This disclosure relates to a system configured to detect the presence of secretions in a subject's airway during respiratory therapy. The system can determine whether the subject requires airway clearance. A pressure generator generates a pressurized flow of breathable gas. Sensors generate output signals relating to one or more gas parameters of the pressurized flow of breathable gas. The system can determine a first parameter that is an indication of the volume of breathable gas within the airway of the subject and a time derivative of the first gas parameter to generate a plot of these parameters. The plot includes a perimeter and an area that can be used to determine whether to effectuate initiation of airway clearance based on a complete breathing cycle that includes at least one inhalation and exhalation.