Asthma and COPD are the most frequent chronic respiratory diseases. Although they have different characteristics, some individuals share features of both diseases, which has been called the asthma-COPD overlap syndrome (ACOS). Although there is not a universally accepted definition for ACOS, it was initially defined as symptoms of increased variability of airflow in association with an incompletely reversible airflow obstruction. The most recent COPD guidelines include this phenotype and its

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... gnostic features. Patients with ACOS are usually characterized by increased reversibility of airflow obstruction, eosinophilic bronchial and systemic inflammation, and increased response to inhaled corticosteroids, compared with COPD patients. The relevance of the ACOS is the need to identify patients with COPD who may have underlying eosinophilic inflammation that responds better to inhaled corticosteroids. Until new diagnostic tools are developed, a previous diagnosis of asthma in a patient with COPD can be a reliable criterion to suspect ACOS in a patient with COPD but a comprehensive approach may be required in most cases for a definitive diagnosis of ACOS.