Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1304 page)

   
Normal response:
Water deprivation causes kidney to increase urine osmolality to 1,000–1,200 mmol/kg. ADH does not cause further increase in urine osmolality because endogenous ADH is already at maximum.

   Use

   To distinguish the major forms of DI—neurogenic, nephrogenic, and polydipsic.

   Steps:

   Have the patient should stop drinking 2–3 hours before coming to the office or clinic; overnight fluid restriction should be avoided, because potentially severe volume depletion and hypernatremia can be induced in patients with marked polyuria.

   Collect 7–10 mL of heparinized blood for immediate measurements of serum sodium concentration and osmolality. Also ask the patient to void his/her bladder, record the urine volume, and send urine specimen for immediate measurement of osmolality.

   Repeat step 2 every hour until (a) plasma sodium concentration or osmolality rises above the upper limit of normal range or (b) urine osmolality rises above 300 mOsm/kg H
₂
O.

   If (a) occurs before (b), primary polydipsia, partial neurogenic, and partial nephrogenic DI are excluded, and a dDAVP (synthetic analog of ADH) challenge test should be done as follows:

•   Inject 2 μg of dDAVP subcutaneously.

•   Ask the patient to empty bladder at 1 and 2 hours after the injection; measure the urine osmolality. Also measure the patient’s plasma ADH level.

○   If either urine samples has an osmolality >50% higher than the value immediately before injection, the patient probably has complete neurogenic DI.

○   If both urine samples have osmolality increase of <50% than the value immediately before injection, the patient is very likely to have complete nephrogenic DI.

   If (b) occurs before (a), complete neurogenic and nephrogenic DI are excluded. Further differentiate among partial nephrogenic DI, partial neurogenic DI, and primary polydipsia will require trained personnel and specialized measurements.

   Interpretation

   
Complete DI:
Water deprivation increases plasma osmolality but urine osmolality stays <290 mmol/kg and does not increase following dDAVP challenge.

   
Partial DI:
Water deprivation causes some increase in urine osmolality to 400–500 mmol/kg (less than normal).