Exhaled nitric oxide in patients with chronic obstructive pulmonary disease: a systematic review and meta-analysis.

Fractional exhaled nitric oxide (FENO) is a useful and noninvasive biomarker for eosinophilic airway inflammation, particularly in asthma. However, its utility in chronic obstructive pulmonary disease (COPD) remains controversial. In this study, we performed a systematic review and meta-analysis to evaluate FENO levels in COPD.