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.
Diagnosis & workup
Diagnostics & workup: - Pre and post-bronchodilator spirometry (FEV1 reversibility >= 12% and >= 200 mL) - Methacholine challenge test if spirometry normal (PC20 < 4 mg/mL diagnostic) - Peak expiratory flow diary over 2 weeks (variability > 10%) - FeNO measurement (> 50 ppb steroid-naive; > 25 ppb in treated patients suggests ongoing eosinophilic inflammation) - Allergy testing: skin prick test or serum specific IgE panel - CBC with differential for eosinophil count (> 0.3 x 10^9/L supports diagnosis)