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

BACKGROUND: 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. OBJECTIVE: To investigate the relationship between asthma control and sputum eosinophils in clinical practice. METHODS: 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. RESULTS: A multivariate analysis showed that asthma control was independently associated with individual fluctuations in 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. CONCLUSIONS: At the individual level, asthma control was associated with fluctuations in sputum eosinophil count over time.