Exhaled breath condensate (EBC) collection and exhaled nitric oxide measurement
EBC was collected by Turbo-Deccs system (Medivac, Parma, Italy). Approximately 1 ml of EBC was collected and immediately stored at -80℃ for further analysis. Following EBC collection (after 30 minutes of rest), fractional exhaled nitric oxide (FeNO) was measured according to published standards by using NIOX MINO Airway Inflammation Monitor (Aerocrine, Solna, Sweden).