Differential diagnosis of psychogenic, central, and nephrogenic DI using water restriction and desmopressin stimulation

Table 6.3-1. Differential diagnosis of central DI, nephrogenic DI, and primary polydipsia using water deprivation and desmopressin stimulation
	Primary polydipsiaa	Central (neurohypophyseal) DI	Nephrogenic DI
Water restriction test (fluid deprivation test)
Urine specific gravity	>1.005	<1.005	<1.005
Urine osmolality	>500-600 mOsm/kg	<250 mOsm/kg	<250 mOsm/kg
Plasma copeptin level	Initially low, then increasing	Low	High
Desmopressin stimulation test (desmopressin 1 microg SC or IV)
Urine specific gravity	No indication for the testb	Increased by ≥50%	Low, not increasing
Urine osmolality	No indication for the testb	Increase by 100% in complete central DI and 15% to 50% in partial central DI	No elevation
a Primary polydipsia is usually psychogenic but may also be related to medications causing dry mucous membrane, structural hypothalamic thirst center lesions, or perceived health benefit of drinking large volumes of fluids. b As the results of the fluid deprivation test are normal.
ADH, antidiuretic hormone; DI, diabetes insipidus; IV, intravenous; SC, subcutaneous.