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Fasting blood test: how long should you fast and how do you prepare properly?
Find out how long you need to fast before a blood test, whether you can drink water and what to avoid ahead of a fasting blood test.
Have you got a blood test coming up and aren't sure whether it's a fasting blood test or you can have breakfast before going to the medical centre? It's one of the most common questions we're asked when people come to us for a blood test.
Although many people think you always need to fast beforehand, the truth is that not all blood tests require the same preparation. Some tests can be done after eating, while others need a period of fasting to give results that can be interpreted correctly.
So how many hours should you go without food? Can you drink water? What about coffee, medication or exercise?
We explain what you need to bear in mind before having a blood test and why it's important to follow the instructions for the tests you've been asked to have.
How long do you need to fast before a blood test?
When a blood test requires fasting, you will usually be asked to go without food for between 8 and 12 hours, although this can vary depending on the tests being carried out.
During that time you can generally drink plain water, but you should avoid food and other drinks that could alter certain blood values.
For example, if you're booked in to have your blood taken at 9am and you've been told to fast for 10 hours, you'll need to stop eating at 11pm the night before.
However, you shouldn't automatically apply the same timing to every blood test.
Some tests don't require fasting, while others may come with specific instructions. So, before you go to the medical centre, check how long you need to go without food and whether there are any other instructions you should follow.
Why do you need to fast for some blood tests?
After we eat or drink, our bodies absorb various nutrients that pass into the bloodstream.
This process can temporarily change the concentration of certain substances, such as glucose or triglycerides.
If a test needs to measure those levels in a fasting state, having eaten recently can make the results harder to interpret.
This doesn't mean that every blood test done after eating is wrong. It means that some tests require specific conditions so that the result can be interpreted in line with the purpose of the test.
That's why it's important to follow the instructions you're given before your blood is taken.
What blood tests require fasting?
Whether you need to fast depends on the tests requested, the method used and the reason for the blood test.
Here are some common examples.
Fasting glucose test
Glucose is the amount of sugar present in the blood.
When a basal or fasting glucose test is requested, you need to stick to the fasting period you've been given so that the result can be interpreted correctly.
However, there are other tests related to glucose control that don't require the same conditions.
So if you've been asked to have tests to check your blood glucose or to monitor diabetes, you should confirm what preparation applies to your particular test.
Cholesterol and triglyceride tests
A lipid profile measures different values related to the fats in the blood, such as cholesterol and triglycerides.
Nowadays, many lipid profiles can be done without fasting, depending on the purpose of the test and the doctor's assessment.
However, in some cases a fasting blood sample may be requested, particularly when triglycerides need to be studied or results need to be obtained under specific conditions.
So it's best not to assume that every cholesterol test means skipping breakfast.
Other tests that may need specific preparation
Some metabolic, hormone or specialised tests may require fasting, need to be done at a specific time or have other requirements beforehand.
Sometimes the preparation depends on the combination of tests included in the same request.
If you have any doubts about your blood test, check the instructions from the laboratory or the professional who requested it before changing your usual habits.
What blood tests don't require fasting?
Many routine tests can be done without having to go several hours without food.
For example, a standard full blood count doesn't usually require fasting on its own.
The same goes for some hormone tests and other measurements, although there may be requirements relating to the time the blood is taken, medication or your clinical situation.
Also, a blood test can include several different tests.
Even if one of them doesn't require fasting, another on the same request might.
The safest approach is to check the instructions for your blood test as a whole, rather than deciding how to prepare based only on the name of one of the tests.
Can you drink water before a fasting blood test?
Yes. When a fasting blood test is requested, you can generally drink plain water.
In fact, staying well hydrated can make it easier to take blood, as it helps to maintain enough fluid volume in the blood vessels.
It's best to drink water as normal, without drinking excessive amounts just before the test.
However, if you've been given specific instructions about fluid intake, you should follow them.
It's important to distinguish plain water from other drinks that contain sugar, caffeine or other ingredients that can affect certain tests.
Can I drink coffee before a blood test?
If you've been told to fast, it's best not to have coffee before your blood is taken, even black coffee without sugar, unless the professional in charge has specifically told you otherwise.
Coffee contains substances that can affect certain processes in the body, and it isn't considered equivalent to water when fasting for a blood test.
You should also avoid coffee with milk, tea, juice, soft drinks and energy drinks when the instructions say to fast.
