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Fasting for a Blood Test

What is fasting for a 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. While you're fasting for a blood test, you also should not:

  • Chew gum
  • Smoke
  • Exercise

How long do I have to fast before the test?

You usually need to fast for 8 to 12 hours before a test, but your provider will tell you exactly how long to fast. For certain tests, you may be asked not to drink alcohol for a longer period of time.

If your blood test requires fasting, you can usually schedule the test early in the morning. That way, most of your fasting time will be while you are sleeping.

Why do I need to fast before my blood test?

When you eat and drink, your bloodstream absorbs sugar, fat, protein, and other nutrients in those foods and beverages. That could affect the results of certain blood tests and lead to the wrong diagnosis.

What types of blood tests require fasting?

Some examples of common blood tests that usually require fasting are:

Other types of tests may require fasting, so always ask your provider if you aren't sure whether you should fast before your test.

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. But you can drink water. It's good to drink water before a blood test. It helps keep more fluid in your veins, which can make it easier to get a sample of your blood.

Can I continue taking medicine during a fast?

Ask your provider whether it's okay to take your usual prescription medicines and/or over-the-counter medicines. But don't stop taking your medicines unless your provider tells you to stop.

Let your provider know about any vitamins and supplements that you take, too. That way you'll be sure to avoid taking anything that could affect your test results.

What if I make a mistake and have something to eat or drink besides water during my fast?

Tell your provider if you eat or drink before your test. You may need to reschedule the test for another time when you can fast to make sure you get accurate results.

When can I eat and drink normally again?

You'll be able to eat 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?

If you don't know whether you should fast for your blood test, ask your provider at least a day before your test so you'll have time enough time to fast.

If you fast before a test when you don't need to, that may affect your results, too. That's why it's important to find out for sure whether you need to fast.

Always ask your provider any questions you have about how to prepare for your test. Taking the right steps before a blood test helps make sure your results will be accurate.


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.