Water deprivation test

Investigation Protocol

Indications

To establish the underlying cause for polydipsia and polyuria and to distinguish between patients with central diabetes insipidus (DI), nephrogenic DI, and psychogenic polydipsia.

Precautions and preparation

Thyroid and adrenal function need to be documented as normal or adequately replaced prior to starting the test. Urea and electrolytes and serum calcium must also be tested and potassium and calcium confirmed as normal before proceeding.

The patient can eat and drink and take all their usual medication prior to the test, excluding demopressin which must have been stopped for at least 24 hours prior to the test. Patients are also asked to avoid cigarettes, alcohol and caffeine for 24 hours prior to the test.

Procedure

Time=0, 8am, start the test. Weigh the patient, calculate and document 97% of this weight on the patient's chart. Stop all fluid intake including water. The patient must be closely supervised at all times to prevent any fluid intake. 

Ask the patient to empty their bladder, measure the volume, document and discard this urine at the start of the test. All urine passed will be measured throughout the test. Cannulate for blood sampling if requested.

T=30 mins, 8.30am, take blood sample into a plain clotted tube for urea and electrolytes and urgent osmolality labeled P1.

T=60 mins, 9am, take urine sample in plain universal pot for osmolality labeled U1.

If the first urine osmolality is >600mOsmol/kg this is normal. The test should be stopped and the patient may go home. If any subsequent urine osmolality is >750 this is also normal and so the test should be stopped, the patient reassured, offered a drink and sent home.

If the first urine osmolality is <300mOsmol/kg and the first plasma osmolality is >300mOsmol/kg, patient can proceed immediately to DDAVP (usually given at 4.30pm) and continue the test from there.

T=2.5 hours, 10.30am, empty bladder (measure, document, then discard sample).

T=3 hours, 11am, take blood sample and label as P2.

T=3.5 hours, 11.30am, take urine sample, label as U2 and weigh patient again.

T=5.5 hours, 1.30am, weigh patient again and empty bladder (measure, document, then discard urine).

T=6 hours, 2pm, take blood for osmolality and label P3.

T=6.5 hours, 2.30pm, take urine sample, label U3, and weigh patient again.

T=7.5 hours, 3.30pm, urine sample, label U4, and blood sample, label P4 and weigh patient again. 

At this stage, a doctor should review the patient and results. If 3% of body weight is lost or the patient is distressed, measure osmolalities urgently. If plasma osmolality has risen to >300mOsmol/kg, proceed to give DDAVP. If the urine output has not decreased, and the urine:plasma  ratio is less than two, but plasma osmolality remains below 295, continue the test.

T=8 hours, 4pm, take a further urine sample, labelled U5, and blood labelled P5 and weigh patient again.

At 4.30pm, give desmopressin (DDAVP) 2mcg intramuscularly as prescribed. The patient may now drink normally, but fluid intake should be restricted to 1 litre during the remainder of the test. The patient should be advised not to drink more than a further 1 litre of fluid until after midnight.

T=1 hours post DDAVP, take urine for osmolality, label U6.

T=2 hours post DDAVP, take urine for osmolality, label U7. The test may be terminated at this stage, at the discretion of senior nurse or consultant. However, if the diagnosis is not clear, continue.

T=3 hours post DDAVP, take urine for osmolality, and label U8.

T=4 hours post DDAVP, take urine for osmolality, label U9.

At any stage in the test, if 3% of body weight is lost or the patient becomes distressed for example with a severe headache, ask  a doctor to review, and measure osmolalities urgently. If plasma osmolality has risen to >300mOsmol/kg, proceed to give DDAVP, allow patient to drink, and continue the test from there. If plasma osmolality is still <275, this indicates that the patient was fluid overloaded prior to the test, and the test should be continued.

After DDAVP, the urine volume is expected to fall dramatically in the presence of cranial DI. If this occurs, send urine for urgent osmolality and if this has risen dramatically, the test can be stopped at t=1 hour post DDAVP. If the urine output is unchanged or osmolality has not risen post DDAVP, continue serial sampling until it does, or to t=4 hours post DDAVP, whichever occurs sooner.

Printer Friendly Version

Patient information sheet

Water deprivation test

Your Doctor has referred you to have a water deprivation test in this department, which is performed over a period of a day (8am to 7.30pm). This is to establish whether or not you are producing enough of a hormone called anti-diuretic hormone, from a small gland called the pituitary. 

You may eat and drink, and take your usual medications on the morning you come for the test. The only exception to this is if you are taking a tablet or spray of desmopressin, which should be stopped 24-36 hours before the test after discussion with your doctor. 

You need to attend the clinical investigation unit at 8am. Urine and blood samples will be taken then and at periodic intervals throughout the day and taken to the biochemistry department for analysis.

You will not be allowed to drink from the time you arrive in the unit and will be allowed dry food only until 4.30pm at the latest.

Depending on your results, the test may be much shorter than this, but please allow the whole day in case you have to stay. If the test has not proved normal by 4pm, you will be given an injection of anti-diuretic hormone. You will then be allowed to drink but you will be required to remain in the unit for further urine and blood samples to be taken and the test should then end at 7pm.

Please do not drink alcohol the night before you have this test and avoid smoking, and drinking tea, coffee, cola and energy drinks for 24 hours before the test. Caffeine, alcohol and smoking can all interfere with the results.

You may wish to bring a book or magazine to read. We do have a television and a radio; alternatively you may like to bring your own personal radio/cd player (including your own headphones). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged from staying in the unit as this can be disruptive to other patients having tests, however if this poses a particular problem for you please discuss this with the senior nurse in the unit. You should be able to undertake normal activities after the test is finished and it should not interfere with your ability to drive home, although you may prefer someone to bring you in and collect you.

If you have any queries about the test, please contact:

Printer Friendly Version