Blood tests are common procedures done to help diagnose certain diseases and measure things like your cholesterol or levels of essential vitamins and minerals.
While most times you don't need to prepare in any way for a blood test, there are certain types that require you to fast (i.e., avoid food and some drinks) before your appointment. This is so the results can be as accurate as possible.
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Here, learn all about fasting for blood work, which tests require you to fast and foods to avoid before a blood test.
- Not all blood tests require you to fast beforehand.
- Fasting blood tests require you to avoid all foods and drinks other than water for eight to 12 hours beforehand.
- You may also have to avoid alcohol, caffeine, tobacco, exercise and certain medications prior to a fasting blood test.
How Long Should You Fast Before Blood Work?
How long you have to avoid eating before lab work will depend on the type of test you are getting. The average fasting time, however, is usually between eight and 12 hours, per the National Library of Medicine (NLM).
Some of the tests that may require fasting include:
1. Lipid Panel
The traditional lipid panel requires a 12-hour fast, which means no food or drink other than water can be consumed 12 hours prior to having the blood drawn, per the NLM.
This lipid test, also known as a cholesterol panel, checks for blood levels of total cholesterol, triglycerides as well as LDL and HDL cholesterol — commonly called "bad" and "good" cholesterol, respectively, per the Mayo Clinic.
However, an emerging practice is to order cholesterol tests that do not require fasting, because the only component of the lipid test influenced by recent food intake is the triglyceride, or fat level in the blood, per a March 2016 review in the Journal of the American College of Cardiology.
Talk to your doctor about whether and how long you need to fast before your lipid panel.
2. Blood Glucose Test
Avoiding all food and liquid other than water is also required at least eight hours before a fasting blood glucose test, which is used to screen for prediabetes and diabetes, per the Cleveland Clinic.
Fasting is also required prior to an oral glucose tolerance test, which is used in pregnancy to screen for gestational diabetes, per the Mayo Clinic.
A blood glucose test can also be done without fasting, but the fasting test is more commonly used for diagnostic purposes, per the National Institutes of Health.
3. Other Tests
Ask your doctor if the tests prescribed to you require fasting. There may be others, besides glucose and cholesterol, that require fasting.
When you eat, your blood levels of vitamins, minerals, electrolytes and other nutrients are temporarily increased, which can show on a blood test. This may not always be most efficient at helping doctors diagnose your issue, especially if the test requires fasting.
Even so, most lab tests do not require fasting for accurate results. Your doctor may, however, have a preference on whether you should avoid specific foods or nutrition supplements prior to having certain tests done.
Can You Drink Water While Fasting?
Yes, you can drink water while fasting for blood work, and you should, because it's essential for staying hydrated.
You cannot, however, drink other beverages like coffee or tea. If you accidentally drink coffee before your blood test, let your doctor know. They may need to reschedule the test, per the NLM.
Foods to Avoid Before a Blood Test
It's not just certain foods that affect blood test results. All foods will affect your nutrient levels, thereby altering the results of your fasting blood test.
This means you'll want to avoid all food and drinks other than water for eight to 12 hours before your test. This includes:
- Soda (even diet soda)
Other things that can "ruin" a blood test result (i.e., you should avoid them) include the following, per the NLM:
- Chewing gum
- Tobacco products
What's the Best Thing to Eat the Night Before a Fasting Blood Test?
Ultimately, the best thing to eat the night before a fasting blood test (that is, more than 12 hours before your test) is something that will keep you full and satiated throughout the fast, such as a meal that features lean protein, healthy fats and complex carbohydrates. (Think: A turkey burger with avocado and lots of non-starchy veggies.)
Keep in mind: What you eat the day before a blood test won't affect your results, according to Texas A&M University, as long as you stop eating before the fasting window.
Can You Take Medication Before a Fasting Blood Test?
Most of the time, it's OK to take your usual medications before a blood test. But there are some medications that can alter the results of a test, like steroids and other vitamins or herbal supplements. Steroids, for example, can alter your cholesterol levels, per University Hospitals.
Be sure to mention all medications or supplements you're taking to your doctor prior to the tests, so they can account for varying levels of certain substances in your blood.
What About Non-Fasting Blood Tests?
Many different blood tests can be done without you having to fast beforehand. These can include tests that check on your kidneys, liver and thyroid function, per Harvard Health Publishing.
Tests that check your vitamin levels, like vitamin D for instance, often don't require you to fast either, per Mount Sinai.
In these cases, you don't have to avoid any particular foods or drinks before your test.
If you're unsure whether or not you need to fast before blood work, talk to your doctor, who can give you some guidance.
Improper test preparation can alter your blood test results and may even lead to an incorrect diagnosis or unnecessary treatment.
For the most accurate blood test results, make sure to follow your doctor's instructions.
- University Hospitals: "How To Get the Most Accurate Blood Test Results"
- Mount Sinai: "25-hydroxy vitamin D test"
- Harvard Health Publishing: "Ask the doctor: What blood tests require fasting?"
- National Library of Medicine: "Fasting for a Blood Test"
- Mayo Clinic: "Cholesterol Test"
- Journal of the American College of Cardiology: "Fasting or Nonfasting Lipid Measurements: It Depends on the Question"
- Cleveland Clinic: "Blood Glucose (Sugar) Test"
- Mayo Clinic: "Glucose Tolerance Test"
- National Institutes of Health: "The A1C Test & Diabetes"
- Texas A&M University: "The do’s and don’ts of fasting before routine bloodwork"
- Lab Tests Online: Test Preparation: Your Role
Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.