Can I leave water out during the fast?

In the vast majority of cases yes, and it is usually better. Fasting is about food, not water, and a well hydrated animal makes the draw easier and some values more reliable. There are rare exceptions where the clinic asks you to remove water too for certain specific tests: in that case they will tell you explicitly. If nothing was said, leave water available.

My cat stole a bite overnight: should I cancel the appointment?

Do not cancel on your own, but tell whoever does the draw straight away. Depending on how much it ate and which tests are ordered, you may go ahead anyway, postpone only some values, or reschedule. The call is the clinic's, and it needs the honest fact: how much and at what time. Hiding the slip only leads to misreading the results.

Should I still give the medicines on the morning of the test?

It depends on the drug and is not for you to decide alone. Some are given as normal, some are paused, some must be taken right at the time of the draw because the value is read against the dose. Heart, thyroid and epilepsy medicines often need a precise instruction. Ask the clinic in advance, drug by drug, and bring the time of the last dose with you.

Exactly how many hours of fasting are needed?

For a healthy adult dog or cat, 8 to 12 hours usually does it, which in practice means removing food after dinner and going in the morning. But small puppies, diabetics and frail seniors follow different rules, and some tests need no fast at all. Ask the clinic for the time that applies to your tests rather than applying a generic number.

Health management25 minLevel: BeginnerUpdated on August 18, 2026

Preparing the fast before your pet's blood test

How many hours of fasting are really needed, when water is allowed and which medicines not to stop: how to reach the draw with reliable values and no risks.

Audience: Pet owners
Species: Dog, Cat
Scope: Valid everywhere, Italy, European Union

What you need before you start

La prescrizione o il messaggio della clinica con l'elenco degli esami richiesti
L'orario esatto dell'appuntamento per il prelievo, così da calcolare a ritroso il digiuno
L'elenco dei farmaci in corso, con dose e orario, da mostrare a chi esegue il prelievo

A blood test is only as good as the preparation before it. A meal given too close to the draw can cloud the sample, raise blood sugar and skew the blood fats, to the point where some values become unusable and the draw has to be repeated. At the same time, a fast that is too long, or applied to the wrong animal, can be dangerous, especially in a small puppy or a diabetic on treatment. The truth is there is no single fast that fits everyone: it depends on the tests ordered, the species, the age and the medicines in use. This guide helps you work out when a fast is genuinely needed, for how many hours, how to handle water and drugs, and how to organise the night before and the morning itself so the draw is useful the first time round.

Why fasting changes the results

After a meal the blood briefly fills with sugars and fats absorbed from the gut. Excess fat makes the sample milky, a state called lipaemia that interferes with the analyser and can distort many values, not just the fats. Blood sugar rises, and in a fasted animal a high reading means something different from the same reading two hours after lunch. That is why the clinic asks for a fast: not out of fussiness, but because many reference ranges are built on the animal with an empty stomach.

How many hours by species and age

The intervals below are the ones most used in practice for healthy adults, as a first draft to confirm with the clinic. The rough starting rule is to remove food in the evening and bring the animal in the morning, but the exceptions matter: a puppy of a few weeks and a diabetic on treatment do not follow the healthy adult pattern.

Indicative fasting times before a blood draw. The binding reference remains the clinic that ordered the tests.
Animal	Indicative fast	Water	Note
Healthy adult dog	8-12 hours	Always available	Usually enough to remove food after dinner
Healthy adult cat	8-12 hours	Always available	Remove treats too and block access to another cat's food
Puppy or small kitten	Only on advice	Always available	A long fast can drop blood sugar, never improvise it
Diabetic on treatment	To be agreed	Always available	Insulin and meal must be managed with the clinic, not alone
Frail senior animal	8-12 hours with care	Always available	Tell the clinic if it skips meals or already eats little

Water and medicines: what not to stop

Fasting is about food, not water. In the large majority of cases water stays available throughout: a dehydrated animal is harder to draw from, and some values, such as the kidney ones, shift precisely with hydration. On medicines, though, the rule is not to decide alone.

