How to Prepare for a Blood Test: Fasting, Water, Exercise and Supplements

Preparing for a blood test mostly means following the specific instruction you were given, drinking water, telling the person taking your blood about your medicines and supplements, and avoiding a hard workout the day before; most routine tests do not need fasting at all. That sounds almost too simple, but the confusion around blood test preparation is real, and it produces avoidable repeat tests, fainting in the chair, and results that look worse than they are.

I have had bloods taken more times than I can count since my blood pressure was first flagged, and I have managed to get the preparation wrong in most of the available ways. I once fasted for a test that did not need it, skipped water “to be safe”, and needed two attempts to find a vein. Another time I went straight from a long run to the clinic and spent a week worrying about a flagged enzyme. None of this was dangerous. All of it was unnecessary.

Fasting: when it matters and what it means

You only need to fast if you have been asked to, and when you are, it means nothing to eat or drink other than water. The NHS describes fasting as not eating or drinking anything other than water for a short time before some tests, and says the GP, nurse, or specialist should tell you if you need to do anything to prepare ¹.

Many of the tests in a routine checkup do not need it. A full blood count, kidney and liver panels, thyroid tests, and HbA1c are usually taken without fasting; what each of them measures is covered in the guides to the full blood count, kidney and liver function tests, and thyroid function screening. A fasting glucose, by definition, does need a fast, which is one reason HbA1c has become such a common choice for diabetes screening, as explained in blood glucose and diabetes screening.

Booking letters do not always keep up. If yours says “fast from 10pm” for a panel that you have been told does not need it, ring and ask. A template that was written for one test is often sent out for all of them.

Cholesterol tests and the end of the routine fast

For most people, a cholesterol test no longer needs fasting, because a normal meal changes the results only slightly. A 2016 joint consensus from the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine concluded that fasting is not routinely required for a lipid profile ². In the 1 to 6 hours after a normal meal, triglycerides rose by up to about 0.3 mmol/L (26 mg/dL), and total cholesterol, LDL, and non-HDL cholesterol changed by up to about 0.2 mmol/L (8 mg/dL), while HDL barely moved ².

A fasting sample may still be asked for in specific situations, such as very high triglycerides or when another fasting test is being taken at the same time ². If you are asked to fast for cholesterol, follow the instruction; just know that it is no longer the default everywhere. The cholesterol and lipid screening guide explains what each line of the result means.

Water, food, and what breaks a fast

Water is allowed before almost every blood test, and it usually makes the test easier. Fasting instructions specifically exclude water from the “nothing to eat or drink” rule ¹. Being dehydrated tends to make veins flatter and harder to find, which is how I ended up with two attempts and a bruise.

What counts as breaking a fast is less clear for drinks like black coffee. The safest reading of a fasting instruction is water only. If you would rather have your morning coffee, ask when you book; for tests that do not need fasting, it is usually fine.

Medicines and supplements

Do not stop prescribed medicines before a blood test unless you have been told to, but do tell the person taking your blood what you take, including supplements. Some tests need changes: the NHS notes that you may need to stop certain medicines, such as anticoagulants, before some tests, and only the person ordering the test can tell you whether that applies ¹. Stopping something on your own initiative can do more harm than the test does good.

Supplements deserve a special mention because people rarely think of them as medicines. High-dose biotin, often sold for hair, skin, and nails, can interfere with tests that rely on a biotin-based reaction, including some hormone, heart, tumour, and infection markers, and can push results falsely up or down ³. Thyroid results can be affected, including a falsely low TSH, and the U.S. FDA has reported a death linked to a falsely low troponin result ³. Singapore’s Health Sciences Authority flags products containing 150 micrograms or more per dose as a potential concern, and notes that some hospital laboratories ask patients to stop biotin for at least 12 hours before a blood draw ³. Ask your clinician or the lab what they want you to do.

Exercise, alcohol, and the day before

A rest day before a planned blood test avoids some of the commonest false alarms. Strenuous exercise can raise markers that leak from working muscle, including some enzymes that also appear on liver panels, and a heavy session the day before can make a result look abnormal for an entirely harmless reason. It happened to me after a long run, and it is a pattern readers on this site have described more than once.

The same logic applies to anything that temporarily shifts your body chemistry: a heavy night of drinking, a viral illness, or an unusually large meal before a non-fasting test. None of these makes a test invalid, but if your result comes back slightly out of range, it helps to be able to tell your clinician what the day before looked like. Reference ranges are built from people tested in ordinary conditions, and a result just outside the range is often not clinically important ⁴. The guide to understanding your blood test results goes into how to read a flagged value.

On the day

The test itself usually takes about 10 minutes, and the main things to bring are your instructions, a list of medicines, and a full water bottle. Blood is usually taken from the inside of the elbow with a needle, sometimes after a tourniquet is placed on the arm; you will feel a scratch, and some bruising or swelling afterwards is common and fades over a few days ¹.

A few practical habits help. Wear a top with sleeves that roll up easily. Mention if you have fainted during blood tests before, so you can lie down. If you have fasted, bring something to eat for afterwards. And ask when and how results will come back: they often arrive within a few days and usually within a few weeks, and if you hear nothing after a few weeks, it is worth contacting the clinic rather than assuming all is well ¹.

A short checklist

Before your next blood test, run through five questions.

1. Do I need to fast for this specific test, and for how long?
2. Am I drinking enough water?
3. Have I told them about every medicine and supplement I take, including biotin?
4. Have I avoided a hard workout and a heavy night in the day or so before?
5. Do I know how and when the results will come back?

It is not a glamorous list, but getting it right means the number on the printout is about you, not about what you did the night before.

This article is general information, not medical advice. Always follow the specific instructions from the clinician or laboratory arranging your test, and ask them if anything is unclear.