Accuracy of blood eosinophil count in predicting sputum eosinophils in children with problematic severe asthma (PSA)

<b>Background:</b> Paediatric severe asthma is characterised by airway eosinophilia. In adults, blood eosinophils predict sputum eosinophils (Wagener AH et al. Thorax 2015;70:115-20) and determine eligibility for biologics targeting eosinophils. However, there is little evidence of a relationship between blood and bronchoalveolar lavage or endobronchial biopsy eosinophils in children (Ullmann N et al. Allergy 2013;68:402-6). Induced sputum (IS) is a non-invasive method to assess airway inflammation in children. <b>Methods:</b> 31 children underwent structured multidisciplinary assessments at a paediatric severe asthma service over 2 years. Blood and IS were obtained on the same day. Demographic characteristics, medications, measures of asthma control, spirometry and FeNO were analysed. Patients with both high blood eosinophils (BE) (&gt;0.3x10⁹/L) and high sputum eosinophils (SE) (&gt;2.5%) were compared to those without concordant blood and sputum eosinophilia. <b>Results:</b> 15/31 (48.3%) had both high BE and SE. 10/31 (32.2%) had high BE alone, 2/31 (6.4%) had high SE alone while 4/31 (12.9%) had low BE and SE. At 0.3x10⁹/L, blood eosinophils had sensitivity of 88% and specificity of 21% in predicting airway eosinophils, while at blood eosinophils of 0.95x10⁹/L sensitivity was 93% and specificity 80%, AUC=0.92, p&lt;0.01. There was no correlation between sputum eosinophils and either blood eosinophils (p=0.47) or FeNO (p=0.97). <b>Conclusions:</b> In contrast to adults with severe asthma, blood eosinophils at 0.3x10⁹/L do not reflect airway eosinophils assessed in IS in paediatric severe asthma. A higher cut-off of blood eosinophils may be a more appropriate biomarker of airway eosinophilia in children.