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Preparing for a blood test: fasting explained

Guide Published

Some blood tests need fasting or a change to what you eat, drink or take beforehand; the person who arranges the test should say which applies.

Key points

  • Whether a blood test needs preparation depends on the test, and the clinician who arranges it should say what applies.
  • For some tests the preparation is a period without food or drink other than water, and the person arranging the test will say if anything else applies.
  • Food and drink can change some results because nutrients from a meal may still be in the blood at the time of the test.
  • Results usually reach the GP or hospital team, and timing varies from a few days to a few weeks depending on the test.

Does every blood test need preparation?

Whether a blood test needs preparation depends on the type of test. The NHS says the healthcare professional arranging the test will tell you if you need to prepare.

For some tests, preparation means not eating or drinking anything other than water for a set time beforehand.

Why does fasting matter?

Fasting means going without food and drink, apart from water, for a period before the test. The reason is that nutrients from food enter the bloodstream and may still be there when the sample is taken, which can change the results of some blood tests.

The length of the fast depends on the test being done. NHS hospital leaflets give different lengths for different tests, so the person arranging the test can say which applies.

What counts as water only, and what about medicines?

Practices that publish fasting advice usually mean water only. Tea, coffee, including black coffee, and sugar-free drinks are not counted as water, and some practices also list vaping and nicotine patches.

People are advised not to change their medicines unless a doctor has told them to. People with diabetes are advised to check with their doctor before fasting for a test.

What if I did not fast, or I am not sure?

If you are not sure whether fasting applies, the practice or clinic can tell you. A test can still go ahead without fasting, but the doctor may ask for a repeat test after fasting if the result needs it.

What happens to the sample, and when do results come back?

Timing varies. Some results are ready within a few days, and others take a few weeks. Results go to the GP or the hospital team that arranged the test, and some people can also see them in the NHS App.

The NHS advises contacting the GP surgery or specialist if you have not heard anything after a few weeks. A GP appointment is the route for questions about what a result means.

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Educational content. This article summarises public guidance and announcements in plain English for education. It is not medical advice and it cannot say what is right for you: speak to a clinician about your own health, call NHS 111 if you are unsure what to do and 999 in an emergency.