A test for diabetes insipidus in which fluid and food intake is withheld completely for (usually) 24 hours, with regular measurement of plasma and urinary osmolality and body weight. Normally (and in a person with psychogenic polydipsia) the output of vasopressin will be increased in order to concentrate the urine as the plasma osmolality rises; correspondingly, the urine osmolality also rises and its volume diminishes. In a patient with diabetes insipidus, however, the urine osmolality will remain low and of high volume while the patient steadily dehydrates. The test must be abandoned if the patient loses 3% of body weight.
Subjects: Medicine and Health.