Chronic Obstructive Pulmonary Disease (COPD) is a common disease and prevalence is increasing worldwide. It is characterized by persistent airway obstruction that is partially reversible but it is considered preventable and treatable disease now. Airflow limitation is associated with chronic and abnormal inflammatory response in the airways and the lung to noxious stimuli . Airway obstruction is defined by a reduction of expiratory airflow. Generally, forced expiratory volume in 1 s/forced volume capacity (FEV1/FVC) ratio of less than 70% after bronchodilator has been used to identify COPD patient. The use of lower limit of normal (LLN) values has been proposed to define airflow limitation by spirometry, but current Global initiative for chronic Obstructive Lung Disease (GOLD) and American Thoracic Society/European Respiratory Society guidelines continue to recommend the fixed ratio criteria instead of an LLN for the diagnosis of COPD . Patients with COPD have shown a great deal of heterogeneity and can be classified according to their clinical and radiologic parameters, biomarkers, lung function impairment and prognosis . Traditionally, COPD has been classified as chronic bronchitis (CB) and emphysema. CB is defined as the presence of a chronic productive cough for 3 months in each of two consecutive years. Emphysema is defined as the destruction of alveolar walls and permanent enlargement of the airspaces distal to the terminal bronchioles. Current GOLD guidelines do not include the use of these terms in the definition of COPD. Asthma and COPD represent different disease entity with different pathogeneses and risk factors. Sometimes clinical manifestations of both diseases may overlap in a patient with airway obstruction and cannot be classified as COPD or asthma only. Large population studies show that some of the patients with airway obstruction are classified with more than one diagnosis. Therefore, overlapping diagnoses of asthma and COPD has been proposed and it is called COPD and Asthma Overlap Syndrome (ACOS) .
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