Diagnose diabetes insipidus (DI) and ascertain the type (central vs nephrogenic, partial vs complete), in a patient with polyuria and polydipsia.
Rule out osmotic diuresis as a cause of polyuria and polydipsia before performing the water deprivation test. This can be done with a 24-hour urine collection for osmoles. Urine osmoles > 900 mOsm/day indicate osmotic diuresis. Consider osmoles such as glucose, urea, and electrolytes.
Baseline Na > 145 with urine osmolality < plasma osmolality — this is consistent with diabetes insipidus, water restriction is risky and unnecessary. Response to DDAVP will indicate whether this is central or nephrogenic.
Baseline hyponatremia — this is more consistent with psychogenic polydipsia or resolving SIADH and goes against diabetes insipidus.
The test may take several hours, so it is usually scheduled to start in the morning.
- Ask the patient to empty their bladder at baseline.
- At baseline then every hour, obtain the following stat:
- Body weights
- Plasma electrolytes and osmolality
- Urine volume and osmolality
- Plasma ADH level — draw at baseline only and keep sample, use only if water deprivation is otherwise equivocal
Monitor results of above tests for one of the following conditions:
a. Urine osmolality > 600 mOsm/kg — DI is ruled out, stop the test.
b. Plasma osmolality ~ 300 mOsm/kg with Na > 145 mmol/L — proceed to step 4
c. Plasma osmolality rises despite stable urine osmolality for 2-3 consecutive readings — proceed to step 4
d. Patient loses > 5% of body weight from baseline — proceed to step 4
If (b), (c), or (d) from above, administer ddAVP 2 µg intravenously, and continue hourly measurements above, for up to 2 hours.
If urine osmolality reaches > 600 mOsm/kg before ddAVP administration, then central and nephrogenic DI are ruled out (thus, the test is terminated and ddAVP administration is unnecessary).
If DI is confirmed and ddAVP is administered, the type of DI is determined by the response of urine osmolality to ddAVP administration:
|U.Osm after ddAVP|
|Doubles (100% increase)||Complete Central DI|
|45% increase or 300 mOsm/kg absolute||Partial Central DI|
|15-45% increase and < 300 mOsm/kg absolute||Partial Nephrogenic DI|
|< 10% increase (i.e., does not increase)||Complete Nephrogenic DI|