Water Deprivation Test

Indication

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.

Contraindications

Procedure

The test may take several hours, so it is usually scheduled to start in the morning.

  1. Ask the patient to empty their bladder at baseline.
  2. 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
  3. 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

  4. If (b), (c), or (d) from above, administer ddAVP 2 µg intravenously, and continue hourly measurements above, for up to 2 hours.

Interpretation

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