disproportionately reduced compared to FVC. Causes: asthma, COPD, bronchiectasis. Classified by FEV1 % predicted: GOLD criteria for COPD: GOLD 1 (mild) ≥80%; GOLD 2 (moderate) 50-79%; GOLD 3 (severe) 30-49%; GOLD 4 (very severe) <30%. Restrictive pattern: FEV1/FVC is normal or increased (≥0.70); both FEV1 and FVC are proportionally reduced; lung volumes are decreased. Requires full pulmonary function testing (PFT) with lung volumes to confirm: TLC (total lung capacity) <80% predicted. Causes: interstitial lung disease, chest wall abnormalities, neuromuscular disease, obesity. Mixed pattern: features...