<qa>  What is the need for surrogate markers of early obstructive lung disease?Functional abnormalities associated with lung disease such as cystic fibrosis CF occur in early childhood, but have historically gone undetected until the onset of clinical symptoms, at which point irreversible lung damage may have already occurred 1-3. Consequently, over the last ten years the focus of clinical care has shifted to early intervention and prevention of these structural changes. To facilitate early intervention there is a pressing need for surrogate markers of early obstructive lung disease that are also sensitive enough to detect treatment effects 4. 