Posted on December 12, 2022

With certain blood tests, you may be instructed to fast for up to eight hours before your appointment. Fasting before a blood draw means you don’t eat or drink anything except water.

Don’t wait until the day of your blood draw to ask if you should fast. That could cause your appointment to be rescheduled. If at any point you’re unsure if fasting is required, contact your doctor.

Frequently asked questions about fasting for blood work and other lab tests

Why do I have to fast before certain lab tests?

Nutrients and ingredients in the food and beverages you consume are absorbed into your bloodstream and could impact factors measured by certain tests. Fasting improves the accuracy of those tests.

Why is water okay to drink while I’m fasting?

Water hydrates your veins. Hydrated veins are easier to find. And that means easier to draw from. Drink plenty of water before having any blood test.

How long do I have to fast for a blood test?

It depends on the test. Fasting for a lab test typically lasts eight hours. Your doctor should give you any special instructions related to your tests, including fasting requirements. Always follow her or his instructions.

What types of blood tests require fasting?

Glucose testing that checks blood-sugar levels and tests that determine your cholesterol, triglycerides, and high-density lipoprotein (HDL) levels usually require fasting. Other lab tests may require fasting, which is why you should ask your doctor. If you think fasting might be a problem, schedule your appointment for the early morning and bring a snack for after the appointment.

Can I eat before other types of blood tests?

If it’s a test that does not require fasting then, yes, please eat something before having your blood drawn.

Can I continue taking medications before a blood test?

Unless your doctor says otherwise, take your usual medications. If you’re taking vitamin/mineral supplements, ask whether you should continue those as well.

How soon can I eat after a blood test?

Once you’ve had your blood drawn, you can eat and drink something immediately. It’s a good idea to bring a snack to your appointment. You can also schedule your blood test for the early morning, to minimize the length of time you’ll go without food.

https://www.questdiagnostics.com/patients/get-tested/prepare/fasting