Fasting blood test: how many hours, and which tests still need it

The short answer

If your GP or the hospital has asked for a fasting blood test, the usual rule is no food and no drink except plain water for 8 to 12 hours. In practice that means nothing after your evening meal until the needle is in. For a fasting glucose, Lab Tests Online UK puts the minimum at 8 hours, generally 8 to 10.

Most of the time, though, you do not need to prepare at all. An NHS hospital leaflet says exactly that: in the vast majority of cases there is nothing to do before a blood test. The full blood count on most forms can be taken at any time of day, and since NICE changed its advice a cholesterol check no longer needs a fasting sample by default either.

The reason fasting exists is mechanical. A meal changes the blood for hours, starting with sugar and fats. A fasting glucose taken after breakfast measures the breakfast, not how your body handles sugar on its own.

One rule beats every table, this one included: follow the instruction on your blood test form or from your surgery. If it says fasting and you have eaten, say so before the sample is taken. A fasting glucose read on a sample taken after food is not slightly off — it cannot be interpreted.

What ‘fasting’ means on a blood test form

Nothing to eat, and nothing to drink except water, for the stated number of hours. East Kent Hospitals’ biochemistry leaflet spells out what that excludes on the morning itself: tea, coffee and fizzy drinks, black coffee and sugar-free diet drinks included, chewing gum, smoking, vaping and nicotine patches.

This is less obvious than it sounds. When researchers questioned 150 outpatients arriving for blood tests, only 39% could define fasting correctly, and 46% thought it was enough for the last meal to have been the day before, whatever the hour.

Part of the confusion is not yours. Instructions vary between surgeries and hospitals: one GP practice asks for nothing except water after midnight, a hospital leaflet lets you eat your dinner the night before. Laboratory specialists have pointed out that definitions of fasting also vary across Europe, which is why a European working group set out to harmonise them.

The practical version: finish eating in the evening, drink water as usual overnight and in the morning, and take nothing else until the sample has been taken.

Which tests need a fast: the UK picture

Use this as a guide and check it against your own form: protocols still differ between NHS laboratories, and not all of them have followed the newer advice on cholesterol.

East Kent Hospitals, for example, still lists 12 to 14 hours for lipids, triglycerides and cholesterol, alongside 8 hours for fasting glucose and for testosterone. When a single line on your form needs a fast, the whole appointment becomes a fasting one, even if the urea and electrolytes next to it would not have needed one.

Is 8 hours of fasting enough for a blood sugar test?

Yes. Lab Tests Online UK, the patient site of the Association for Laboratory Medicine, describes a fasting glucose as one taken after at least 8 hours with nothing to eat or drink except water, generally 8 to 10. East Kent Hospitals’ table also lists 8 hours. An overnight fast from your evening meal covers it comfortably.

The precision matters because the UK cut-offs sit close together. A fasting plasma glucose of 7.0 mmol/L or more is in the diabetes range, and NICE treats 5.5 to 6.9 mmol/L as a sign of high risk. A sample that was not truly fasting can push a result across one of those lines.

HbA1c works differently: it needs no preparation, and 48 mmol/mol or more is the diagnostic threshold, while 42 to 47 mmol/mol is called non-diabetic hyperglycaemia, often shortened to prediabetes. If your form lists both, the fasting glucose sets the rule for the morning.

Do you need to fast for a cholesterol test?

Usually not. NICE’s lipid guideline (NG238, December 2023) defines a full lipid profile as total cholesterol, HDL cholesterol and triglycerides, with non-HDL and LDL cholesterol calculated from them, and states that a fasting sample is not mandated. Lab Tests Online UK dates the change in UK practice to 2014.

The differences are small. After an ordinary meal, the largest average changes are about +0.3 mmol/L for triglycerides and −0.2 mmol/L for total and LDL cholesterol, while HDL cholesterol and lipoprotein(a) are largely unaffected, according to a 2016 joint statement from the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine. Non-fasting values predict heart disease and stroke just as well.

A fast comes back in one situation in the NICE guideline: when triglycerides come back between 10 and 20 mmol/L, the measurement is repeated as a fasting lipid test, at least 5 days later and within 2 weeks.

If your surgery or hospital still asks you to fast for cholesterol, follow that instruction for your sample. It is their protocol, and there may be a reason specific to you, such as very high triglycerides on an earlier test.

Water yes, tea and coffee no

Plain water is allowed throughout the fast, still or sparkling, including on the morning of the test. Tea, coffee, juice, milk and fizzy drinks are not, and East Kent’s leaflet names black coffee and sugar-free diet drinks specifically.

Coffee surprises people, black and unsweetened included. In a crossover trial, healthy men given caffeinated coffee an hour before a meal had a blood sugar response 147% to 216% larger than when they were given decaffeinated coffee, and insulin sensitivity fell by up to 40%. In another trial, coffee drunk half an hour before a glucose drink raised insulin compared with water.

The evidence is not one-sided. In a 2023 study of 27 volunteers, one cup of coffee an hour before the test caused no clinically significant change in routine chemistry and blood count results. That is reassuring if you slipped, but it has not changed the rule, which exists to protect the fasting glucose.

Alcohol often comes with its own, longer window: Nuffield Health advises avoiding it for 24 hours before a fasting test. If nobody has told you, ask.

The night before

Eat a normal evening meal. No single food is banned, but laboratory specialists note that the body’s response to a meal varies with what it contains and when it was eaten, and with smoking, coffee and alcohol. An unusually heavy, late or boozy evening is the one to avoid.

Then count backwards from the appointment. For an 8.30am slot and a 12-hour instruction, your last food is at 8.30pm the evening before; for an 8-hour instruction, 12.30am. Booking the first appointment of the morning puts most of the fast inside your sleep.

Medicines: do not stop anything on your own

Take your usual tablets with water unless whoever requested the test has told you otherwise. An NHS hospital leaflet is explicit: if you are unsure about a medicine, keep taking it as prescribed until you have checked, because the doctor can often read the result with the medicine taken into account. Your regular tablets and supplements all belong on the list you mention at the appointment.

The clearest exception concerns diabetes. Insulin and some diabetes tablets, such as gliclazide and glimepiride, can push blood sugar too low, and the NHS lists missed or delayed meals among the triggers. If you take them, speak to your GP or diabetes team before you start fasting, as East Kent’s leaflet asks.

Supplements deserve a word too. If iron studies are on the form, Lab Tests Online UK advises no iron tablets for 24 hours beforehand, and you may be asked to fast for 12 hours. Whatever you take, tell the person taking your blood.

If you ate or drank by mistake

Tell the phlebotomist or practice nurse before the sample is taken, not after. Hospital guidance is plain about why: not following the instructions can affect the result and may mean the test has to be repeated.

It is rarely a wasted trip. Many lines on a routine form do not need a fast, and the staff can tell you whether those can go ahead while the fasting ones are rebooked. What you should not do is stay quiet: a glucose read as fasting when it was not can suggest a problem you do not have, or hide one you do.