Van Rossem I, Vandevoorde J, Hanon S, et al. - In view of the recent inclusion of blood eosinophil counts (BEC) in the 2019 Global Initiative for Obstructive Lung Disease (GOLD) guideline as an easily accessible theragnostic biomarker for Chronic Obstructive Pulmonary Disease (COPD), this retrospective analysis is done to characterise the stability of blood eosinophils over time. The percentage of patients with BEC persistently below or above the 2019 GOLD guideline thresholds (100 and 300 cells/μL) was documented. To ascertain the intra-patient variability, the mean, standard deviation (SD) as well as relative standard deviation (RSD) of the BEC were computed for each patient. This study involved 98 patients, with 1,082 eosinophil measurements; BEC ranged between 0 and 1,504 cells/μL. BEC persistently below 100 cells/μL were detected in four (4.1%) patients, and measurements persistently above this threshold were obtained from 34 (34.7%). The mean intra-patient RSD was estimated to be 0.46. Overall, variability in BEC can be seen in individual COPD patients, and thus, applying a single measurement for guiding treatment decisions remains questionable.