Good preparation makes the blood draw easier and the results more accurate. Here's everything you need to know before you go.
Check whether fasting is required
Your test request form or the lab's instructions will state whether fasting is needed. Fasting tests — glucose, triglycerides, fasting insulin — require 8–12 hours without food. Non-fasting tests (full blood count, HbA1c, thyroid, kidney function) don't. If you're unsure, call the lab or your GP's reception before the appointment.
Hydration and medications
- Drink plenty of water — well-hydrated veins are easier to locate and draw from; this won't affect most test results
- Take your regular morning medications with water unless your doctor has specified otherwise
- Avoid biotin supplements for 24–48 hours before certain hormone and cardiac tests — high-dose biotin can interfere with immunoassay methods
- Avoid intense exercise the night before — vigorous activity temporarily raises creatinine, potassium and some cardiac markers
- Avoid alcohol for 24 hours before liver function tests
At the appointment
Wear a short-sleeved or loose-fitting top for easy access to the arm. Tell the phlebotomist if you've had lymph node surgery on either arm (the blood draw should avoid that arm), if you have difficult veins or previous bad experiences, or if you feel faint. It's normal to feel slight lightheadedness — sit for 5 minutes after the draw before standing.