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Many studies have assessed sputum eosinophil percentage as a phenotypic descriptor in asthma patients. Adjusting inhaled corticosteroid (ICS) dose based on sputum eosinophilia can lead to better clinical outcomes than using empirical clinical guidelines alone.1 ,2 While techniques for collecting and analysing sputum eosinophils are available in specialty clinics, sputum analysis remains time consuming, onerous to the patient, labour intensive and is not always successful, which precludes its broader clinical generalisability to primary care settings where most asthma patients are managed. Thus, there is a need for less invasive and more technically straightforward means of phenotyping asthma patients to enable therapeutic management along biological, rather than empirical, guidelines. In particular, accurate phenotyping of severe asthma patients refractory to ICS treatment has proven to be critical for demonstrating the efficacy of emerging experimental therapies that target specific inflammatory pathways.3
Sterk and colleagues compared three biomarkers, peripheral blood eosinophils, fractional exhaled nitric oxide (FeNO) and serum periostin levels, for assessing how reliably each reflects sputum eosinophilia in a cross-sectional study.4 They report that blood eosinophils performed favourably, with a receiver operating characteristic area under the curve of 0.89 in a test cohort and 0.85 in a replication cohort. Using a threshold of 270 eosinophils/μL of blood, they report positive and negative predictive values of 79% and 91% for predicting sputum eosinophilia of ≥3%, which is in line with a recent similar study in Clinical and Experimental Allergy by Zhang and colleagues who identified 260 eosinophils/μL of blood as an optimal cut-off.5 Overall, the finding that blood eosinophil counts are a reasonably accurate biomarker of sputum eosinophil percentage in two rigorously conducted independent studies is encouraging, as blood eosinophil counts are readily obtained from a complete blood count with differential, a test that is convenient and widely available …