Most blood draws require little more than showing up hydrated and wearing the right clothes, but a few simple steps beforehand can make your results more accurate and the experience more comfortable. Whether you’re fasting for a cholesterol panel or just getting routine bloodwork, here’s what to do in the hours and days leading up to your appointment.
Know Whether You Need to Fast
Fasting is the single most important preparation step, and not every blood test requires it. The tests that typically do include blood glucose (blood sugar), a lipid panel (cholesterol and triglycerides), and a basic metabolic panel. For these, you’ll need to avoid all food and beverages except plain water for 8 to 12 hours before your draw. That means if your appointment is at 8 a.m., you’d stop eating by midnight the night before.
Plain water is not just allowed during a fast, it’s encouraged. Staying well hydrated plumps your veins and makes it easier for the phlebotomist to find a good one on the first try. Coffee, tea, juice, and anything with calories or sweetener will break your fast and can throw off your results. If you’re unsure whether your specific test requires fasting, check the lab order or call the office that ordered it.
Ask About Your Medications and Supplements
Most routine medications can be taken before a blood draw, but there are exceptions depending on the test. If you take medication in the morning, ask the provider who ordered your bloodwork whether you should take it before or after the draw. Some medications are specifically being measured (like thyroid hormone levels), so the timing of your dose matters.
Supplements deserve special attention, particularly biotin. Biotin is found in many hair, skin, and nail supplements, sometimes in high doses, and it can interfere with a surprisingly long list of lab tests. It can skew thyroid function results so dramatically that healthy patients have received false readings mimicking Graves’ disease. It also affects tests for hormones like cortisol, testosterone, and estradiol, as well as cardiac markers and even some cancer biomarkers. Stop taking biotin at least 48 hours before your blood draw. If you take a daily multivitamin, check the label, as many contain biotin.
Skip the Hard Workout
Intense exercise in the hours before a blood draw can temporarily shift several lab values. Strenuous activity raises cortisol levels by 30 to 50 percent above resting levels, and those levels can stay elevated for 90 minutes or more into recovery. Exercise also affects markers of muscle damage, electrolytes, and blood sugar. A gentle walk is fine, but save the long run or heavy lifting for after your appointment. If you exercise in the morning, schedule your draw for a rest day or go to the lab first.
Dress for Easy Access
Blood is almost always drawn from a vein in the inner elbow area, so wear a short-sleeved shirt or one with sleeves you can push up easily past your elbow. Tight long sleeves that bunch up above the draw site can act like a second tourniquet and make the process harder. If it’s cold out, layer a loose sweater or jacket over a short-sleeved top.
Bring the Right Paperwork
If you’re going to a standalone lab rather than your doctor’s office, you’ll typically need a lab order (sometimes called a requisition) and a photo ID. Many labs now receive orders electronically, but it’s worth confirming before you arrive. The lab will match your identity to the order, including your full name and date of birth, so bring your insurance card if applicable. Having everything ready speeds up check-in and reduces the chance of a mix-up with your samples.
How to Prevent Feeling Faint
Feeling lightheaded or dizzy during or after a blood draw is common and usually caused by a drop in blood pressure triggered by the sight of blood or needles. This is a vasovagal response, and it’s not dangerous, but it’s unpleasant. The good news is that a few physical techniques are very effective at preventing it.
Drinking plenty of water beforehand helps maintain your blood volume, which is your first line of defense. At the draw itself, if you start feeling warm, dizzy, or nauseous, cross your legs and tense the muscles in your arms, chest, abdomen, and buttocks simultaneously. Don’t hold your breath. This combination raises your blood pressure and cardiac output enough to counteract the fainting reflex. If the dizziness doesn’t pass, sit down and lower your head. These maneuvers are simple, safe, and well-studied.
Looking away from the needle and the blood collection tubes also helps. Focus on something across the room, take slow breaths, or chat with the phlebotomist. Let them know ahead of time if you have a history of fainting so they can have you lie down for the draw.
Preparing a Child for a Blood Draw
Kids do better when they know what to expect. Explain the process in age-appropriate terms: a small pinch, a stretchy band on their arm, and then it’s done. Be honest. Don’t promise it won’t hurt, because it might, and losing trust makes the next visit harder. Instead, say it may pinch for a moment but the feeling goes away quickly. Reassure them that even adults get nervous about blood draws.
For younger children, practicing on a stuffed animal at home can take the mystery out of the procedure. Practice deep breathing together or counting slowly to ten. On the day of the draw, schedule it for a time when your child isn’t already tired or hungry. If fasting isn’t required, feed them beforehand to reduce lightheadedness. Bring a favorite toy, book, or tablet as a distraction, and plan a small reward for afterward. During the draw itself, encourage your child to look at you rather than the needle, and provide physical comfort by holding their hand or, for babies, holding them in your lap if the lab allows it.
What to Do Right After the Draw
Once the needle is out, the phlebotomist will press gauze to the site or ask you to apply firm pressure yourself. Keep that pressure steady for at least 2 to 3 minutes. This is the step most people rush, and skipping it is the main reason people end up with bruises. After the gauze comes off, a small bandage goes on. Leave it in place for a few hours.
Avoid heavy lifting or vigorous use of that arm for the rest of the day. If you were fasting, eat a snack soon after. If you felt lightheaded, sit in the waiting area for 10 to 15 minutes before driving.