LAUSR

After a lifetime of being seen but not noticed, the blood eosinophil count has now taken centre stage as the most clinically accessible biomarker to aid in the management of asthma and COPD. Phenotyping has identified options for targeted therapy, and an increasing number of studies describe the role of blood eosinophils in diagnosing the eosinophilic phenotype, prognosticating future exacerbation risk, and predicting treatment response to corticosteroids and biological treatments that are directed against eosinophilic pathways. However, the pace is moving so fast that it is easy to forget about the basic measurement properties of a test in a world of ever increasing favourable associations. So what are the measurement issues around the blood eosinophil count? These relate to analytical variation, within-subject biological variation, and between-subject biological variation. When making a clinical decision for an individual patient, it is the first two that are important. A range of methods are available for measuring blood eosinophils. At present, the gold standard is manually counting 400 cells from a blood smear; however, in practice this has been largely replaced by automated fluorescence flow cytometric methods. There is generally good agreement between these measurement techniques, and provided the laboratory uses an effective quality control process, the laboratory results can be considered a precise measure of the blood eosinophil count at the time when the sample was collected. It is within-subject biological variability that is most relevant for managing airway disease in a clinical setting. We measure a blood eosinophil count looking for a level above a certain threshold, usually 300 or 400 cells/μL, which indicates a likely response to targeted therapy. A single blood eosinophil measurement has been found to be sufficient to identify responders to mepolizumab in clinical trial populations. Stability over time of blood eosinophil counts has been assessed in COPD and asthma. In COPD, there was good agreement between two measurements of blood eosinophil count over a median of 28 days (Ri, intraclass correlation coefficient = 0.8; 95% CI: 0.66–0.88; P < 0.000). Similarly, there was an acceptable stability of blood eosinophils over 1 year in COPD (Ri, 0.64), which improved with exclusion of patients using oral corticosteroids and antibiotics (Ri, 0.70). However, these are average or group results, and in clinical practice we manage individual patients according to their individual test result. This means that 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. There are a number of factors that contribute to within-subject variability of blood eosinophil measurements. The crucial ones to consider are those that lower the count and could lead to a false-negative test for diagnosis of the eosinophilic phenotype. These include eating, exercise, medications, and the time of testing. Consuming a light meal was found to reduce blood eosinophils between 2 and 4 h after eating, with an average 23% reduction at 4 h (from 130 to 100 cells/μL). Exercise also reduces blood eosinophil counts. For example, a symptom-limited exercise test in COPD patients reduced blood eosinophils at 2 h, with normalization by 24 h. There is significant diurnal variability of blood eosinophil counts, with peak levels recorded around midnight, and lowest levels at midday. A study of within-subject biological variation of hourly blood eosinophil counts in healthy individuals found 20.9% variation in blood eosinophil counts. Betweensubject diurnal variation was greater at 46.6%. In another study of healthy males, 21% had blood eosinophils >300 cells/μL at 1 am, whereas only 7% had a level above this threshold at 12 pm. 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. These studies mainly involved healthy volunteers, and the changes in blood eosinophil count were small. Asthmatic patients are reported to have a high degree of within-day variability in blood eosinophil counts which is not related to an obvious diurnal pattern and requires more study. Oral corticosteroid medication very significantly reduces blood eosinophil counts, and this suppression lasts greater than 24 h after a single dose. An effect of inhaled corticosteroid is also documented, but of lesser magnitude. An oral prednisone dose of 15 mg had an equivalent effect to 1.84 mg/day of inhaled budesonide on the blood eosinophil count. The blood eosinophil count has emerged as an important biomarker in airway disease because it is clinically accessible, reliably identifies the eosinophilic phenotype in airway disease, and can be used to select