Interpreting the response to desmopressin
After water deprivation, desmopressin, a synthetic analog of arginine vasopressin (AVP), the antidiuretic hormone, is given to see whether the kidneys can respond. In arginine vasopressin resistance (formerly nephrogenic diabetes insipidus), the kidneys cannot respond to vasopressin. Urine stays dilute during water deprivation and still does not concentrate after desmopressin. Urine osmolality that stays near 160 mOsm/kg after desmopressin indicates vasopressin resistance.
Comparing the three main causes of polyuria
In arginine vasopressin deficiency (formerly central diabetes insipidus), the pituitary does not release enough vasopressin. The kidneys are normal, so urine concentrates markedly when desmopressin is given, for example from 160 to 540 mOsm/kg. In primary polydipsia, excessive drinking dilutes the urine, but vasopressin release and kidney response are intact, so the urine concentrates during water deprivation itself, for example to above 700 mOsm/kg. In vasopressin resistance, neither deprivation nor desmopressin produces concentrated urine.
| Condition | During water deprivation | After desmopressin |
|---|
| Primary polydipsia | Urine concentrates | Little further change |
| AVP deficiency (central) | Stays dilute | Marked rise in concentration |
| AVP resistance (nephrogenic) | Stays dilute | Stays dilute |
Why the other results do not indicate resistance
Urine osmolality rising above 700 mOsm/kg during water deprivation shows intact vasopressin release and response, as in primary polydipsia. A marked rise from 160 to 540 mOsm/kg after desmopressin shows that the kidneys respond, which indicates vasopressin deficiency. A fall in serum sodium below 135 mEq/L after desmopressin reflects water retention while the client continues to drink, not kidney resistance. Watch out! Current terminology uses AVP deficiency and AVP resistance; older sources call them central and nephrogenic diabetes insipidus.
Exam takeaway
The key question in the test is where the defect lies: in drinking, in hormone release, or in the kidney's response. Key point! No concentration after desmopressin means the kidney cannot respond to vasopressin: arginine vasopressin resistance.