Clinical characteristics of eosinophilic and noneosinophilic exacerbations of COPD: relationship with sputum eosinophils and FENO

<b>Background:</b> Blood eosinophil count is a marker of steroid responsiveness in stable chronic obstructive pulmonary disease (COPD). The aim of this study was to assess the relationship between blood eosinophils and clinical outcomes in patients experiencing a severe acute exacerbation of COPD (AECOPD). <b>Methods:</b> Clinical parameters were measured in AECOPD patients (n=183), first at the time of hospital admission, and again at discharge following treatment. Patients were divided into eosinophilic (n=38) and noneosinophilic (n=145) groups based on blood eosinophil count results on admission. In a subset of patients (n=84) eosinophils in the sputum were also determined. Data are presented as mean±SEM or median with IQR. <b>Results:</b> Eosinophilic AECOPD patients (≥2% and/or ≥200 cells/µL) had higher fractional exhaled nitric oxide (FENO) (15.7 [5.4-27.8] vs. 7.6 [5.0-17.1] ppb, p&lt;0.05), but lower C-reactive protein (22.4±6.1 vs. 54.6±7.7 mg/L, p&lt;0.05) levels compared to noneosinophilic subjects. Moreover, the magnitude of the increase in forced expiratory volume in one second (ΔFEV₁) after treatment was greater in this group (11.0 [5.0-15.1] vs. 7.2 [2.0-13.5] %, p&lt;0.05). Although blood eosinophils were related with sputum eosinophils (r=0.36, p&lt;0.005), the predictive accuracy of blood eosinophil count for sputum eosinophilia (≥2%) was only modest (ROC AUC: 0.70, p&lt;0.05). FENO was a better predictor of airway eosinophilia (ROC AUC: 0.80, p&lt;0.005). <b>Conclusions:</b> Eosinophilic AECOPD patients have lower systemic inflammatory marker levels and more improvements in airflow limitation posttreatment. The surrogate markers to detect airway eosinophilia are not of equal value.