Is a single blood eosinophil count a reliable marker for "eosinophilic asthma?"

"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.

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

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

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.

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.