Leave water available until you head out, unless the clinic tells you otherwise explicitly.
Do not stop an ongoing treatment on your own: some drugs are given as usual, some at the time of the draw, some paused, and only the clinic knows which.
For heart, epilepsy and thyroid medicines, ask ahead whether and when to give them on the test day.
If a drug is meant to be taken with food, ask how to handle it during the fast rather than quietly skipping it.
Bring the medicine list with dose and time of the last one given, because some values are read against when the dose was taken.

The night before and the morning of the draw

The trickiest part is not the fast itself, but stopping the animal from finding food on the sly. A mouthful stolen from another animal's bowl, a treat handed over out of habit on the way out, a scrap found on the floor: it takes very little to undo the whole preparation. Run the night before and the morning like a small procedure.

Work out the time of the last meal
Start from the appointment time and subtract the agreed fasting hours. If the draw is at 9 and 12 hours are needed, the last meal is by 9 the evening before. Set a reminder so you do not forget it.
Clear away bowls, treats and scraps
After the last meal, remove the food bowls, close the treat bags and check no scraps are left on the floor or the table. Leave only water.
Separate animals that share the home
If you have several animals, the one that must fast should not be able to reach the others' bowls. In a multi cat home this is where the preparation most often falls apart.
Do not hand out treats as you leave
The automatic little snack before getting in the car ruins twelve hours of fasting in a second. Warn anyone in the house who might give one without thinking.
Bring food for afterwards
Pack the meal or a few treats to give right after the draw. That way the animal eats as soon as it is done, rather than staying empty during the journey home.
Note any slip in the rule
If the animal stole food or vomited during the fast, tell whoever does the draw. It is information that changes the reading, not a fault to hide.

Recording everything for comparison over time

A single blood test says far less than a series of comparable ones. For the comparison to hold, though, the conditions have to be similar: same fast, same time of day, same hydration. Recording how you prepared each draw makes later checks genuinely comparable and helps the vet tell a real change apart from a mere effect of the preparation.

Note the date, the time of the draw and the actual fasting hours for each test.
Record the drugs given that day, with dose and time, next to the results.
Keep the report together with the note on preparation, so the comparison is at hand.
Where you can, repeat the same time of day for later checks of the same value.

Frequently asked questions

Can I leave water out during the fast?: In the vast majority of cases yes, and it is usually better. Fasting is about food, not water, and a well hydrated animal makes the draw easier and some values more reliable. There are rare exceptions where the clinic asks you to remove water too for certain specific tests: in that case they will tell you explicitly. If nothing was said, leave water available.
My cat stole a bite overnight: should I cancel the appointment?: Do not cancel on your own, but tell whoever does the draw straight away. Depending on how much it ate and which tests are ordered, you may go ahead anyway, postpone only some values, or reschedule. The call is the clinic's, and it needs the honest fact: how much and at what time. Hiding the slip only leads to misreading the results.
Should I still give the medicines on the morning of the test?: It depends on the drug and is not for you to decide alone. Some are given as normal, some are paused, some must be taken right at the time of the draw because the value is read against the dose. Heart, thyroid and epilepsy medicines often need a precise instruction. Ask the clinic in advance, drug by drug, and bring the time of the last dose with you.
Exactly how many hours of fasting are needed?: For a healthy adult dog or cat, 8 to 12 hours usually does it, which in practice means removing food after dinner and going in the morning. But small puppies, diabetics and frail seniors follow different rules, and some tests need no fast at all. Ask the clinic for the time that applies to your tests rather than applying a generic number.

What to do next

Ask the clinic which tests need fasting and for how many hours, then count back from the appointment to fix the time of the last meal. Leave water available unless told otherwise, never stop medicines on your own, and the night before clear bowls, treats and scraps while separating the animals. Bring food for after the draw and record the fast and the drugs so later checks stay comparable over time.