How to Prepare for a Blood Test

When you actually need special preparation

According to MedlinePlus, most blood tests need no special preparation at all — blood can be drawn at any time of day, regardless of when you last ate. But some tests do require fasting, because food temporarily changes the composition of your blood. MedlinePlus lists these among the tests that typically require fasting:

• the glucose (blood sugar) test
• the lipid panel — total cholesterol, LDL, HDL and triglycerides
• the basic metabolic panel (electrolytes, kidney markers)
• in some cases, liver and kidney function tests

The exact list of tests and whether fasting is required is always specified by the doctor who ordered the test or by the laboratory itself. If your referral form doesn't mention it, it's worth asking in advance rather than finding out once you're already at the clinic.

How long to fast, and what "fasting" actually means

MedlinePlus defines fasting quite specifically: no food or drink for several hours before the test, except plain water. The usual duration is 8–12 hours, with the exact timing for each test set by your doctor. In practice, this usually means eating your last meal in the evening and having the test done the next morning. During the fasting period, MedlinePlus also advises avoiding:

• chewing gum
• smoking
• physical exercise

All three can affect the result even when you haven't eaten — chewing gum and smoking can trigger the release of saliva and stomach acid, while intense exercise temporarily changes the concentration of certain substances in the blood. If you're unsure how many hours to fast before a specific test assigned to you, the doctor who ordered it or the laboratory will give you the most accurate answer, since the requirement depends on the particular type of test. If you'd like a quick estimate, you can also check the "How Long to Fast" calculator.

Alcohol is a separate question. The Norwegian medical encyclopedia Store medisinske leksikon says to avoid alcohol for at least 24 hours before a triglyceride test or a blood glucose test done to diagnose diabetes. For triglycerides, the reason is that alcohol actually raises them, so the result will not reflect your usual state. For glucose, alcohol affects glucose metabolism: according to MedlinePlus, while the liver is breaking down alcohol it releases less glucose into the blood, so blood glucose can drop. The fasting time (usually 8–12 hours) is set by the doctor or the laboratory.

Water, medication, and exercise before the test

Yes, you can drink water — MedlinePlus is clear that fasting means no food or drink *except plain water*. It should be plain water with nothing added: the source specifically warns against water with lemon or artificial sweeteners, since these can also affect the result. Drinking plain water before the test is not just allowed but actually helpful, as it makes the vein easier to find and the blood easier to draw. As for medication, you shouldn't stop taking anything on your own — MedlinePlus states this directly: don't stop any medication unless your doctor has clearly told you to. Mention all prescription and over-the-counter medications, as well as vitamins and supplements, to your doctor in advance; they will assess whether any of them could affect the specific test result and give separate instructions if needed. The decision about taking a medication before a test is always made by the doctor, not the patient. If the test measures the level of the medicine itself in the blood, Oslo University Hospital notes that the blood is usually taken just before the next dose — in that case the doctor gives you the timing. Exercise is one of the things MedlinePlus advises avoiding during the fasting period, both because it can trigger hunger and thirst and because intense exertion temporarily changes the composition of your blood — so if a test requires fasting, it's worth skipping your workout beforehand. Oslo University Hospital also advises avoiding physical activity beyond ordinary walking before the blood is taken, and training or hard physical work in the days before as well. Right before the sample is taken, it is recommended to sit quietly for at least 15 minutes so that the blood volume settles.

What happens during the blood draw itself

The most common method is a venous blood draw: a phlebotomist or nurse uses a needle to take a blood sample from a vein in your arm. According to MedlinePlus, the procedure usually takes less than five minutes and may cause brief discomfort. For children or newborns, a finger-stick or heel-stick method is sometimes used instead. When blood oxygen levels need to be measured, an arterial blood sample is taken, which can hurt more; afterward, it's recommended to avoid strenuous exercise with that arm for a few hours. Some mild pain or bruising may remain at the puncture site afterward — this usually clears up quickly. If you fasted for the test, you can eat right away once the sample is taken; MedlinePlus suggests bringing a snack in case you feel lightheaded after a longer fast. If you realize before the sample is taken that you accidentally ate or drank something you shouldn't have, tell your doctor or the phlebotomist right away — depending on the test, it may need to be postponed to another day so the result is accurate. It's not worth hiding this: an inaccurate result can wrongly look abnormal, or the opposite — mask a real abnormality — which can affect your doctor's later decisions.

Why proper preparation matters for your result

Proper preparation doesn't make the test itself less painful or risky, but it helps get an accurate result the first time, without needing to repeat the test unnecessarily. This matters especially for markers that are directly affected by recently eaten food, such as glucose: after eating, blood glucose temporarily rises, so a fasting result lets your doctor evaluate the marker against the reference range meant for fasting rather than for the fed state. For most tests, the time of day doesn't matter — what matters is the length of the fast rather than the exact hour. For some tests, though (certain hormone tests, for example), your doctor may specify a particular time of day or other additional conditions — in that case, these instructions will always be on your referral form or given to you directly. If your referral doesn't mention a specific time or extra conditions, follow the general rule — the fasting period and plain water only — and check directly with your doctor or the lab beforehand if anything is unclear. Once you have your result, what a specific number and reference range mean is explained in more detail in the article on how to read your blood test results.