Correlation between fractional exhaled nitric oxide and sputum eosinophilia in exacerbations of COPD

Measurements of eosinophils in induced sputum and fractional exhaled nitric oxide (FeNO) are noninvasive biomarkers for assessing airway inflammation phenotypes in chronic obstructive pulmonary disease (COPD). Nevertheless, the clinical application of the correlation between FeNO levels and sputum eosinophilia is controversial. The study aimed to investigate the correlation and predictive relationship between FeNO levels and sputum eosinophils in patients with COPD exacerbation. It also examined the relationship between FeNO levels and blood eosinophil percentage.

A total of 163 patients with COPD exacerbation were included in the cross-sectional study. All patients underwent the following on the same day: FeNO test, spirometry, bronchodilator reversibility test, induced sputum, and routine blood test. They were classified as eosinophilic group or noneosinophilic group based on sputum eosinophilic percentage (≥2.5%)/FeNO levels (≥32 parts per billion [ppb]).

FeNO levels and blood eosinophilic percentage were higher in patients with sputum eosinophilia (n=62) compared to those without (31.35 ppb versus 21.43 ppb, P=0.015; 2.71% versus 0.98%, P<0.0001, respectively). Sputum eosinophilic percentage was higher with raised FeNO (n=34) compared to those with FeNO <32 ppb (5.12% versus 3.12%, P=0.007). Eosinophils in induced sputum correlated with both FeNO levels (ρ=0.221, P=0.005) and blood eosinophilic percentage (ρ=0.399, P<0.001). There was no relationship between FeNO and blood eosinophilic percentage. Blood eosinophilic percentage was predictive of sputum eosinophilia (95% confidence interval [CI] =0.65-0.81, P<0.001) at a cutoff point of 0.65% (sensitivity =73%, specificity =61.3%). FeNO levels were predictive of sputum eosinophilia (95% CI =0.53-3,071, P=0.012) at a cutoff point of 17.5 ppb (sensitivity =65.1%, specificity =56.4%).

The clinical relevance of this study provides evidence that inflammatory biomarkers, including sputum eosinophilic percentage, FeNO level, and blood eosinophilic percentage, can be used to positively diagnose eosinophilic COPD. The FeNO level and blood eosinophilic counts/percentage, which determine an optimal cutoff for sputum eosinophilia, need more studies.