Pathophysiology
Clinical meaning
Asthma diagnosis requires demonstration of variable expiratory airflow limitation. Spirometry showing bronchodilator reversibility (FEV1 increase >= 12% AND >= 200 mL after 200-400 mcg salbutamol) is the gold standard. If spirometry is normal, provocation testing with methacholine (PC20 < 4 mg/mL is diagnostic, 4-16 mg/mL is borderline) or exercise challenge (FEV1 decrease >= 10-15%) can confirm bronchial hyperresponsiveness. Peak flow variability > 10% in adults (> 13% in children) over 2 weeks supports the diagnosis. FeNO > 50 ppb in steroid-naive adults strongly supports eosinophilic asthma. The clinician must distinguish asthma from COPD, vocal cord dysfunction, and cardiac dyspnea using clinical history and objective testing. GINA guidelines emphasize that asthma should never be diagnosed on symptoms alone.