The water deprivation test has long been the ΓÇÿcornerstoneΓÇÖ for differentiating between the causes of polyuria. The protocol for a modified Dashe dehydration test, combined with an assessment of response to exogenous vasopressin, is shown in Figs 5.16 & 5.17. Patients with moderate or severe cranial diabetes insipidus are clearly distinguished from others by the observation of consistently hypotonic urine during fluid restriction, together with concentrated urine after the administration of exogenous vasopressin. Hypotonic urine following exogenous vasopressin strongly suggests nephrogenic diabetes insipidus.
