Is Fractional exhaled nitric oxide (FeNO) test reliable in the differentiation of chronic obstructive pulmonary disease (COPD) and ACOS (asthma-COPD overlap syndrome)?

Introduction: ACOS shares clinical and spirometric features of asthma and COPD. Several ACOS definitions exist. FeNO is a marker of eosinophilic airway inflammation commonly used to diagnose and monitor allergic asthma. Recently, it was proposed to set apart COPD and ACOS. Aim of the study: Evaluate FeNO contribution to distinguish COPD from ACOS. Methods: This study included spirometry files with forced expiratory volume in one second (FEV 1 ) over forced vital capacity (FVC) after bronchodilator 1 and/or FVC. Subjects were interviewed and underwent FeNO measurement. Among the population, a group underwent a blood test for eosinophils counting. Results: Thirty-two patients were enrolled (28 males). Mean age was 57.93 [36-79]. All subjects were active or past cigarette smokers or exposed to other toxics or wood smoke. Twenty subjects were suspected to have asthma, twelve to have COPD. Only 4 subjects had an increased FeNO (10.93 parts per billion [1 – 48]). Thus, the other 28 subjects were classified as COPD. There was a negative correlation between FeNO level and smoking (p=0.001). There was a positive correlation between FeNO and FEV 1 improvement after bronchodilator in percentage (21.31% [0-52]) (p=0.001) and in milliliter (345.31 ml [0-880]) (p=0.0003). Eosinophils were counted in 17 cases (219.79/mm3 [8-616]) and were positively correlated to FeNO (p=0.032). Conclusion: Reversibility in FEV 1 and/or FVC associated to FeNO and eosinophils counting seem to be more reliable to differenciate COPD and ACOS. More studies are needed to help for diagnosis in daily practice.