Lung function tests and methacholine challenge tests
Following FeNO measurement, lung function tests were performed with the spirometer-Lungest 1000 (MES, Krakow, Poland) according to ERS/ATS14 standards. These children had not received any anti-asthmatic medication, including oral β-2 agonists, theophylline, steroids, or antihistamines, for at least two days.