How to Prepare for a Blood Test
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
The night before, hydrate well and follow your fasting instructions — usually 8 to 12 hours with only water for glucose and lipid tests. Keep taking prescribed medications unless told otherwise, skip alcohol, and bring your ID and insurance card. Most draws take minutes.
What should I do the night before?
Hydrate well and, if your tests require fasting, stop eating at the time your lab specifies — usually 8 to 12 hours before the draw. Skip alcohol the night before, since it can affect liver enzymes, glucose, and triglycerides. Get a normal night of sleep; being rested makes the draw easier and your results more representative. Lay out your ID and insurance card so the morning is smooth.
Which tests require fasting?
Fasting is required for fasting glucose, a lipid panel (especially for accurate triglycerides), and some metabolic panels. It is generally not needed for a CBC, TSH, HbA1c, or most hormone tests. Confirm with your doctor or the lab which of your tests need fasting — do not assume all of them do. If only one test needs it, you still fast for the whole draw.
Can I take my medications?
Usually yes. Keep taking your prescribed medications unless your doctor explicitly tells you to hold them. Some meds genuinely affect certain tests, so your doctor will flag those in advance. If you are unsure, call the office — but do not stop a medication on your own, especially one for blood pressure, diabetes, or the heart. Bring a list of everything you take, including supplements.
What about coffee and alcohol?
Skip alcohol for at least 24 hours before the draw, since it can skew liver and lipid results. For coffee, plain black is sometimes allowed during a fast, but many labs prefer you skip it entirely. Coffee with cream or sugar breaks a fast. When in doubt, water only. If you normally drink caffeine, a headache from skipping it is common but harmless.
What should I bring?
Bring a photo ID, your insurance card, and a list of current medications and supplements. If you have a lab order, bring that too. Wear a top with easy sleeve access so the phlebotomist can reach your arm, and tell them about any history of fainting, difficult draws, or bleeding disorders before they start. A few minutes of preparation makes the whole visit smoother.
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Frequently asked questions
Can I exercise before a blood test?
Light activity is usually fine, but intense exercise can temporarily raise some enzymes and muscle markers. Ask your lab if your tests are sensitive to it.
What if I feel faint during the draw?
Tell the phlebotomist before you feel faint. They can draw you lying down, and you should stay seated and eat or drink something after.