Reply to: Asthma–Chronic Obstructive Pulmonary Disease Overlap versus Chronic Obstructive Pulmonary Disease: Comparing Apples and Oranges

asthma-COPD overlap syndrome (ACOS), including persistent yet reversible airflow limitation (post-bronchodilator FEV 1 to FVC ratio of ,70% and FEV 1 improvement of .12%and .400ml from baseline after bronchodilator therapy).However, the term "syndrome" was dropped because it is not a single phenomenon but is caused by a variety of mechanisms.Growing evidence shows distinctions between ACO and COPD.Karayama and colleagues (2) evaluated 167 patients with COPD and divided them into a COPD group and an ACO group.The authors analyzed respiratory resistance and reactance and three-dimensional computed tomography data between COPD and ACO.They found that patients with ACO had higher respiratory resistance and reactance during tidal breathing and a smaller gap between the inspiratory and expiratory phases than patients with COPD, suggesting that patients with ACO had greater airway narrowing and more severe small airway disease than those with COPD.Similarly, in the review by Leung and Sin, the authors concluded that patients with ACOS had greater decrement in quality of life and their healthcare utilization was significantly higher compared with asthma or COPD alone (3).In addition, the FEV 1 annual decline was 46.5 ml/yr in the COPD group and 36.5 ml/yr in the ACO group in the study by Mannino (4).Baarnes and colleagues defined ACOS as post-bronchodilator FEV 1 /FVC , 0.70, combined with wheeze and/or significant bronchodilator reversibility.In the cohort, the authors discovered that, compared with COPD only, patients with ACOS had more dyspnea and lower FEV 1 % predicted, whereas no difference was found in bronchodilator reversibility (5).In conclusion, the abovementioned study indicates that ACO and COPD exhibit differences in lung function.Upon reviewing the article by Gerstein and colleagues (1), it is inadvisable for the author to classify patients with ACO into the COPD group, because they exhibit differences in lung function and varying degrees of disease severity.This could affect the reliability of the results.In addition, considering the unequal distribution of ACO cases between the undiagnosed and diagnosed groups, it may result in a lack of comparability between these groups.However, there are still some issues that require further discussion.In some aspects, the outcomes of ACO and COPD are consistent.Modak and colleagues used the Healthcare Cost and Utilization Project Nationwide Readmissions Database and analyzed outcomes of index admissions and 30-day readmissions in asthma, COPD, and ACO.They concluded that although ACO was linked to higher rates of baseline comorbidities, an extended length of stay, and increased healthcare costs during the index admission compared with asthma or COPD, this did not result in higher in-hospital mortality, complication rates, or an increased risk of readmission (6).To sum up, despite its limitations, ACO differs from COPD in symptoms.Therefore, we recommend that patients with ACO should be separated from COPD and analyzed as an independent group to preserve the homogeneity of the study population.

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