How long to fast before a blood test: times, rules and what breaks a fast

There is no universal number. The NHS says that for some tests you may need to fast, meaning not eating or drinking anything other than water for a short time beforehand, and that the healthcare professional arranging the test should tell you. East Kent Hospitals puts it plainly: your doctor will tell you how long to fast, because it is different for each test.

If you have not been told and the form does not say, the safest step is to ask the service that requested the test before the day. Most routine blood tests, such as a full blood count, do not need fasting, and Lab Tests Online UK says so for the full blood count and for a standard cholesterol test.

What the trusts publish is a range, and most fasting tests sit between about 8 and 14 hours. In practice that means eating your evening meal at a normal time, going without breakfast, and attending an early morning appointment.

The table sets the published figures side by side. It is a reference to show how the numbers vary, not a rule to apply to yourself. Different laboratories choose their own protocols, and your request form is the authority for your test.

East Kent describes the pattern clearly. You can eat dinner the night before. You do not eat breakfast on the morning of your blood test. You drink only water on the morning of the test. You do not drink tea, coffee or fizzy drinks, which includes black coffee and sugar-free drinks, and you do not chew gum or smoke, which includes vaping and nicotine patches.

University Hospitals Plymouth gives a slightly longer list: no food of any kind, no drink other than plain unflavoured water (tea or coffee with or without milk or sugar are excluded), no chewing gum even if sugar-free, no mints, cough drops, linctus or throat lozenges, and no smoking. It states that if you do any of these within the fasting window, the test will be unable to be performed.

Why is the restriction so strict? East Kent explains that food and drink contain vitamins, minerals, fats, carbohydrates and proteins which can cloud your test results and affect your blood-level readings. In other words, a result taken after eating may not be comparable with the reference range for a fasted sample.

Water is permitted. East Sussex Healthcare writes that patients must have nothing to eat and only drink plain, unflavoured water. Staying hydrated may also make the blood easier to take, which East Sussex mentions as a benefit for people who are known to be hard to bleed.

Regular medicines are a separate question. East Kent says not to change your medication unless your doctor tells you, and that if you need to take a medicine with food and this will interfere with the timing of a fasting blood test, you should speak to your doctor. Do not decide yourself to skip a prescribed tablet.

Diabetes needs particular care because fasting can affect blood sugar control. East Kent says if you are diabetic you should speak to your doctor before you begin fasting for any test, and East Sussex lists shorter times for people using insulin (10 hours) than for people not using it (14 hours) for glucose and lipid tests. These are decisions for your own diabetes team.

Lab Tests Online UK says that for a cholesterol test no special preparation is needed and you can eat and drink normally, although a full lipid profile may need an 8 to 12 hour fast if your doctor asks. For a full blood count it says you do not need to do anything special, and the same is said for bilirubin.

This is why many people attend for blood tests in the afternoon without any restriction. The NHS lists common blood tests such as full blood count, blood glucose, urea and electrolytes, lipid profile, total protein and thyroid stimulating hormone, but it does not give a fasting rule for each. Where a fast is needed the form or the person arranging the test says so.

Because fasting needs vary even for the same type of test, it is worth asking rather than assuming. If your form is silent and you are unsure, contact the service that requested the test before the day.

Do not hide it. Tell the phlebotomist or the surgery. Plymouth says the test will be unable to be performed if the fasting rules are broken, so the sensible outcome is to rebook rather than to have a result that cannot be interpreted. Ashford and St Peter's says that not following the instructions may lead to delay or the need for blood to be taken again.

A fasting appointment is best booked early in the day, which East Sussex Healthcare recommends, to keep the fast manageable. If fasting makes you feel faint or unwell, say so, and tell the phlebotomist if you have fainted during blood tests before.