 Lung Clearance Index LCI has been shown to be more sensitive and perhaps more applicable in early stages of lung disease when compared to currently utilised physiological outcome measures, and yet it is not approved by regulatory agencies as a recognised surrogate outcome measure. This may highlight some of the lack of understanding within MBW; what is the best equipment to use, what is the minimal value of change to show an improvement in LCI, what is LCIs correlation with clinical outcome measures and what does the progression of LCI tell us about lung disease. These questions will be addressed with a view to contributing a body of data to guide decisions around its utility in both interventional drug trials and for monitoring in a clinical setting. 