Is a Single Blood Eosinophil Count a Reliable Marker for "eosinophilic Asthma?"

Introduction: "Eosinophilic asthma" refers to an asthma phenotype characterized by predominance of eosinophils in the bronchial airways and corticosteroid responsiveness. Recent clinical trials of eosinophil-blocking agents have utilized a blood eosinophil count of 300 or 400 eosinophils/mm(3) or higher to identify subjects with moderate to severe asthma. We observed multiple instances of counts which varied widely in the same patient within the same day.

Objectives: To determine whether there is significant variability in blood eosinophil counts taken throughout the day in the same patients with moderate asthma.

Methods: Twelve subjects had serial blood eosinophil counts obtained within a 24-hour period.

Results: Twelve subjects were enrolled: seven subjects had moderate asthma, three subjects had mild asthma, and two control subjects had no asthma. The variability of blood eosinophil counts ranged from 17% to 396%. No specific diurnal pattern was found among the subjects. The highest variability were seen in three moderate asthmatics (396%, 170%, and 154%) and one mild asthmatic (164%) while the other subjects had variability of 84% or less.

Conclusions: This study showed significant variability in blood eosinophil counts within a 24-hour period in the same subjects. The highest variability was seen in moderate asthmatics. These findings would appear to place the utility of a single eosinophil count in question.