Investigating Fractional Exhaled Nitric Oxide in Chronic Obstructive Pulmonary Disease (COPD) and Asthma-COPD Overlap (ACO): A Scoping Review.

Chronic obstructive pulmonary disease (COPD) is the most common fixed airflow limitation. Individuals may present with the features of both asthma and COPD called asthma-COPD overlap (ACO) with more severity and worse health-related quality of life than COPD or asthma. One of the promising biomarkers that could be used in clinical practice to differentiate ACO from COPD is fractional exhaled nitric oxide (FENO). The role of Fractional exhaled nitric oxide (FENO) in COPD/ACO remains unknown. This scoping review aims to investigate the role of FENO measurement to differentiate COPD from ACO, to anticipate disease severity/progression and treatment response. A structured comprehensive literature search was performed in major databases including Medline, EMBASE, CINAHL, Cochrane Library, Web of Science, and BIOSIS from 2005 onwards. Thirty-eight studies were retrieved. Based on the synthesis of the reviewed literature, six themes emerged. Thirty-four articles covered more than one theme. From which, 24 articles were on modifying factors in FENO measurement, 18 on FENO in COPD compared with healthy subjects, and seven on FENO in ACO compared with COPD, 22 on FENO and disease severity/progression,12 on FENO and biomarkers, and eight on FENO and treatment response. FENO measurement cannot be used alone in the clinical settings of COPD patients. Although FENO level is higher in ACO patients than COPD-only, it is still unclear if there is a FENO cut-off that can be used to make the diagnosis of ACO and/or to guide therapy with inhaled corticosteroids/glucocorticoids in COPD patients.