Why do I need to fast before my blood test?
If your health care provider has told you to fast before a blood test, it means you should not eat or drink anything, except water, for several hours before your test. When you eat and drink normally, those foods and beverages are absorbed into your bloodstream. That could affect the results of certain types of blood tests.
What types of blood tests require fasting?
The most common types of tests that require fasting include:
- Glucose tests, which measure blood sugar. One type of glucose test is called a glucose tolerance test. For this test you will need to fast for 8 hours before test. When you arrive at the lab or health care facility, you will:
- Have your blood tested
- Drink a special liquid containing glucose
- Have your blood re-tested one hour later, two hours later and possibly three hours later
Glucose tests are used to diagnose diabetes.
How long do I have to fast before the test?
You usually need to fast for 8–12 hours before a test. Most tests that require fasting are scheduled for early in the morning. That way, most of your fasting time will be overnight.
Can I drink anything besides water during a fast?
No. Juice, coffee, soda, and other beverages can get in your bloodstream and affect your results. In addition, you should not:
- Chew gum
These activities can also affect your results.
But you can drink water. It's actually good to drink water before a blood test. It helps keep more fluid in your veins, which can make it easier to draw blood.
Can I continue taking medicine during a fast?
Ask your health care provider. Most of the time it's OK to take your usual medicines, but you may need to avoid certain medicines, especially if they need to be taken with food.
What if I make a mistake and have something to eat or drink besides water during my fast?
Tell your health care provider before your test. He or she can reschedule the test for another time when you are able to complete your fast.
When can I eat and drink normally again?
As soon as your test is over. You may want to bring a snack with you, so you can eat right away.
Is there anything else I need to know about fasting before a blood test?
Be sure to talk to your health care provider if you have any questions or concerns about fasting.
You should talk to your provider before taking any lab test. Most tests don't require fasting or other special preparations. For others, you may need to avoid certain foods, medicines, or activities. Taking the right steps before testing helps ensure your results will be accurate.
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- Harvard Health Publishing: Harvard Medical School [Internet]. Boston: Harvard University; 2010–2018. Ask the doctor: What blood tests require fasting?; 2014 November [cited 2018 Jun 15]; [about 4 screens]. Available from: https://www.health.harvard.edu/staying-healthy/ask-the-doctor-what-blood-tests-require-fasting
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- Providence Health and Services [Internet]. Providence Health and Services; c2018. Providence Laboratory Services: Fasting Instructions [cited 2018 Jun 15]; [about 4 screens]. Available from: https://oregon.providence.org/~/media/Files/Providence%20OR%20Migrated%20PDFs/Patients%20Toolkit/Labs_Services_Fasting_Brochure%20_7_08.pdf
- Quest Diagnostics [Internet]. Quest Diagnostics; c2000–2018. For Patients: What to know about fasting before your lab test [cited 2018 Jun 15]; [about 3 screens]. Available from: https://www.questdiagnostics.com/home/patients/preparing-for-test/fasting.html
- University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2018. Health Encyclopedia: Cholesterol in the Blood; [cited 2018 Jun20]; [about 2 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=85&contentid;=P00220
- UW Health [Internet]. Madison (WI): University of Wisconsin Hospitals and Clinics Authority; c2018. Health Information: Health Facts for You: Getting Ready for Your Fasting Blood Draw [updated 2017 May 30; cited 2018 Jun 15]; [about 3 screens]. Available from: https://www.uwhealth.org/healthfacts/lab/7979.html