Blood eosinophil counts do not reliably reflect airway eosinophil concentrations in children with severe, therapy-resistant asthma, and therefore cannot be used to make therapeutic decisions, researchers report.
The findings are a disappointing set back in the development of a blood test to substitute more invasive techniques to measure airway inflammation, which has previously shown promise for adults with severe asthma.
"Our data suggest that if blood eosinophilia is present, it is highly probable that airway eosinophilia ([bronchoalveolar lavage] and biopsy) is also present, but if the blood eosinophil count is normal, it is not possible to predict whether airway eosinophilia is present," say Sejal Saglani (Imperial College London, UK) and colleagues.
The study, reported in Allergy, included 88 children aged 6-17 years with severe, therapy-resistant asthma. Patients underwent blood tests, exhaled nitric oxide measurement, sputum induction, bronchoalveolar lavage, and endobronchial biopsy.
Of 86% of children with normal blood eosinophil counts, 84% had evidence of airway eosinophilia detected either by bronchoalveolar lavage or endobronchial biopsy.
Conversely, all 12 children with elevated blood eosinophil levels had eosinophilia on brochoalveolar lavage, and nine had biopsy eosinophilia.
The authors showed that blood eosinophil levels of more than 0.2 x 109/L predicted eosinophilia on bronchoalveolar lavage or endobronchial biopsy with negative predictive values of only 65% and 33%, respectively.
Saglani and colleagues explain that the management of severe, therapy-resistant asthma is optimal when guided by inflammatory phenotype in adults. While there is currently no comparable evidence to support this premise in children, the authors say that treating children on the basis of airway inflammation could avoid unnecessary treatment with increasingly potent anti-inflammatory medications.
However, their current results suggest that invasive methods will continue to be needed to determine pediatric inflammatory phenotypes.
"As peripheral blood counts are not reliable in characterizing airway inflammation in severe asthmatic children exposed to high dose inhaled and maintenance oral steroid therapy, bronchoscopy with [bronchoalveolar lavage] should be considered," they conclude.
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