Fasting before certain blood tests is important to help make sure that your test results are accurate.
Some blood tests 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.
The vitamins, minerals, fats, carbohydrates, and proteins that make up all food and beverages can affect blood-level readings, clouding your test results.
Not all blood tests will require you to fast beforehand. Blood tests that you will likely need to fast for include:
- cholesterol test
- triglyceride level test
- high-density lipoprotein (HDL) level test
- low-density lipoprotein (LDL) level test
- lipoprotein panel (which includes all of the above tests)
Some tests, such as a fecal occult blood test, don’t require fasting but do limit certain foods. Red meat, broccoli, and some medications may cause a false positive test.
How long you need to fast will vary depending on the test. For most tests, a doctor will ask you not to consume anything but water for 8–12 hours leading up to the test.
Schedule your test as early in the day as possible. The hours you spend sleeping count as part of the fasting period, as long as you don’t break your fast with coffee or food once you’re awake.
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 it will increase how much you pee. This can have a dehydrating effect. The less hydrated you are, the harder it can be for a healthcare professional to find a vein for the blood draw. This can make the test harder or more stressful for you.
Tea and energy drinks are also off-limits before a fasting blood test.
If you have any concerns about alcohol consumption, discuss them with your doctor when you schedule your test.
It’s fine to drink water before a blood test unless your doctor instructs you otherwise. 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. You shouldn’t consume carbonated beverages, flavored or otherwise, during a fast.
Water hydrates your body and makes your veins plumper and more visible. Stay hydrated in the 2 days before your test. Consider drinking several glasses of water right before the blood draw to make it easier for the healthcare professional to find a vein.
If you have to fast before a blood test, you should also avoid the following:
- Chewing gum: Even sugar-free gum can affect several key results, according to
- Smoking: Traces of nicotine can stay in your blood for up to
5 days. Smoking can also affect lipoprotein levels in your blood. Check with your doctor about whether you’ll need to refrain from smoking, vaping, or chewing tobacco beforehand and for how long.
- Exercise: Blood tests provide the most accurate results when you’re at rest. Some experts recommend refraining from strenuous exercise for at least
24 hoursbefore a blood test.
- Certain medications: You can usually take your regular medications before a test. However, you may need to refrain from medications that require you take them with food. Be sure to check with a doctor before you stop taking any prescribed medications.
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 for parents
- 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 tablet.
- 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.
Some standard tests during pregnancy may 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 people. Let your doctor know if you experience uncomfortable or concerning symptoms while waiting to have your blood drawn.
If you’re seeing a doctor other than your obstetrician-gynecologist, be sure to let them know about your pregnancy before your blood test.
What happens if you don’t fast before a blood test? Should you still do the test?Anonymous
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.
Some blood tests may require you to fast for 8–12 hours beforehand. Check with your doctor for specific instructions, as some tests may have further restrictions.
Talk with a doctor if you think you may have trouble fasting before the test. Some organizations, like the Department of Veterans Affairs, no longer require fasting for most blood tests.
Always follow your doctor’s advice when preparing for a test. They can advise what’s best for you and the specific test you need.