WATER DEPRIVATION TEST

WATER DEPRIVATION TEST:

The aims of this test are:

• To establish the diagnosis of diabetes insipidus (DI).

• To distinguish between nephrogenic and central DI.

Principle:

In a normal individual even after a period of water deprivation, homeostatic mechanisms to maintain serum osmolality and euvolemia within the normal range are activated. Urine output decreases and urine becomes progressively concentrated, but serum sodium (reflecting intravascular volume status) and serum osmolality remain normal.

Patients with DI in whom the renal concentrating ability is impaired can maintain euvolemia only by increasing their water intake. When deprived of water, urine output continues to be high and urine remains dilute. The serum sodium and serum osmolality rise.

Precautions and planning of the water deprivation test:

The test should be performed in a day care where a period of observation for 8–10 hours is possible, or in an in-patient setting

Patients with DI may get rapidly dehydrated on water deprivation; hence resource and expertise for fluid resuscitation should be readily available during the test.

Test procedure: Many protocols for water deprivation test are described. Most protocols entail hourly sampling and can be performed if a heparinized IV cannula is left in place and test results are obtained within 1 hour. Hence a modification of this test may be adopted in practice.

Step 1: Thyroid and adrenal function should be assessed before the water deprivation test.

Water deprivation should be started at midnight. No water or drinks or food should be allowed during the period of testing. Baseline weight is recorded; baseline samples are collected for S. sodium, S. osmolality, urine osmolality

Starting at 9 am, hourly estimations of the above parameters are done. The results are procured as soon as possible

Step 1 stopped at 12 noon (after 12 hours of water deprivation) and Step 2 is started

Step 1 may be terminated at any stage and Step 2 is started if:

Urine osmolality is more than 600 mOsm/L as DI is ruled out

If there is weight loss of more than 5% body weight or there is hemodynamic compromise

If S. osmolality exceeds 300 mOsm/kg at any stage, DI is diagnosed and no further testing is required

Serum arginine vasopressin levels may be collected, if available.

Step 2: Child is allowed some water orally. Desmopressin 5 mcg is administered intranasally or 0.3 mcg is given subcutaneously. Hourly samples are collected for the next 4 hours (Table).

Limitations of Water Deprivation Test:

It cannot be performed in neonates and infants due to higher risk of dehydration during the test. In such situations, estimation of serum and urine osmolality may be a useful guide to diagnosis, coupled with a therapeutic trial of desmopressin when clinical suspicion of DI is high.

Interpretation of the water deprivation test:

S. osmolality (mOsm/kg)	U. osmolality (mOsm/kg)	U. osmolality after dDAVP	Diagnosis
<295	>600	—	Normal response
>295	<300	>600	CDI
>295	300–600	>600	Partial CDI
>295	<300	Rise <100 above baseline	NDI

CDI: Central diabetes insipidus      NDI: Nephrogenic diabetes insipidus