Blood test preparation: what to ask before you go

In short

• Find out the exact name of each test and get instructions for that test.
• Fasting is needed only for some tests. Do not fast out of habit.
• Tell your clinician about every prescription and over-the-counter medicine, vitamin and supplement, and do not stop any unless you are told to.
• If you could not follow an instruction, say so before the blood is drawn, because even a small change can affect results.
• Ask when and how you will get your results, and who will explain them.

Why preparation matters

A blood test shows what is in a sample at one moment. Food, drink, exercise, smoking, some medicines and supplements can change certain results, so the person reading your results needs to know the conditions. That does not mean that every slip ruins a test. It means honest information helps your clinician interpret the numbers or decide to reschedule.

MedlinePlus says that even a minor change from the instructions can have a big effect on your results. If you could not follow the instructions exactly, tell your provider or the lab staff. Do not hide a cup of coffee, a snack or a supplement.

First, confirm which test was ordered

Simply being told to get blood drawn is not enough information. Your order might include a complete blood count, blood chemistry, cholesterol, hormone tests or a drug level, and the requirements differ. Ask which tests are ordered and whether any of them needs fasting, a particular time of day, or limits on food, drink, exercise or medicines. Be sure you understand your instructions before the day of your test.

If what your clinician told you differs from the lab's sheet, do not choose between them yourself. Contact the office that ordered the test or the lab, ideally the day before rather than on the morning of your appointment. Save the instructions on your phone or on paper.

Fasting for a blood test

Fasting means not eating or drinking anything except plain water for several hours before the test. MedlinePlus says you usually need to fast for 8 to 12 hours, and that your provider gives the exact length for your test. Tests that often need fasting include blood glucose and cholesterol (lipid panel) tests, and some liver and kidney tests and the basic metabolic panel may also need it.

• Do not eat or drink anything except plain water during the fast.
• Avoid chewing gum, smoking and exercise while fasting, as MedlinePlus advises.
• Flavored or carbonated waters may contain sugar or sweeteners, so they do not count as plain water.
• Ask before you fast if you have a condition or take medicines that make fasting difficult, and make a safe plan first.
• Do not reuse an old fasting rule for a new test.

Plain water is allowed during a fast. MedlinePlus notes that some blood tests call for extra water to help keep fluid in your veins. Ask directly whether you can have water, how much, and until what time. If you have been told to limit fluids for a heart, kidney or other condition, do not change that limit for a test without checking.

Medicines, vitamins and supplements

Make a list of everything you take: prescription and over-the-counter medicines, vitamins and supplements. MedlinePlus advises telling your provider about all of them and not stopping any unless your provider says so. The NHS notes that you may be asked to pause certain medicines, such as blood thinners, for some tests, and that this is for your clinician to decide.

If you have no instructions, ask the clinician who ordered the test ahead of time. Staff in the blood-draw room may not be able to change your treatment plan.

The day before and the morning of

• Follow only the restrictions that apply to your test. Depending on the test, you might be told to skip certain foods or drinks, avoid smoking or strenuous exercise, or not overeat the day before.
• Do not start a cleanse, crash diet or unusual workout to get ready.
• Bring your order, ID and the address and time of your appointment.
• Wear a top whose sleeve rolls up easily above the elbow.
• Note the time of your last meal, drink and medicines if it matters for the test.

If you develop a fever or other new symptoms before the test, call to ask whether the plan still applies.

If you are afraid of needles or feel faint

Tell the person drawing your blood beforehand if you have felt faint or very anxious before. The NHS says you can tell staff you are anxious about needles and bring someone with you for support. A blood draw usually takes about ten minutes, you will feel a scratch as the needle goes in, and mild bruising or swelling should settle within a few days.

MedlinePlus says it is common to feel light-headed after a blood test that requires fasting, so you may want to bring a snack that you can eat right away once you are allowed to.

If you broke the instructions

Do not cancel without a word and do not try to make up for it. Tell the staff exactly what happened and when: a coffee, a meal, a workout or a medicine. The clinician or lab decides whether to proceed, note the circumstance or reschedule. If you realize afterwards, tell the clinician before your results are interpreted.

Getting and discussing your results

Before you leave, ask how long results take and how you will get them. The NHS says results can take a few days and often a few weeks, and that if you have not heard anything after a few weeks you should contact the practice. Your clinician or nurse should explain your results and next steps, and if you do not understand them, ask for someone to go through them with you.

A result outside the reference range is not a diagnosis by itself. It is read with your symptoms, history and medicines, and your treatment should not change just because of a line in a patient portal.

When to see a doctor, and red flags

Contact the clinician who ordered the test if the instructions are unclear, conflict with your medicines, or would require fasting when you have diabetes, are pregnant or tend to faint. Also call if you become unwell before the test.

Red flags: do not wait for a test or a fasting window if you have chest pain, severe trouble breathing, fainting, signs of a stroke or heavy bleeding. Call 911. After a blood draw, get urgent help for bleeding that will not stop with firm pressure, rapidly growing swelling, severe pain or numbness in the hand.

Questions and answers

Sources

1. MedlinePlus — How to Prepare for a Lab Test
2. MedlinePlus — Fasting for a Blood Test
3. NHS — Blood tests

Sources checked on 29 October 2026.