Stability of sputum eosinophil cell counts in COPD

<b>Background:</b> Blood eosinophil counts in COPD show greater stability over time at lower counts (&lt;100cells/µL). This has not been studied in sputum samples. We tested the hypothesis that low sputum eosinophil counts are more stable over time. <b>Methods:</b> Sputum differential cell counts (DCC) during stable state were obtained from 94 COPD patients at baseline and 6 months. Patients were stratified based on baseline sputum eosinophil count into ‘low’, ‘intermediate’ and ‘high’ groups: eos^LOW (&lt;1%), eos^INT (1-3%) and eos^HIGH (&gt;3%). <b>Results:</b> The majority of eos^LOW remained in the same category after repeat sampling (67.6%), with less for eos^INT and eos^HIGH (25.7 and 59.1% respectively). Repeatability coefficient analysis (RCA) showed that 95% of repeat sputum eosinophil counts were within 2.07%, 21.79% and 14.84% at repeat visits for eos^LOW, eos^INT and eos^HIGH groups respectively. The mean within subject difference eos^LOW group was lower compared to other groups although this difference was not statistically significant (p=0.065 and 0.061 for eos^LOW vs eos^INT and eos^HIGH respectively). <b>Table 1:</b> Number of patients per sputum eosinophil group at 0 and 6 months <b>Conclusion:</b> These data highlight a COPD subgroup with low sputum eosinophil counts which are highly stable over time. This subgroup contrasts with those with higher sputum eosinophil counts, which are more temporally dynamic.