How to Prepare for a Blood Test, Test by Test

Can you eat and drink before a blood test?

Plain water is allowed before almost every blood test, even during a water-only fast, and some orders ask you to drink extra because fuller veins are easier to draw from. Coffee, juice and sports drinks do not count as water. If your care team has limited your fluids, their instructions come first.

Jump to tests with special preparation, medicines and biotin, exercise, alcohol and illness, the appointment or after the draw.

Whether you can eat depends on the test. HbA1c and a complete blood count usually need no fasting, a lipid panel may or may not, and the tests that do need a fast are in the table below. Coffee, alcohol and what to do if you ate by mistake are covered in fasting before a blood test.

Tests that need special preparation

These are the instructions most often missed, because "blood test" sounds like a single procedure. Check your order for these names.

For a cycle-timed test, bring the date your last period started. Hormonal contraception or hormone therapy is worth mentioning when the test is ordered, because it can change which test is worth doing.

Medicines, vitamins and biotin

Keep taking prescribed medicines unless the clinician who ordered the test says otherwise, and tell the lab everything you take, including over-the-counter products. If you did not follow an instruction exactly, say so rather than hoping it does not matter.

Biotin, common in hair, skin and nail supplements, is the one to name. Many lab tests use biotin as part of how they detect what they measure, so extra biotin from a supplement can push results falsely low or falsely high. Thyroid tests such as TSH and free T4 are affected most often, and PTH, prolactin, testosterone and cortisol tests can be too.

In practice, a 10 mg supplement taken with breakfast the day before is about 24 hours old at a 9 a.m. appointment, well past the 8-hour minimum; the same dose taken that morning is not. Prescribed high-dose biotin should only be paused on the prescriber's advice.

Exercise, alcohol and recent illness

Most of what you did in the past week makes no difference. The few things that do are easier to mention at the appointment than to explain later.

A normal gym session before a routine panel is no reason to reschedule. A half marathon on Saturday can still show in a Monday CK, and a chest infection two weeks ago can explain a high inflammation marker on an otherwise ordinary report.

At the appointment

The NHS puts a routine blood test at about ten minutes, most of it spent checking who you are and finding a vein. If needles make you anxious or you have fainted before, tell the staff before they start: you can ask to lie down, bring someone with you, and plan not to drive straight afterwards. It helps to bring:

• sleeves that roll above the elbow, or a short-sleeved layer;
• the lab order, photo ID and an insurance card if the lab asks for one;
• a list of your medicines and supplements, with the time of each last dose;
• the time you last ate or drank anything but water, if you were fasting.

Keep your hand relaxed once the tourniquet is on. In the study behind this advice, 10 to 15 fist clenches raised potassium in that arm by up to 1.7 mmol/L while the other arm stayed normal, and a tourniquet left on for a long time has a similar effect.

After the blood draw

Press on the site for a minute or two, and eat if you were fasting. Some aching is normal, and a bruise or swelling should settle within a few days. Contact the clinic if pain or swelling gets worse instead of better, or if numbness or tingling spreads down the arm.

Routine chemistry and blood counts often come back within a day or two; cultures and specialized tests take longer. How long blood test results take breaks this down by test.