Fig. 5. 19 
Measurements used to differentiate between the causes of polyuria.
 (a) After osmotic stimulation with five per cent hypertonic saline patients with cranial diabetes insipidus (CDI) exhibit values to the right of the normal range while those with              nephrogenic diabetes insipidus (NDI) and primary polydipsia will show values within the normal limits. (b) After overnight dehydration. NDI is usually distinguishable from primary polydipsia by the inappropriately high levels of plasma vasopressin relative to urine osmolality.