Patients with cystic fibrosis need extra salt as they have increased losses through sweat. Current advices are to follow and supplement if needed, however the way to follow salt depletion is unclear.~The best way is to calculate fractional salt excretion. This needs a simultaneous blood and urine sample for electrolytes and creatinin. Urinary surrogate markers have been validated on only 10 patients.~At the annual check-up patients with cystic fibrosis receive these measurements. The investigators will collect the measurements and calculate fractional excretion and the possible surrogate markers on urine. Further this will be correlated to nutritional and pulmonary status and patient subgroups at risk will be identified.
The results of the annual check-up will be collected to evaluate the best urinary marker for fractional sodium excretion and salt status will be correlated to clinical outcome measures.