Primary Outcome Measures:
- Discriminative value of copeptin during different diagnostic tests. [ Time Frame: beginning and end of protocol, up to 8hours ]
Other: Water deprivation test
Classical water deprivation test alone
Other: Water deprivationtest
classical water deprivation test plus plasma copeptin cut-off levels
Other: Hypertonic saline infusion
hypertonic saline infusion test plus plasma copeptin measurement
Purpose of this study is to compare the overall diagnostic accuracy of the three following diagnostic test procedures in diabetes insipidus (central, nephrogen) and primary polydipsia: a) classical water deprivation test alone, b) classical water deprivation test plus plasma copeptin cut-off levels, c) hypertonic saline infusion test plus plasma copeptin measurement.
The investigators hypothesize that firstly b) and c) is better as a). Secondly the investigators hypothesize that c) is non-inferior to b).