Before a blood test, many people wonder whether they need to fast, whether they are allowed to drink water and whether they can take their medicines as usual.
The short answer: Not every blood test requires fasting. If fasting is required, the usual period is around 8–12 hours. During that time you should normally drink only plain water and follow the instructions given by your doctor or the testing centre.
Frequently asked questions
How long do you need to fast for a blood test?
The usual period is 8–12 hours, depending on the type of test. Do not decide to fast for longer on your own, as this can cause tiredness, dizziness or low blood glucose.
Can you drink water while fasting for a blood test?
Normally you can drink a moderate amount of unsweetened plain water. You should not drink coffee, tea, milk, fruit juice or soft drinks.
Do you need to fast for a lipid profile?
Not in every case. Your doctor may ask you to fast if triglycerides need to be assessed or if an earlier result needs clarifying.
Do you need to fast for an HbA1c test?
Normally not. However, if HbA1c is done together with a fasting blood glucose or another test, you may still be asked to fast.
Can you take your medicines before a blood test?
Do not stop taking your medicines on your own. Ask your doctor if you are taking insulin, diabetes medicines, anticoagulants, thyroid medicines, corticosteroids or any long-term treatment.
“Before having a test, ask your doctor how you need to prepare.”
Why do some tests require fasting?
After you eat or drink, sugars, fats and other nutrients are absorbed into the blood. This can temporarily change some of the readings.
Fasting as instructed limits the effect of your most recent meal and helps your doctor assess the results more accurately.
Fasting is not necessary for every test, however. According to the NHS, how you prepare depends on the type of test, your state of health and the medicines you are taking.
Which tests may require fasting?
Tests that commonly require fasting include:
- Fasting blood glucose
- Glucose tolerance test
- Some lipid profile tests, particularly triglycerides
- Certain specialised hormone or metabolic tests
The exact requirements can differ from person to person and from one testing centre to another. If your request form does not state this clearly, you should check before the day of your blood draw.
Which tests usually do not require fasting?
Tests that usually do not require fasting include:
- Full blood count
- HbA1c
- Blood group
- Some thyroid tests
- Some liver function, kidney function or immunology tests
Even so, a health-check package may include several tests. You may still be asked to fast if the package includes a blood glucose or lipid profile check.
What can you drink while fasting?
The most suitable choice is usually unsweetened plain water, if the testing centre allows it.
You should avoid:
- Coffee, including black coffee
- Tea
- Milk and plant-based milks
- Fruit juice
- Soft drinks and energy drinks
- Vitamin or electrolyte waters
- Alcohol
Sugar-free drinks can still affect some of the readings. “Sugar-free” therefore does not mean “allowed while fasting”.
Can you smoke or chew gum?
You should avoid smoking, vaping and chewing gum before your blood is taken.
You should also avoid high-intensity exercise on the morning of the test, as vigorous physical activity can temporarily change some of the readings.
Do you need to stop your medicines before a blood test?
Do not stop taking your medicines on your own.
Some medicines should be continued as normal, while for others the timing of the dose may need adjusting. Your doctor will decide on the basis of the medicine, your underlying conditions and the test to be carried out.
Do tell your doctor if you are taking:
- Insulin or diabetes medicines
- Anticoagulants
- Blood pressure or heart medicines
- Thyroid medicines
- Corticosteroids
- Hormone medicines
- Vitamins, supplements or herbal remedies
If a medicine has to be taken with food, ask your doctor what to do before the day of the test.
What if you accidentally eat or drink coffee?
Tell the healthcare staff straight away, before your blood is taken. Do not keep it to yourself, because the results may be affected.
Depending on the type of test, the healthcare facility may:
- Still take the sample and record that you have not fasted
- Carry out only the tests that do not require fasting
- Adjust the time of the blood draw
- Rebook you for another day
Who needs to take care when fasting?
The following groups should ask their doctor before fasting:
- People with diabetes
- Women who are pregnant or breastfeeding
- Children
- Older people or those who are frail
- People who have had low blood glucose before
- People taking insulin or glucose-lowering medicines
- People with endocrine conditions or metabolic disorders
If you develop trembling hands, sweating, dizziness, confusion, a racing heartbeat or the feeling that you are about to faint, tell the healthcare staff immediately.
Checklist before a blood test
The evening before
- Confirm whether the test requires fasting
- Check what time the fast should start
- Eat a normal evening meal, without too much fat
- Avoid alcohol and excessive exercise
- Prepare a list of the medicines you are taking
The morning of the test
- Do not eat if you have been asked to fast
- Drink only plain water, if this is allowed
- Do not drink tea, coffee, milk or fruit juice
- Do not smoke or chew gum
- Do not skip your medicines on your own
- Tell the healthcare staff if you have eaten or drunk something by mistake
What should you do after your blood is taken?
After your blood has been taken, you should:
- Press gently on the cotton wool at the puncture site
- Sit and rest if you feel dizzy
- Have a light snack once the test is finished
- Drink enough water
- Avoid driving yourself if you feel unwell
A small bruise at the puncture site will usually clear up on its own. If bleeding carries on, there is a lot of swelling and pain, or any unusual signs appear, contact your healthcare provider.
Appointment booking for blood tests on Halza
Preparing properly reduces the chance of needing a repeat sample and helps your doctor interpret the results in the right context.
With Halza, you can:
- Search for testing services at partner healthcare providers
- Look through test packages that suit your needs
- Book in advance so you can plan your time
- Get help confirming your appointment
- Store and track your health results in the app
Book your blood test on Halza to receive the right preparation guidance before the day of your sample.
The information in this article is for reference only and does not replace the advice, diagnosis or instructions of a doctor. Fasting times and the way medicines are taken may differ depending on the test and on your state of health.
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