Key Takeaways
- Most blood tests do not need fasting.
- When fasting is needed, the clinician or business that requested the test will tell you for how long.
- Water is usually allowed during a fast and helps the phlebotomist find a vein.
- If you ate by mistake, tell the phlebotomist before the draw.
Why Some Tests Need Fasting
After a meal, sugars, fats and other substances from food pass into the blood. For a few tests that would hide your usual level, so the laboratory needs a sample taken after a period without food. Triglycerides, one of the fats in a lipid profile, are the best-known example.
Tests That Often Need Fasting
- Fasting glucose
- Fasting insulin
- Some lipid profiles
- Some iron studies
- Glucose tolerance tests
Tests That Usually Do Not
- Full blood count
- Thyroid function
- HbA1c, which reflects glucose over recent months rather than your last meal
- Kidney and liver function
- Vitamin D
Instructions can differ between laboratories, so the instruction you were given wins over any list.
What Can You Drink During a Fast?
Water is usually fine and helps. Avoid tea, coffee, juice, squash, fizzy drinks and alcohol unless you have been told they are allowed, and avoid chewing gum.
What If I Ate by Mistake?
Tell the phlebotomist before the draw. Sometimes the test can go ahead with a note on the form; sometimes it needs to be rebooked. Being honest is better than a result that does not reflect your usual level.
Tips for Fasting Comfortably
- If you can, choose an early appointment so most of the fast happens while you sleep.
- Eat a normal meal the evening before.
- Have a glass of water when you wake up.
- Keep a snack ready for straight after the draw.
Booking blood draws for your patients?
Lola Dispatch is an invitation-only platform healthcare businesses use to book mobile blood draws with approved phlebotomists. It does not take bookings from the public.