Patients with asthma and chronic obstructive airways disease (COPD) undergo routine testing of their lung function in the diagnosis, progression, management and, response to treatment of their disease. Standard lung function obtains measurements based on the forced flow of air moving within the airways. Such measurements give a reasonable assessment of disease affecting the large airways, but not an accurate estimate of small airways disease. Small airways are less than 2mm in diameter. However, both asthma and COPD have disease that involves not only the large but also the small airways that has important clinical consequences. Indeed, COPD predominantly affects the small airways.