Most blood test preparation comes down to a few simple steps: fasting for the right number of hours, drinking plenty of water, and avoiding a short list of substances that can skew your results. The specifics depend on which test you’re getting, but the basics apply broadly and are easy to get right once you know them.
Know Whether You Need to Fast
Fasting is the single most important preparation step, and it typically means going 8 to 12 hours without eating or drinking anything except plain water. Not all blood tests require fasting, so confirm with your provider ahead of time. The tests that most commonly require it include lipid panels (cholesterol), fasting glucose, and certain metabolic panels.
A lipid panel usually calls for 10 to 12 hours of fasting. If your provider told you to fast and you accidentally ate something, let them know before the draw rather than staying quiet. The results won’t be as useful without a proper fast, and you may need to reschedule rather than waste the appointment.
Fasting means more than just skipping food. You should also avoid chewing gum, which can trigger digestive enzymes that affect your blood chemistry. Flavored water, including lemon-flavored or artificially sweetened sparkling water, can also break a fast. Stick to plain water only.
Drink More Water Than Usual
Staying well hydrated is one of the most overlooked parts of blood test prep, and it makes a real difference in your experience. Drinking water keeps your veins fuller and more visible, which helps the phlebotomist find a good vein on the first try. Dehydrated veins are smaller and harder to access, leading to multiple needle sticks or a longer, more uncomfortable draw.
Aim to drink a few extra glasses of water the evening before and the morning of your test. This is especially important if you’re fasting, since you won’t be getting fluids from food. There’s no need to overdo it, but being slightly better hydrated than normal makes the process smoother for everyone.
Skip Alcohol, Smoking, and Coffee
Alcohol should be avoided for at least 24 hours before a fasting blood test. It affects liver enzymes, triglycerides, and blood sugar levels, all of which show up on common panels. Even a single drink the night before can throw off your lipid results.
Smoking and vaping can also alter test results, so avoid them the morning of your draw if possible. Coffee is a common stumbling point for people who fast overnight and reach for a cup first thing. Even black coffee is not permitted during a fast for blood work. Beyond potentially affecting results, coffee is a diuretic that increases urination and can leave you mildly dehydrated, making your veins harder to access and potentially concentrating certain substances in your blood.
Watch Your Exercise Timing
A hard workout within 24 hours of your blood test can raise markers that your doctor might otherwise flag as concerning. Strenuous exercise releases muscle enzymes into your bloodstream, and these overlap with markers used to evaluate heart and liver health. After intense activity like a long run or heavy lifting, some of these enzyme levels can spike to more than double their normal range and stay elevated for a full day or longer.
Liver-specific markers can also rise after vigorous exercise because blood flow to the liver temporarily decreases during hard effort. In trained athletes, most markers return to normal within about 24 hours. In people who exercise less regularly, the effect can last even longer. The simplest approach is to take a rest day the day before your blood draw. Light walking or gentle activity is fine.
Pause Biotin Supplements
If you take biotin (sometimes labeled as vitamin B7 or vitamin H), stop taking it at least two days before your blood test. Biotin is found in many hair, skin, and nail supplements, sometimes at doses far higher than what your body needs. It interferes with the lab technology used to measure thyroid hormones and certain cardiac markers, producing falsely high or falsely low readings. The American Thyroid Association specifically recommends a minimum two-day washout before thyroid testing. Check your multivitamin label too, since many contain biotin.
Ask About Your Medications
In most cases, you can take your regular prescription medications with a sip of water even while fasting. However, some medications directly affect the markers being tested. If you take thyroid medication and you’re having thyroid levels checked, or you’re on cholesterol-lowering drugs and getting a lipid panel, your provider may want you to delay your dose until after the draw. The key is to ask in advance rather than guessing. When you schedule your test or receive your lab order, confirm whether any of your medications should be held that morning.
Practical Tips for the Day Of
Wear a short-sleeved shirt or a top with sleeves that push up easily past your elbow. The draw site is almost always the inside of your arm, and tight sleeves that bunch up can get in the way. Bring a valid photo ID and your insurance card. You’ll typically be asked to confirm your name, date of birth, and the reason for your visit.
If needles make you anxious, you’re far from alone, and a few strategies can make the experience more manageable. Let the staff know you’re nervous when you arrive. Phlebotomists deal with this regularly and can help by chatting with you, covering the puncture site so you don’t see it, or positioning you so you’re more comfortable. Deep breathing works well in the moment: breathe in through your nose for four seconds, hold for four, breathe out through your mouth for four, and hold again for four. Looking away from the needle and bringing a friend to talk to are simple distractions that genuinely help. If needle anxiety is severe enough that it keeps you from getting necessary medical care, exposure therapy with a mental health professional is an effective treatment option.
A Quick Prep Timeline
- Two or more days before: Stop biotin supplements if you’re having thyroid or cardiac markers tested.
- 24 hours before: Avoid alcohol and skip intense exercise. Take a rest day.
- 8 to 12 hours before: Begin your fast if required. Only plain water is allowed.
- Morning of: Drink extra water, skip coffee and cigarettes, wear a short-sleeved shirt, and bring your ID.
Most of the preparation is about removing things that interfere with your results rather than adding anything special to your routine. A quiet evening, a good night’s sleep, and plenty of water will cover the majority of what your lab work needs to be accurate.