Time of Day Affects Eosinophil Biomarkers in Asthma: Implications for Diagnosis and Treatment

Asthma is characterised by strong time of day rhythms; symptoms worsen around 04:00 (1), coincident with increased airway narrowing, reflected by a reduced peak expiratory flow (PEF) or Forced Expiratory Volume in 1 second (FEV1) (2). Clinically useful biomarkers in asthma include sputum and blood eosinophils (3) and results from several small studies looking at circadian variation in airway inflammation in asthma are conflicting (4-9). Sputum eosinophil percentage is used to guide management decisions in severe asthma clinics (3); any diurnal variation in sputum eosinophilia could therefore influence the management of patients. We studied circadian variation of blood and sputum eosinophils within a mild/moderate, atopic asthma cohort compared to healthy controls. We then retrospectively compared sputum eosinophil counts from a severe asthma clinic cohort in relation to time of day of collection (morning versus afternoon).