Validity of fractional exhaled nitric oxide and small airway function indices in diagnosis of cough-variant asthma

Objective: Fractional exhaled nitric oxide (FeNO) is a non-invasive method used to reveal the eosinophilic inflammation of airway and to diagnose and manage asthma. The use of FeNO measurement or its combination with the small airway function in differentiating CVA patients from chronic cough patients is unexplored. Methods: From December 2012 to June 2016, patients with chronic non-productive cough for more than 8 weeks based on physician's opinion were subjected to FeNO, pulmonary function, and bronchial hyperresponsiveness (BHR) tests. Receiver operating characteristic (ROC) curves were obtained to evaluate the clinical value of FeNO and small airways indices in CVA diagnosis. The optimal cutoff point for the level of FeNO was also determined. Results: The FeNO value in the CVA group was significantly higher than that in the non-cough-variant asthma (NCVA) group [42(41) parts per billion (ppb) vs. (16(11) ppb), P < 0.05]. The area under the ROC curve was estimated to be 0.874 for FeNO. An optimal cutoff point of 25 ppb had respective sensitivity and specificity values of 81.3% and 84.0% in differentiating CVA patients from NCVA patients. The CVA group showed small airway dysfunction, and the small airway resistance was significantly higher than in the NCVA group. The area under the ROC curve of FeNO combined with maximal midexpiratory flow (MEF) (% predicted) was 0.883, and that combined with MEF50 (% predicted) was 0.886. Conclusions: The measurement of FeNO is a non-invasive, reproducible, and sensitive method of differentiating CVA patients from NCVA patients. A combination of the level of FeNO (25 ppb) and the abnormal small airway function suggested higher CVA possibility, thereby resulting in a rapid diagnosis. Unnecessary treatments are avoided. This finding provides a new perspective for the management of patients with CVA.