8-HOUR WATER DEPRIVATION TEST
INTRODUCTION
Polyuria due to lack of ADH usually exceeds 5 litres daily. In diabetes insipidus there is a tendency for the serum osmolality to be increased above normal; the reverse is true in compulsive water drinkers although there is overlap with normals. Other causes of polyuria, including diabetes mellitus and chronic renal failure (caused by osmotic diuretics, glucose and urea), hypercalcaemia (which causes nephrogenic diabetes insipidus due to calcium interfering with ADH action), and hypokalaemia, should be excluded before undertaking this test.
PRECAUTIONS
Please refer to the Trust’s phlebotomy procedure for sample collection precautions.
Caution: Pre-renal uraemia is a hazard in patients with renal impairment.
This test should only be carried out in a well-hydrated patient who is under careful supervision because those with ADH deficiency may become dangerously dehydrated, whereas compulsive water drinkers may steal water or other fluids during the test.
Weigh the patient before and hourly during the test to determine whether the patient is taking fluid or becoming dangerously dehydrated. Consider terminating the test if more than 3% body weight is lost.
PATIENT PREPARATION
The patient is encouraged to drink fluids overnight. A light breakfast is allowed without tea or coffee, but must refrain from smoking prior to and during the test. The test lasts 8 hrs (usually 08.30 – 16.30hrs) during which no fluids are allowed although some dry food is permitted.
SAMPLES
Eight plain containers (500ml) for the urine collections should be obtained prior to the test from Pathology despatch.
Urine is collected hourly into the containers supplied and the volumes recorded. The following timed specimens should be sent to Clinical Biochemistry Department for osmolality determinations:
Urine: |
First hour |
08.30 – 09.30 |
Third to fourth hour |
11.30 – 12.30 |
|
Sixth to seventh hour |
14.30 – 15.30 |
|
Seventh to eighth hour |
15.30 – 16.30 |
A serum sample (6ml SST yellow top tube) is taken at the mid point of each urine collection period:
Serum: |
09.00 hours |
12.00 |
|
15.00 |
|
16.00 |
DDAVP TEST
If no antidiuresis has been obtained after 8 hours water deprivation DDAVP (Desmopressin) may be given to distinguish between nephrogenic and cranial diabetes insipidus.
DDAVP (20ug intra-nasally or 2ug intramuscularly) is given immediately on completion of the water deprivation test. Urine is then collected each hour for a further 4 hours for osmolality determinations. The patient may drink water but no more than twice the volume excreted during the deprivation test is allowed for the next 24 hours, due to danger of water overload.
RESPONSES
In normal patients the serum osmolality should not exceed 295 mosm/kg and the urine osmolality exceeds 600 mosm/kg at some time during the test. The serum osmolality exceeds 295 mosm/kg in cranial and nephrogenic diabetes insipidus. In the former condition, the urine osmolality remains less than 300 mosm/kg during water deprivation but exceeds 600 mosm/kg following DDAVP administration. In nephrogenic diabetes insipidus the urine osmolality fails to exceed 600 mosm/kg following DDAVP administration. Some patients show intermediate values and partial defects, primary polydipsia and the adequacy of the test should be considered.
Reviewed by: Tony
Everitt, Consultant Biochemist
Last edited 07/08/08