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
