Water deprivation test

Your doctor has recommended that you have a water deprivation test.

Why do I need a water deprivation test?

This test is used to diagnose or to exclude deficiency, or resistance to, the hormone arginine vasopressin (AVP). AVP, also known as antidiuretic hormone (ADH) controls your body’s fluid balance. The rare conditions of AVP deficiency and AVP resistance were previously collectively termed diabetes insipidus.

AVP (or ADH) is produced by a gland called the hypothalamus before being transported to and stored in another gland called the pituitary, (a small gland, about the size of a pea, which sits at the base of the brain behind the bridge of your nose). If there is not enough AVP being produced, or the kidneys are not able to respond to the hormone, then your body cannot hold on to enough fluid and your urine becomes very dilute.

If your body is losing too much water, you will become very thirsty and will need to drink large amounts of fluid to try and re-hydrate yourself.

What do I need to do before the test?

One week before: If you are taking desmopressin tablets or spray then please discuss this with the doctor ordering the test. If you are taking certain other medication (including water tablets, carbamazepine, phenytoin, lithium and medicines called “flozins” [also called SGLT-2i or SGLT-2 inhibitors]), your doctor may advise you to stop these up to one week before the test, if safe and possible to do so. Alternative medication may be needed during this period. All this will be discussed at your appointment with the doctor who has asked for this test.

Please call us on 01223 217848 if you have any questions.

One day before: Avoid coffee, tea and only drink clear fluids as required to quench your thirst. Please fill in the food and drink sheet (enclosed) for the 24 hours prior to the test and bring it with you to your appointment. (This is meant as a useful tool, it is not necessary to alter your normal habits to fill every time space.)

From 22:00 the evening before: No smoking or alcohol at all.

On the morning of the test: Have a light breakfast such as toast around 06:00-06:30 but no tea or coffee. Continue normal steroid replacement and drink guided by your thirst, but try to avoid drinking large volumes of fluid if possible.

What happens in the test?

You will be allocated a bed for the day; however, you will not be required to undress. You will be weighed, asked to pass urine and then have a small cannula inserted into your arm so we can take blood samples throughout the day.

The water deprivation test involves not drinking anything and having your urine measured every hour, being weighed and some blood taken through the cannula every two hours. If you have diabetes insipidus, then even if you have stopped drinking you will continue to pass larger than average amounts of dilute urine, as your body will not be able to concentrate the urine.

Can I eat and drink anything?

You will not be able to eat, drink or smoke for the first eight hours of the test.

How long does this test take?

You will be with us for up to 12 hours for the complete test. Drinking and eating is allowed after 8 hours. Your cannula will be removed from your arm before you go home.

Are there any risks with this test?

There is a risk of dehydration with this test. This is why the nursing staff will be monitoring you very closely to observe for any signs of dehydration. If you are becoming significantly dehydrated or if you feel unwell the test will be stopped.

If the test shows that you are unable to normally concentrate your urine within eight hours then you will have an injection called Desmopressin, (anti-diuretic hormone). This is to test whether your body can respond and concentrate your urine. We will continue to monitor your urine every hour after this injection. You will be able to eat and drink freely once the Desmopressin has been given.

When can I have the results of this test?

You be able to discuss the results with the doctor at your next endocrine clinic appointment.

Any other questions?

Please ring the endocrine nurses office on 01223 217848. Monday to Friday 09:00-16:30.