“…As a signicant biological signaling molecule, 1,2 fractional exhaled nitric oxide (FeNO, below 100 ppb) reects the eosinophilic inammation of the airways, which could evaluate respiratory diseases, such as asthma and chronic cough, and predict the treatment responses. [3][4][5] Moreover, some studies have shown that FeNO level is a predictor of COVID-19 disease severity. [6][7][8][9] Obviously, the detection of ppb-level nitric oxide (NO) has its unique credit in the clinical diagnosis of respiratory diseases.…”