Within-day variability of blood eosinophils in stable COPD patients: a prospective trial

Introduction: Blood eosinophil counts (BEC) can be used as a biomarker for guiding maintenance therapy with inhaled corticosteroid (ICS) in COPD patients. Aims and objectives: To evaluate the within-day variability of blood eosinophils in stable COPD patients with respect to the BEC GOLD thresholds (100 and 300 cells/µL). Methods: Patients were recruited at the outpatient clinic of a University Hospital in Brussels. Patients with asthma, active malignancy, recent use of systemic corticosteroids or an exacerbation <4 weeks ago were excluded. For each patient, BEC was determined at three time points of the same day (08h00,12h00,16h00). Results: Fifty COPD patients were included (32 male, 23 current smokers, mean age 68.7±6.7(SD) years). The proportion of patients crossing at least one threshold throughout the day was 50% (34% crossed 100 cells/µL, 18% crossed 300 cells/µL, 2% both). The median within-day within-patient coefficient of variation of BEC was 28% (IQR: 17-38%). There was a highly significant diurnal effect on BEC (Friedman; p<0.001), with highest mean value at 8h00 (263±142(SD) cells/µL), lowest at 12h00 (171±119(SD) cells/µL) and intermediate at 16h00 (198±120(SD) cells/µL). All pairwise comparisons between the time points showed significant BEC differences. Limiting the analysis to the subgroups of patients with at least one exacerbation in the previous year (n=24) or patients not under ICS therapy (n=21), did not alter the outcomes. Conclusions: In up to half of the COPD patients in this study, BEC crossed at least one GOLD threshold throughout the day. This within-day variability of blood eosinophils could cause therapeutic decisions to differ, depending on the time of day of blood sampling.