Everything You Need to Know About Fasting Before a Blood Test

Medically reviewed by Deborah Weatherspoon, PhD, RN, CRNA on March 22, 2017Written by Corey Whelan

How do you prepare for a blood test?

Some blood tests will require you to fast beforehand. In these cases, your doctor will instruct you not to eat or drink anything, except water, in the hours leading up to the test.

Fasting before certain blood tests is important to help make sure that your test results are accurate. The vitamins, minerals, fats, carbohydrates, and proteins that make up all food and beverages can impact blood-level readings, clouding the results of your test.

Not all blood tests will require you to fast beforehand. Blood tests that you will likely need to fast for include:

If your doctor has prescribed a new blood test for you, or doesn’t mention whether or not you should fast or for how long, ask them if fasting is required. Some tests, such as a fecal occult blood test, don’t require fasting but do limit certain foods. Red meats, broccoli, and even some medications may cause a false positive test. Always follow your doctor’s advice when preparing for a test.

How long should you fast before a blood test?

The amount of time you need to fast for will vary depending on the test. For most tests, you will be told not to consume anything but water for eight hours leading up to the test. For a few tests, a 12-hour fast may be needed.

Tip

  • Schedule your test as early in the day as possible. The hours you spend sleeping are considered part of the fasting period, as long as you don’t break your fast with coffee or food once you’re awake.

Can you drink coffee if you’re fasting before a blood test?

Even if you drink it black, coffee can interfere with blood test results. That’s because it contains caffeine and soluble plant matter, which might skew your test results.

Coffee is also a diuretic, which means that it will increase how much you pee. This can have a dehydrating effect. The less hydrated you are, the harder it can be for the nurse or other medical professional who’s doing your blood test to find a vein. This can make the blood test harder or more stressful for you.

Can you drink alcohol if you’re fasting before a blood test?

Some blood tests, such as those that assess liver health or triglyceride levels, may require you to not drink any alcohol for a full 24 hours. Trace amounts of alcohol can remain in your bloodstream for several days. If you have any concerns about alcohol consumption, discuss this with your doctor when you schedule your test.

Also ask your doctor if you can smoke cigarettes before the test, or if you should refrain from smoking during your fast.

Is it OK to drink water before a blood test?

It’s fine to drink water before a blood test, unless you’re instructed otherwise by your doctor. This is different from some surgical procedures, which may require you to have a totally empty stomach.

Tap or bottled water are both OK, but leave the squeeze of lemon for another time. Seltzer and club soda are off-limits. Carbonated beverages, flavored or otherwise, shouldn’t be consumed during a fast, and neither should any type of tea.

Tip

  • Water hydrates your body and makes your veins plumper and more visible. Stay hydrated in the two days before your test. Also try drinking several glasses of water right before the blood draw to make it easier for the nurse or other medical professional to find a vein.

What if your child needs to fast before a blood test?

Just like adults, children may need blood tests that require them to fast beforehand. If so, your child’s pediatrician will let you know how long your child should abstain from eating and drinking.

Tips

  • Schedule your child’s blood test as early in the day as possible.
  • Distract, distract, distract: The hours leading up to the test may be the time to give in and let them watch a nonstop hour of goofy cartoons on TV or play with your iPad.
  • Pack a snack for them to devour as soon as the test is done.
  • If they manage to sneak a snack when you’re not looking, it’s better to reschedule than to get inaccurate readings.

What about fasting for a blood test during pregnancy?

There are several blood tests you may need if you’re pregnant. These are designed to assess any potential health concerns that you or your baby might experience during pregnancy or after you give birth. Some of these tests will require you to fast beforehand. Your doctor will advise you how to prepare for each test.

Fasting is usually safe if you’re pregnant, provided you’re in good health and aren’t having a high-risk pregnancy. For your overall comfort, your doctor may advise you to drink extra water or to remain indoors, especially if the weather is very hot or humid.

Fasting may increase heartburn in some pregnant women. If you experience uncomfortable or concerning symptoms of any kind while you’re waiting to have your blood drawn, let your doctor know immediately.

If you’re seeing a doctor other than your obstetrician-gynecologist, make sure they are made aware of your pregnancy before your blood test.

Q&A: What happens if you don’t fast before a blood test?

Q:

What happens if you don’t fast before a blood test? Should you still do the test?

A:

If you don’t fast before a test that requires it, the results may not be accurate. If you forget and eat or drink something, call your provider and ask if the test can still be done. Some tests can be analyzed with a notation that it isn’t fasting and results may vary. The main thing is to be honest. If you had a snack, a cup of coffee, or even a full breakfast, tell the technician when you have your blood drawn. They should make a note so that the results are reviewed with the food intake as a variable. And if fasting is an absolute must for meaningful results, they should stop and reschedule your blood draw.

Deborah Weatherspoon, PhD, RN, CRNAAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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