Urine and plasma osmolalities are observed in response to fluid deprivation and subsequent administration of desmopressin. Careful supervision is essential and the test should be discontinued if 5% of initial body weight is lost; 3% in children.
Free access to fluid is permitted until the start of the test. During the test, dry food is permitted but not fluid; at timed hourly intervals for up to 8 hours the following are measured:
- urine volume and osmolality
- serum osmolality
2 mcg IM desmopressin is administered at the end of 8 hours. Urine is collected for a further 16 hours during which time fluid intake is allowed but restricted to a maximum of 1.5 times the urine volume voided during the dehydration period. Blood is collected at the end of the 16 hours and plasma osmolality measured.
Results may be interpreted as:
- urine osmolality less than 300 mosmol/kg after fluid deprivation and greater than 800 mosmol/kg after desmopressin suggests cranial diabetes insipidus
- urine osmolality less than 300 mosmol/kg after fluid deprivation and less than 300 mosmol/kg after desmopressin suggests nephrogenic diabetes insipidus
- urine osmolarity greater than 800 mosmol/kg after fluid deprivation and greater than 800 mosmol/kg after desmopressin suggests primary polydipsia