Once your blood has been taken, you can usually have breakfast and go back to eating normally, unless you have other tests to do straight afterwards.
What shouldn't you do before a blood test?
As well as sticking to the fasting rules when required, there are other factors that can affect the results of certain tests.
Doing strenuous exercise
Strenuous exercise can temporarily alter some blood values.
If you have a blood test booked, avoid a demanding workout just before your blood is taken if you've been told to rest or avoid exercise.
If you play sport regularly and aren't sure whether you should change your routine the day before, check with the professional who requested the test.
Drinking alcohol
Alcohol can affect certain results.
Some blood tests require you to avoid alcohol for a period before your blood is taken.
How long you need to avoid it depends on the tests requested, so it's best to follow the instructions you've been given.
Smoking or using nicotine products
If you've been told to fast for your blood test, avoid smoking during that time.
Nicotine and other components of tobacco can affect certain values.
Chewing gum
When a test requires fasting, it's best not to chew gum before your blood is taken.
Even sugar-free gum can contain sweeteners or other ingredients, and it isn't part of the usual conditions for fasting before a blood test.
Changing your medication on your own initiative
This point is especially important.
Do not stop, delay or change any medication on your own in order to have a blood test.
Some treatments can affect certain tests, but it's the healthcare professional who should tell you whether you need to change the time you take them or follow any specific instructions.
If you take medication regularly, ask in advance what you should do on the day of your blood test.
What happens if I take medication before a blood test?
It depends on the medicine and the tests that have been requested.
In many cases, you can carry on with your usual treatment. However, some tests need to be done before or after taking a medicine, or under particular conditions.
It's also important to mention whether you take vitamins, food supplements or herbal remedies.
Some supplements, such as those containing biotin, can interfere with certain laboratory methods.
If you're unsure, tell the centre what you take and check with the professional responsible for your blood test.
It isn't advisable to decide on your own to stop taking a medication simply because you're going to have a blood test.
What if I have diabetes and need to fast for a blood test?
If you have diabetes, especially if you use insulin or medicines that can cause low blood sugar, it's important to ask beforehand how you should prepare.
Going several hours without food while keeping to your usual medication without proper guidance can increase the risk of hypoglycaemia in some people.
So, before your appointment, check with the professional who manages your treatment whether you need to fast, when you should take your medication and what to do if you notice symptoms of low blood sugar.
Don't extend your fast or change your treatment on your own initiative.
What happens if I've eaten before a fasting blood test?
If you've had breakfast or eaten anything during the fasting period you were given, tell the healthcare staff before your blood is taken.
Depending on the tests requested, it may be possible to do part of the blood test, or it may need to be rearranged.
The important thing is to tell them.
Keeping quiet about having eaten can make the results harder to interpret and, in some circumstances, mean the test has to be repeated.
Nor is it a good idea to try to make up for it by fasting for many more hours than you were told to.
Is it better to have a blood test in the morning?
When a blood test requires fasting, booking it for the morning is usually more convenient.
That way, a good part of the time without food passes overnight.
In addition, certain tests need to be done at a specific time because some of the body's values vary throughout the day.
However, not all blood tests have to be done in the morning.
If your test doesn't require fasting or a specific time, it may be possible to have it done at another time, as advised by the centre.
How should you prepare for a blood test? Practical tips
Before you go for your blood test, it's worth checking a few things that can help you avoid uncertainty and possible delays.
- Confirm whether your blood test requires fasting and for how many hours.
- Ask whether you can drink water and whether there are any other instructions to follow.
- Check what to do about your usual medication.
- Mention any food supplements you take.
- Let them know if you have diabetes, are pregnant or have any medical condition that could affect the preparation.
- Avoid strenuous exercise, alcohol or other activities if the instructions for your test say so.
- Tell the staff if you tend to feel faint when having blood taken.
If you need to fast, it can be handy to bring something for breakfast once the test is over, as long as you don't have other tests that require you to keep fasting.
Blood tests in Torre del Mar: get your questions answered before you come in
Preparing properly for a blood test is important to obtain results that can be interpreted in line with the tests requested.
At RincĂłn Salud we have a lab testing service in Torre del Mar that includes a range of laboratory tests.
If you need a blood test and don't know whether you have to fast, you can check beforehand which preparation applies to the tests you've been asked to have.
Our team can guide you through the blood sampling process and help with any questions about the service.
