Variable expiratory airflow, and not airway obstruction, is the defining physiological feature of asthma.

Asthma is usually diagnosed by history and presence of airway obstruction with a significant bronchodilator response on spirometry. However, airway obstruction may not always be present, or a bronchodilator response may not be significant. Contrary to wide perception, airway obstruction is not the defining functional feature of asthma. Definitions of asthma have never included the term 'airways obstruction'. According to the Global Initiative for Asthma, the diagnosis of asthma requires a history of typical variable symptoms and confirmation by variable expiratory airflow. It has emphasized that variable expiratory airflow is the key physiological feature of asthma. Airway obstruction is a manifestation of this variability and represents one end of the spectrum of this variation. When airways obstruction is observed, a positive bronchodilator response is the most convenient method to confirm the diagnosis. However, when airway obstruction or bronchodilator responsiveness is not demonstrable, there are other options to demonstrate variability in not demonstrable, there are other options to demonstrate variability of expiratory airflows and confirm the diagnosis.