Study population
The EBC was obtained from 58 healthy controls and 251 patients with asthma at the Asan Medical Center between September 2014 and December 2019. Healthy controls were screened for respiratory diseases and recruited during regular wellness examinations conducted at Asan Medical Center if they did not have any current or previous history of pulmonary disease, as determined by the medical examination. The eligibility criteria for patients with asthma were as follows11,12: (1) patients aged >18 years; (2) symptoms such as dyspnea, wheezing, or cough for more than 3 months; and (3) airway hyperresponsiveness as indicated by a 20% reduction in forced expiratory volume in 1 s (FEV1) with methacholine 16 mg/mL (PC20) on a provocation test, or airway reversibility with FEV1 >12% (and at least 200 mL) after inhalation of a short-acting beta-agonist. Patients with severe lung damage, bronchiectasis, or a history of lung resection were excluded. The participants’ EBC samples were collected using the RTubeTM (Respiratory Research, Austin, USA). Informed consent was obtained from all participants, and the study protocol was approved by the Institutional Review Board of Asan Medical Center (IRB No. 2006-0388).