**3.1 Overlap of symptoms and airflow limitation between COPD and asthma**

COPD and asthma are the two most prevalent human airway diseases. Although COPD and asthma are pathologically entirely different diseases, it is not so easy to clearly distinguish between these two diseases. In patients with COPD, the initial pathological alterations occur in bronchioles less than 2 mm in diameter ("silent zone" in spirometry). Disorders in the bronchioles are followed by parenchymal remodeling [68], which is different from asthma. On the other hand, disorders in bronchioles probably cause wheezing, which is characteristic to asthma. Clinical manifestations and airflow limitation overlap with COPD and asthma; moreover, eosinophilia and hyperresponsiveness in the airways, which are classically considered to be characteristic of asthma, also overlap with these two diseases. In patients with asthma who have a history of smoking, a differential diagnosis between asthma and COPD can be difficult just in the routine use of spirometry. In these cases that fit the pyrometric criteria for COPD, it may be hard to distinguish clearly between asthma and COPD because of the LLN for FEV1/FVC ratio and the low incidence (approximately 15%) of COPD in smokers. In patients with COPD who have eosinophilia and hyperresponsiveness in the airways, it is also can be difficult to accurately distinguish between COPD and COPD with asthma even though they have symptoms with variability (cough, dyspnea, wheezing), which are clinical features of asthma, but overlap between COPD and asthma.
