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Background: The relationship between lung function decline and eosinophils and neutrophils has important therapeutic implications among asthmatics, but it has rarely been studied in large cohort studies.

Objective: The aim is to study the relationship between blood eosinophils and neutrophils and FEV₁ decline in a long-term follow-up of a population-based adult asthma cohort.

Methods: In 2012-2014, an adult asthma cohort was invited to a follow-up including spirometry, blood sampling, and structured interviews, and n = 892 participated (55% women, mean age 59 y, 32-92 y). Blood eosinophils, neutrophils and FEV ₁ decline were analyzed both as continuous variables and divided into categories with different cut-offs. Regression models adjusted for smoking, exposure to vapors, gas, dust, or fumes (VGDF), use of inhaled and oral corticosteroids, and other possible confounders were utilized to analyze the relationship between eosinophils and neutrophils at follow-up and FEV₁ decline.

Results: The mean follow-up time was 18 years, and the mean FEV ₁ decline was 27 ml/year. The annual FEV₁ decline was related to higher levels of both blood eosinophils and neutrophils at follow-up, but only the association with eosinophils remained when adjusted for confounders. Further, the association between FEV₁ decline and eosinophils was stronger among those using ICS. With EOS <0.3 × 10⁹/L as reference, a more rapid decline in FEV1 was independently related to EOS ≥0.4 × 10⁹/L in adjusted analyses.

Conclusions and clinical relevance: Besides emphasizing the importance of smoking cessation and reduction of other harmful exposures, our real-world results indicate that there is an independent relationship between blood eosinophils and FEV₁ decline among adults with asthma.