How to Prepare for a Blood Test (Fasting, Hydration, Timing)

Getting a blood test is straightforward — but showing up unprepared can mean inaccurate results, a second draw, and wasted time and money. A few simple steps before your appointment make a significant difference in the quality of your results.

Here is everything you need to know to prepare properly.

Not all blood tests require fasting. Whether you need to fast depends entirely on which tests are being run.

• Fasting glucose — eating raises blood sugar, which would make a fasting glucose test meaningless
• HbA1C — technically does not require fasting, but is often ordered alongside fasting glucose
• Lipid panel (cholesterol) — eating, especially fatty foods, temporarily raises triglycerides and can affect LDL calculations
• Comprehensive Metabolic Panel (CMP) — includes glucose, so fasting is recommended
• Basic Metabolic Panel (BMP) — same reason as CMP
• Iron studies — iron levels fluctuate significantly after eating

• Complete Blood Count (CBC) — not affected by food
• Thyroid panel (TSH, T3, T4) — not significantly affected by recent meals
• STD panels — not affected by food
• Vitamin D — not affected by food
• PSA (prostate-specific antigen) — not affected by food
• HbA1C (alone) — reflects 3-month average, not affected by a single meal

When in doubt, fast. If your panel includes any fasting tests, fasting for the entire draw is the safest approach.

A standard fast for blood work is 8–12 hours with no food or caloric beverages. Here is what that means in practice:

• Plain water — stay hydrated (more on this below)
• Black coffee — in moderation; it does not significantly affect most fasting tests, though some labs recommend avoiding it to be safe
• Plain tea — without milk, sugar, or sweeteners
• Prescription medications — take as usual unless your doctor specifically says otherwise

• Any food, including small snacks or gum
• Juice, soda, sports drinks, or flavored water
• Coffee with milk, cream, or sugar
• Alcohol (avoid for at least 24 hours before a lipid panel)
• Supplements, especially fish oil (can affect triglycerides)

Schedule your draw for early morning. If you fast from 9 PM the night before and draw at 7–8 AM, you have completed a 10–11 hour fast with minimal disruption to your day. You can eat immediately after the draw.

This is the most overlooked preparation step — and one of the most important.

Dehydration makes blood draws harder. When you are dehydrated, your veins are smaller and harder to access. Phlebotomists may need multiple attempts, which is uncomfortable and can cause bruising.

Dehydration also affects results. Low fluid volume concentrates your blood, which can artificially elevate certain markers like BUN, creatinine, and hematocrit.

What to do: Drink 16–32 oz of water in the hour or two before your draw. Continue drinking water up until you walk in. Water does not break a fast.

Vigorous exercise in the 24 hours before a blood draw can temporarily affect several markers:

• CK (creatine kinase) — elevated after muscle exertion
• AST — can be elevated after intense exercise (it is not only a liver marker)
• Potassium — can shift temporarily after intense exercise
• WBC count — can be transiently elevated after hard workouts

Light walking is fine. Skip the intense gym session the morning of your draw.

Alcohol affects multiple lab values:

• Triglycerides — alcohol raises triglycerides, sometimes dramatically
• Liver enzymes (ALT, AST, GGT) — even moderate drinking can temporarily elevate these
• Blood glucose — alcohol can cause hypoglycemia, especially in a fasted state
• Uric acid — alcohol raises uric acid levels

For a lipid panel or liver function tests, avoid alcohol for at least 24 hours — ideally 48 hours.

Do not stop prescription medications before a blood draw unless your ordering provider specifically instructs you to. Most medications do not significantly affect routine lab values, and stopping them can be harmful.

If you are taking biotin (vitamin B7) supplements, stop them 72 hours before your draw. High-dose biotin interferes with several immunoassay-based tests, including thyroid panels and some hormone tests, and can cause falsely normal or abnormal results.

Wear a short-sleeved shirt or one with sleeves that roll up easily. Blood is typically drawn from the antecubital vein in the crook of your elbow. Tight sleeves that cannot be rolled up make the draw more difficult.

At LabCorp and Quest Diagnostics locations, the process is quick:

1. Check in at the front desk with your requisition form (emailed to you after ordering)
2. A phlebotomist will call you back, typically within 5–15 minutes
3. The draw itself takes 2–5 minutes
4. You are free to eat and resume normal activity immediately after

Most people feel nothing more than a brief pinch. If you are prone to feeling lightheaded after blood draws, let the phlebotomist know — they can have you lie down and will monitor you for a few minutes afterward.

• Eat something — especially if you fasted. Bring a snack or plan to eat immediately after.
• Keep the bandage on for at least 15–20 minutes to prevent bruising.
• Avoid heavy lifting with the draw arm for a few hours.
• Expect results in 1–2 business days — you will receive an email notification when they are ready.

Proper preparation takes less than a day and makes a real difference in the accuracy of your results. Browse our full test catalog to find the panel that fits your needs — most panels include fasting instructions in the test description so you know exactly what to expect.