The aim of this study was to examine the association between blood eosinophil levels and the decline in lung function in individuals aged >40 years from the general population.

The study evaluated the eosinophil counts from thawed blood in 1120 participants (mean age 65 years) from the prospective population-based Canadian Cohort of Obstructive Lung Disease (CanCOLD) study. Participants answered interviewer-administered respiratory questionnaires and performed pre-/post-bronchodilator spirometric tests at 18-month intervals; computed tomography (CT) imaging was performed at baseline. Statistical analyses to describe the relationship between eosinophil levels and decline in forced expiratory volume in 1 s (FEV₁) were performed using random mixed-effects regression models with adjustments for demographics, smoking, baseline FEV₁, ever-asthma and history of exacerbations in the previous 12 months. CT measurements were compared between eosinophil subgroups using ANOVA.

Participants who had a peripheral eosinophil count of =300 cells·µL^-1 (n=273) had a greater decline in FEV₁ compared with those with eosinophil counts of <150 cells·µL^-1 (n=430; p=0.003) (reference group) and 150–<300 cells·µL^-1 (n=417; p=0.003). The absolute change in FEV₁ was -32.99 mL·year^-1 for participants with eosinophil counts <150 cells·µL^-1; -38.78 mL·year^-1 for those with 150–<300 cells·µL^-1 and -67.30 mL·year^-1 for participants with =300 cells·µL^-1. In COPD, higher eosinophil count was associated with quantitative CT measurements reflecting both small and large airway abnormalities.

A blood eosinophil count of =300 cells·µL^-1 is an independent risk factor for accelerated lung function decline in older adults and is related to undetected structural airway abnormalities.