BACKGROUND: Blood eosinophil count is a marker of eosinophilic airway inflammation and disease severity in asthma. However, blood neutrophil count might also be associated with disease severity. We tested the hypothesis that high blood eosinophil and neutrophil counts are both associated with the risk of asthma exacerbations among individuals with asthma from the general population.

METHODS: From the Copenhagen General Population Study with 81351 participants, we included 4838 with self-reported asthma. We recorded baseline blood eosinophil and neutrophil counts, and asthma exacerbations during follow-up in 2003-2011, defined as moderate (short-course treatment of prednisolone) or severe (hospitalization).

RESULTS: The multivariable-adjusted incidence rate ratios (IRRs) were 1.28 (95% CI, 1.06-1.55) for moderate exacerbations and 1.55 (1.20-2.00) for severe exacerbations for individuals with blood eosinophil counts >0.29 × 10(9)/L (highest tertile) vs individuals with blood eosinophil counts <0.18 × 10(9)/L (lowest tertile). For blood neutrophils, the multivariable-adjusted IRRs were 2.14 (1.74-2.63) for moderate exacerbations and 1.18 (0.89-1.55) for severe exacerbations for individuals with blood neutrophil counts >4.85 × 10(9)/L (highest tertile) vs individuals with blood neutrophil counts <3.77 × 10(9)/L (lowest tertile). Blood eosinophil and neutrophil counts interacted on moderate exacerbations (P = 3 × 10(-4)), but not on severe exacerbations.

CONCLUSIONS: High blood eosinophil counts are associated with an increased risk of both moderate and severe asthma exacerbations, while high blood neutrophil counts are associated with an increased risk of moderate, but not severe exacerbations.