Most blood test preparation comes down to a short checklist: know whether you need to fast, drink plenty of water, and avoid a few things that can skew your results. The specifics depend on which tests your doctor ordered, but the basics apply broadly and take very little effort to get right.
Fasting: What Counts and How Long
If you’ve been told to fast, that means no food or drinks other than plain water for 8 to 12 hours before your draw. The most common tests requiring a fast are blood glucose (blood sugar) tests and cholesterol panels, also called lipid panels. Fasting ensures the lab measures your baseline levels rather than whatever you recently ate.
The “plain water only” rule is stricter than most people expect. Coffee, juice, soda, flavored water, and even water with lemon or lime can introduce substances that alter your results. Coffee is especially tricky: it contains compounds that affect metabolism and acts as a diuretic, which can concentrate certain substances in your blood. Chewing gum, smoking, and exercising during your fast can also interfere with results. If your test doesn’t require fasting, your doctor or the lab order will typically say so. When in doubt, fast anyway; it won’t hurt your results and avoids having to redo the draw.
Drink Plenty of Water
Water is the one thing you should actively consume more of before a blood draw, not less. Staying well hydrated keeps your veins fuller and closer to the surface, making it easier for the person drawing your blood to find a vein on the first try. Dehydration does the opposite: veins shrink, the draw takes longer, and you’re more likely to end up bruised or needing multiple sticks. Plain water has no calories, sugar, or other substances that affect test results, so there’s no downside to drinking a few extra glasses the evening before and the morning of your appointment.
Skip the Hard Workout
Intense exercise the day before a blood test can produce results that look alarming on paper but are actually just a normal response to physical stress. In one documented case, a healthy 22-year-old man showed critically elevated muscle enzymes and liver markers after three hours of heavy weightlifting and endurance training. His results returned to normal after just two days of rest. That’s because strenuous activity causes microscopic tearing in muscle fibers, releasing enzymes into the bloodstream that labs measure as markers of muscle or liver damage.
Exercise also temporarily changes blood sugar levels by driving glucose into muscle cells and raises cortisol, which can shift your white blood cell count. Light activity like walking is fine, but avoid anything intense for at least 24 to 48 hours before your draw if your panel includes metabolic, liver, or muscle-related markers.
Biotin Supplements Need a Pause
If you take biotin (vitamin B7), a popular supplement for hair, skin, and nails, stop taking it at least 48 hours before your blood draw. Biotin interferes with the lab technology used to run a surprisingly wide range of tests. It can skew thyroid panels, hormone levels (testosterone, estrogen, cortisol, and others), vitamin B12, iron storage markers, cardiac function tests, and even some cancer screening markers. The interference can push results falsely high or falsely low depending on the test, which means a normal result could look abnormal or a genuine problem could be hidden. Two days without biotin is enough to clear it from your system.
Keep Taking Your Medications
Unless your doctor specifically tells you otherwise, continue taking your regular medications before a fasting blood test. This is a common source of confusion, especially for people on morning pills who aren’t sure whether “fasting” means skipping their dose. It doesn’t. Most medications should be taken on their normal schedule with a small sip of water. The key exception is any drug your doctor has explicitly asked you to hold, which sometimes happens when the blood test is measuring that medication’s level in your system. Never stop a medication on your own just because you have lab work coming up.
Timing Matters for Some Tests
Certain hormones follow a daily rhythm, peaking in the early morning and dropping throughout the day. Cortisol and testosterone are the most common examples. If your doctor has ordered hormone testing, an early morning draw (typically before 9 or 10 a.m.) gives the most accurate and standardized reading. This is also why most fasting blood work is scheduled first thing in the morning: you sleep through most of the fast, your hormone levels are at their reference-range peak, and you can eat breakfast right after.
If your lab offers walk-in appointments, aim for as early as possible. You’ll also avoid the longer wait times that tend to build later in the morning.
What to Bring and Wear
Wear a short-sleeved shirt or a top with sleeves loose enough to push above your elbow. This sounds minor, but struggling with a tight sleeve while the phlebotomist is trying to find a vein makes the process harder for both of you. Bring your insurance card, a photo ID, and the lab order if your doctor gave you a printed or digital copy. Some labs can pull the order electronically, but having it on hand avoids delays.
If you tend to feel lightheaded during blood draws, mention it before the needle goes in. Most labs can recline your chair or have you lie down, and having a small snack ready in your bag for right after the draw helps you recover quickly, especially if you’ve been fasting.