Read our latest advice on Coronavirus (COVID-19)

Our services are open and safe to attend – we are here to help

Water drinking test (WDT) for glaucoma


More in this section


The WDT is a type of stress-test for glaucoma which helps us assess the changes in eye pressure that may happen during the day. Spikes in eye pressure are associated with an increased risk of developing glaucoma, or of your glaucoma getting worse even if you are already on treatment.

When you drink a lot of water in a short time, some of the water stays inside the eyes, increasing the eye pressure. This excess water leaves through the normal drainage channels of the eye. Depending on how well these channels are working, the eye pressure goes up and then comes back down after a time. The WDT is like a ‘stress test’ for the eye drainage channels, and helps us to understand how well your eyes drain excess fluid.

The WDT has replaced the phasing clinic where we measured eye pressures over a whole day. To limit the amount of time you spend in the hospital, we are doing this ‘mini phasing’, WDT instead.

Why you should have this test

We have arranged this test because we need more information on your eye pressures to find out the risk of you developing glaucoma or of it getting worse. The results of this test will help us decide if you:

  • are at risk of developing glaucoma
  • need to start eye pressure-lowering treatment
  • need to have your glaucoma treatment changing
  • would benefit from laser treatment or surgery.

We will also examine your eyes and do other tests, such as a scan of the optic nerve or visual field test, if needed.

What you need to do before the test

You should not have anything to eat or drink for at least 2 hours before the test. You can have breakfast but must have finished by 7am at the latest, as your test usually starts soon after 9am.

If you are already on glaucoma drops, you must use them as usual unless we ask you to stop them. It is very important that you use all your usual morning drops before you leave home. You can also have your usual general health tablets in the morning.

What you need to bring on the day

It is important that you bring your glasses and a list of your current medications. Please bring something to keep you occupied, such as a book or magazine. You can also bring your own water bottle (1 litre) for this test. There is a water fountain in the department.

What happens during this test?

You will have your vision and baseline (usual) eye pressure checked when you arrive.

After this you will be asked to drink 1 litre of water (800mls if you are lightweight) within 5 minutes. Your eye pressures will then be checked every 15 minutes, at least 3 times, so you will need to stay in or around the department for this to be done. We might also arrange other tests in between the eye pressure checks.

You should not have any food or other fluids (including water, coffee or tea) until all the eye pressure measurements have been done. You can go to the toilet if you need to.

What happens after the WDT?

After the eye pressure checks are done you might have drops put in to dilate (widen) your pupils if we need to examine the back of your eyes in detail or you need a scan.

We will discuss your results with you and decide what treatment might help you. This might involve starting glaucoma treatment, changing your treatment, or discussing laser treatment or surgery. We might decide that your glaucoma risk is low and that you do not need treatment now, but may benefit from monitoring for a few years, or we might discharge you back to your optometrist.

Are there any risks for this test?

If you have been advised to restrict your fluid intake, for example in certain heart conditions or kidney failure, please let us know as we will not do this test and will discuss other options for assessing your glaucoma risk.

Even if the WDT causes your eye pressure to go up, you will not need to restrict the amount of fluid you drink for the rest of the day. Eye pressure does not change much when you drink normal amounts of water (2-3 litres) spaced out over the whole day. It is advisable to avoid drinking large amounts of fluid in a short time, particularly if your glaucoma is advanced or getting worse.



Ref number: 5116/VER1

Date published: December 2020 | Review date: December 2023

© 2020 Guy’s and St Thomas’ NHS Foundation Trust

A list of sources is available on request


Where next?

Contact us

The glaucoma call-back service (24-hour answerphone). We will call you back in a few days.

Tel: 020 7188 9121

Printable PDF

Download Water
drinking test
(PDF 335Kb)

PIF TICK logo - trusted health information

Staying safe in our hospitals

We've made some important changes to help keep you safe. Find out more.

Was this helpful?

If you have any comments about this information, we would be happy to hear from you. Fill in our simple online form or email:


Our information is changing

Find out how we are making our patient information more accessible and inclusive.