Variability of blood eosinophil counts with food, exercise, medications and the time of testing has the potential to affect patients’ eligibility for targeted therapy.

Writing in Respirology, Professor Peter Gibson said ‘the most clinically accessible biomarker to aid in the management of asthma and COPD’ could potentially result in unexpected false negatives.

He said ‘…while blood eosinophil counts are stable over time in a group of people in a clinical research setting, there are sources of individual variability that might alter a result on a given day and this could determine whether or not a person meets the criteria for treatment eligibility.’

A single blood eosinophil count ≥ 300 cells/μL was the PBS Authority threshold for mepolizumab.

Professor Gibson, from the Centre of Excellence in Severe Asthma and the Priority Research Centre for Healthy Lungs at the University of Newcastle, told the limbic there was concern about a lack of awareness of test variability.

“My main concern is that there is considerable intra-subject variability with the peripheral blood eosinophil test, and this is not appreciated when trying to interpret the results.”

He said in the Commentary that a light meal could reduce eosinophils by about 23% within four hours; an exercise test in COPD patients could reduce eosinophils within two hours.

“There is significant diurnal variability of blood eosinophil counts, with peak levels recorded around midnight, and lowest levels at midday.”

A study in healthy individuals had shown within-subject biological variation in hourly eosinophil counts of 20.9%.

“There have not been any studies to look at whether this might be an issue in asthma, resulting in false positive or negatives,” Professor Gibson said.

“This is significant because the amplitude of oscillation for blood eosinophil counts is clinically significant and current threshold levels are not time-specific, meaning that samples collected later in the day may falsely classify a person as non-eosinophilic.”

Oral corticosteroids also significantly reduced eosinophil counts for more than 24 hours and inhaled corticosteroids to a lesser extent.