Asthma Control and Sputum Eosinophils: A Longitudinal Study in Daily Practice

Abstract

Longitudinal trials have suggested that asthma control may be influenced by fluctuations in eosinophilic inflammation. This association has however never been confirmed in daily practice. To investigate the relationship between asthma control and sputum eosinophils in clinical practice. A retrospective longitudinal study was conducted on 187 patients with asthma with at least 2 successful sputum inductions at our Asthma Clinic. Linear mixed models were used to assess the relationship between asthma control and individual changes in sputum eosinophils. Receiver-operating characteristic curves were constructed to define minimal important differences (MIDs) of sputum eosinophils associated with a change of at least 0.5 in Asthma Control Questionnaire (ACQ) score. Then, a validation cohort of 79 patients with asthma was recruited to reassess this relationship and the accuracy of the MID values. A multivariate analysis showed that asthma controlwasindependently associated with individual fluctuationsin sputum eosinophil count (P < .001). In patients with intermittent/persistently eosinophilic asthma, we calculated a minimal important decrease of 4.3% in the percentage of sputum eosinophils (area under the curve [AUC], 0.69; P < .001) or 3.4-fold (AUC, 0.65; P= .003) for a significant improvement in asthma control and a minimal important increase of 3.5% (AUC, 0.67; P=.004) or 1.8-fold (AUC, 0.63; P= .02) for a significant worsening in asthma control. The association between asthma control and sputum eosinophils and the accuracy of the MIDs of sputum eosinophils were confirmed in the validation cohort. At the individual level, asthma control was associated with fluctuations in sputum eosinophil count over time.