Fractional exhaled nitric oxide could identify early spirometry change in clinically suspected asthma patients without airway obstruction

Abstract

Fractional exhaled nitric oxide (FeNO) has been proposed as a non-invasive biomarker for allergic inflammation seen in asthma. The aim of this study was to assess the ability of FeNO to discriminate spirometry and lung volume measurements between those with and without airway obstruction among subjects with clinically suspected asthma. A retrospective study was conducted. Diagnostic evaluations including spirometry and FeNO testing (NO electrochemical equipment: NIOX VERO; Aerocrine AB, Solna, Sweden) were performed in all subjects. Airway obstruction was defined according to the Standardization of Spirometry of the American Thoracic Society (ATS)/European Respiratory Society (ERS), and 2014 recommendations of the Chinese National Guidelines of Pulmonary Function Test. It was used the Student t test for analysis of continuous variables and the χ2 test for analysis of discrete variables including FeNO levels and lung function metrics. Of the 138 subjects with clinically suspected asthma, airway obstruction was found in 61. There was no significant difference in the mean FeNO levels among subjects with or without airway obstruction ( p = 0.241) among un-selected subjects. Likewise, there was no difference in the FeNO levels between aged (>50 years) and younger subjects (⩽50 years) ( p = 0.804). A significant proportion of subjects had a normal FeNO level (<25 part per billion, ppb) in spite of having airway obstruction (39/138), 25 had an elevated FeNO level (⩾25 ppb) in spite of having no airway obstruction (25/138). Additionally, the airway-obstructed subjects with increased FeNO level had comparable spirometry to those with normal FeNO level ( p > 0.05). However, among subjects without airway obstruction, the forced expiratory volume in 1 s (FEV1)/predicted (pred), maximal expiratory flow at 25% of forced vital capacity (FVC) (MEF25%)/pred, maximal expiratory flow at 50% of FVC (MEF50%)/pred and maximum mid-expiratory flow (MMEF)/pred were significantly lower in the FeNO ⩾ 25 ppb group compared to those in the FeNO < 25 ppb group. These analyses indicated that increased FeNO levels could help to determinate early spirometry change within clinically suspected asthma subjects without airway obstruction. It is highlighted the importance of FeNO as a phenotype associated with an increased risk of airway obstruction in some subjects in this study.