Diagnostic tests can only increase or decrease the probability of the asthma diagnosis, so a thorough history is very important. In patients with asthma-like symptoms, spirometric evidence of airway obstruction plus a large bronchodilator response makes asthma much more likely. However, normal spirometry is common in patients with mild asthma who are not symptomatic at the time of testing, and patients with poorly controlled asthma may lack substantial bronchodilator response. Inhalation challenge test often helps confirm asthma in patients with normal spirometry. Adult smokers with intermittent respiratory symptoms may have either asthma or chronic obstructive pulmonary disease (COPD). Normal post-bronchodilator spirometry rules out COPD. In patients with airway obstruction, a low diffusing capacity of the lung for carbon monoxide increases the probability of COPD and makes asthma much less likely. A high exhaled nitric oxide level makes allergic asthma more likely. Response to inhaled corticosteroids makes asthma more likely and COPD less likely.