**2. Definition of ACOS**

ACOS is the coexistence of two distinct diseases in the same individual: asthma and COPD. Whether this concept is clinically relevant or not depends on its capacity to describe an entity with differentiated pathogenic mechanisms, prognostic particularities, and potentially specific treatment options. The recently updated Spanish COPD guidelines [6] acknowledge the existence of a syndrome that overlaps characteristics of COPD and asthma, and it proposes a differential treatment.

When considering how to define this entity, existing definitions of asthma and COPD should be taken into account. The new definition of COPD according to GOLD 2014 includes subtle changes regarding the previous definitions, integrating the findings of recent evidence [7]. For instance, there is no longer mention of reversibility, and it emphasizes the role of exacerbations and comorbidities. Thus, COPD is a common, preventable and treatable disorder, defined by persistent airflow obstruction that is mostly progressive, characterized by a chronic inflam‐ matory response in the airways and lungs to noxious particles and gases; exacerbations and comorbidities generally contribute to the severity of the disease in individual patients.

On the other hand, in 2014, GINA [8] defined asthma as "a heterogeneous disease, usually characterized by chronic airway inflammation. It is defined by the history of respiratory symptoms such as wheeze, shortness of breath, chest tightness and cough that vary over time and in intensity, together with variable expiratory airflow limitation". The similarities between asthma and COPD definitions are obvious, but none of their features are pathognomonic, and all of them might be present in individual patients.

For operative purposes, in 2014, GINA and GOLD published a joint document on ACOS [9]. ACOS was defined as the presence of persistent airflow limitation with several features usually associated with asthma and others usually associated with COPD. The document presents the characteristics of asthma and COPD listed separately and suggests that ACOS may be the diagnosis when a similar number of characteristics of both asthma and COPD are identified in a given patient. The joint task force also recommends a stepwise approach to the diagnosis. It uses clinical, spirometric and radiographic findings to help delineate if an adult patient is most likely suffering from asthma or COPD or fulfills enough shared features to be considered within ACOS. This definition and the diagnostic criteria differ from other guidelines, for instance, the Spanish expert report from 2012 [10].

The different diagnostic criteria proposed so far are discussed in Section 5. However, to obtain any clearer and validated criteria, further research about underlying mechanisms is needed.
