Cutoff point of the fraction of exhaled nitric oxide (FeNO) with the off-line method for diagnosing asthma and the effect of smoking on FeNO

Evidences have shown that the fraction of exhaled nitric oxide (FeNO) is a useful marker of eosinophilic airway inflammation in asthmatics. There is no study to show the FeNO cutoff point for detecting asthma and the influence of smoking, measured by the Sievers Bag collection kit. The study population comprised 60 steroid-naive asthmatic patients (BA) (32 non-smokers, 28 smokers) and 59 patients with chronic cough (CC) without asthma (42 non-smokers, 17 smokers) in our outpatient clinic. We measured FeNO levels, spirometory, bronchial hyperresponsiveness against acetylcholine, and other parameters. The levels of FeNO were significantly increased in asthmatics compared with subjects with chronic cough. According to the ROC curve, the cutoff point of FeNO was 30 ppb (AUC = 0.83, sensitivity 78.1%, specificity 73.5%, p < 0.001) in non-smokers. The levels of FeNO in smokers were not significantly different from those in non-smokers, both bronchial asthma and chronic cough subjects. But the cutoff point of FeNO was 40 ppb (AUC = 0.65, sensitivity 67.8%, specificity 70.6%, p = 0.012). In conclusion, the cutoff point of FeNO was 30 ppb in non-smokers and 40 ppb in smokers. In smokers, FeNO measurement was less useful.