 The life expectancy of patients with Cystic Fibrosis CF has greatly increased over time due to improved clinical care. While this is certainly beneficial to CF patients, the overall stability in lung function has made it more difficult to assess the effect of therapeutic interventions. Currently, FEV1 forced expiratory volume in 1 second remains the primary outcome measure for most clinical trials, but many CF patients have normal pulmonary function and the annual rate of decline is now less than 2 in large specialized centres. Therefore, additional parameters are needed that are more sensitive parameters to define abnormalities in CF patients and be used in therapeutic trials. 