How to prepare for your blood test
Preparation changes results more than most patients expect. A lipid panel drawn after a late dinner, or a cortisol drawn at the wrong hour, can look abnormal for reasons that have nothing to do with your health. Here is what actually matters.
Which tests require fasting
During a fast, water is not only allowed — it helps. Black coffee is usually acceptable but can nudge glucose and lipids slightly, so skip it if your provider is tracking small changes.
- Lipid panel (cholesterol, triglycerides): 9 to 12 hours, if your provider requested fasting.
- Fasting glucose and insulin: 8 to 12 hours.
- Comprehensive metabolic panel: fasting often requested, but not always required — follow the order.
- Iron studies: morning draw, fasting, and hold iron supplements for 24 hours unless told otherwise.
- CBC, thyroid panel, most hormone panels: no fasting needed.
Hydration is the single best thing you can do
Two to three glasses of water in the two hours before your appointment plumps veins and makes the draw faster and less bruise-prone. Dehydration is the most common reason a draw needs a second stick.
Medications and supplements
Take prescribed medications on your normal schedule unless your provider says otherwise — never stop a prescription to prepare for a lab test on your own.
- Biotin (often in hair, skin, and nail supplements) can distort thyroid and troponin assays. Stop 48 to 72 hours before, with provider approval.
- High-dose vitamin C can interfere with some chemistry assays.
- Blood thinners are fine — tell your phlebotomist so they hold pressure longer.
Timing that matters
- Cortisol: morning draw, usually between 7 and 9 a.m.
- Testosterone: morning, before 10 a.m., for the most interpretable value.
- Therapeutic drug levels: usually a trough, immediately before your next dose.
- Female hormone panels: often cycle-day specific — confirm the target day with your provider.
The morning of
- Drink water; skip the workout right before the draw.
- Wear a sleeve that rolls above the elbow.
- Set out your ID and requisition.
- Clear a table with a chair and light.
- Eat something ready to go for right after the draw, especially if you fasted.
Frequently asked
Can I drink water before a fasting blood test?
Yes. Plain water is encouraged during a fast — it improves vein access and does not affect fasting results.
Can I drink coffee before a blood test?
Black coffee is usually acceptable, but it can slightly affect glucose and lipid values. If your provider is tracking small changes, skip it and drink water instead.
Should I stop my supplements before a blood test?
Biotin and high-dose vitamin C can interfere with certain assays; pause them 48 to 72 hours before with your provider's approval. Never stop a prescribed medication without asking your provider.
Next step
A certified phlebotomist comes to your home or office anywhere in metro Atlanta, usually within 24–48 hours.
See the test menu