Identification of COPD Inflammatory Endotypes Using Repeated Sputum Eosinophil Counts

Higher blood and sputum eosinophil counts are associated with a greater response to corticosteroids in COPD. Low blood eosinophil counts exhibit greater stability over time whereas higher counts demonstrate more variability. Stability of airway eosinophil levels is less well understood. We have studied the stability of sputum eosinophil counts. Differential cell count data for COPD patients (n = 100) were analysed. Subjects with two sputum eosinophil counts, 6 months apart, were included in the analysis. Patients were stratified based on baseline sputum eosinophil count into ‘low’, ‘intermediate’ and ‘high’ groups: eosinophil(LOW) (<1%), eosinophil(INT) (1–3%) and eosinophil(HIGH) (≥3%). Sputum eosinophil counts showed good stability (rho = 0.61, p < 0.0001, ICC of 0.77), with 67.4% of eosinophil(LOW) patients remaining in the same category on repeat sampling. Bland–Altman analysis of the whole cohort (median difference between measurements = 0.00%, 90th percentile = −1.4 and 4.7%) showed greater variation at higher counts. This was confirmed by the wider 90th centiles in the eosinophil(INT) (−1.50 to 5.65) and eosinophil(HIGH) groups (−5.33 to 9.80) compared to the eosinophil(LOW) group (−0.40 to 1.40). The repeatability of sputum eosinophil counts was related to the baseline eosinophil count; sputum eosinophil(LOW) COPD patients were relatively stable over time, while the eosinophil(HIGH) group showed greater variability. These results can facilitate the identification of COPD endotypes with differential responses to treatment.