Fractional Exhaled Nitric Oxide in Eosinophilic COPD.

Eosinophilic chronic obstructive pulmonary disease (COPD) is a distinct subtype with clinical and biological differences from noneosinophilic COPD and asthma. Fractional exhaled nitric oxide (FeNO) is an established marker of type 2 airway inflammation in asthma, but its utility in eosinophilic COPD is less well understood. We analyzed baseline data from 176 participants in the Air Purification for Eosinophilic COPD Study, a randomized controlled trial of former smokers with eosinophilic COPD. At enrollment, FeNO and blood eosinophil counts were measured, and participants reported asthma history and severe COPD exacerbations requiring hospitalization in the prior year. Each 50cells/μL higher eosinophil count was associated with a 3.2% increase in FeNO (95% confidence interval [CI]: 0.3-6.1%). Asthma history was associated with a 29.1% higher FeNO (95% CI: 5.2-58.5%). Elevated FeNO, defined as ≥25 or ≥50 parts per billion, was linked to greater odds of an asthma diagnosis and a recent severe exacerbation, although CIs included the null. These findings suggest that FeNO may serve as a practical, noninvasive biomarker of type 2 inflammation in eosinophilic COPD and could help identify patients at higher risk of severe exacerbation.