How Long Should You Fast Before a Blood Test? (2022)

Blood work is the diagnostic testing of a blood sample in a lab. Blood is obtained through a process called a venipuncture, in which a vein is punctured with a needle to draw the blood.

Lab tests are useful for diagnosing, screening, and monitoring medical conditions. While most lab tests do not require fasting, some lab tests do require a period of going without food before testing, since food enters the bloodstream and can affect the results.

This article discusses why your healthcare provider might ask you to fast for blood work and how you can best prepare.

How Long Should You Fast Before a Blood Test? (1)

How to Prepare

Most blood tests actually do not require fasting, but some common ones do.

If your healthcare provider has asked you to fast before a test, it's important that you do so for the most accurate result. Otherwise, you may have to come back for a repeat test.

The type of fasting required for blood work is different from the "NPO after midnight" order that healthcare providers give the day before procedures. "NPO after midnight" means "nil per os," which is Latin for "nothing by mouth."

NPO means no food or beverages, not even water. This is ordered before procedures that require sedation or anesthesia, since the medications used can cause nausea and vomiting, increasing your risk of choking or aspiration (when food enters your airways or lungs accidentally).

General Rules for Fasting

To fast for blood work, do not eat anything for eight to 12 hours before the test. You may drink water, tea, or black coffee (no sugar or milk added). It's best to schedule fasting blood work for the morning, so that you don't have to spend the whole day hungry.

How Long to Fast

Generally, you should fast for eight to 12 hours before lab work that requires it. Fasting for a blood sugar test, which is included in the basic metabolic panel, is generally eight to 12 hours.

You can always clarify how long to fast with your healthcare provider. If you are unsure, aim for 12 hours of fasting. For example, if you schedule your test for first thing in the morning, you should generally not eat anything after dinnertime the night before.

Drinking

Drinking certain liquids is allowed, and even encouraged, before blood work. This is because a 12-hour fast from drinking fluids can make you slightly dehydrated. This causes your veins to flatten and makes them harder to find for a venipuncture.

Drinking should be limited to water, tea, or black coffee with no sweeteners or creams.

Drinking water is recommended before lab tests so that you do not become dehydrated. The phlebotomist (medical professional who performs blood draws) will have an easier time drawing your blood if you are well-hydrated.

(Video) Fasting for a Blood Test | WebMD

Eating

Eating before certain blood work can affect results.

Specifically, eating before a cholesterol panel can raise the triglyceride levels and potentially the LDL (low-density lipoprotein) levels (known as "bad" cholesterol). Individuals who are not on statin medications for cholesterol may not be required to fast, and new guidelines suggest that fasting before a lipid test is optional.

Eating before a blood glucose test will raise your blood sugar. However, another test for diabetes, called the hemoglobin A1c test, does not require fasting, since it looks at a marker of blood sugar control over the past few months rather than directly measuring blood sugar.

Less common tests that require fasting include:

  • Gastrin
  • Proinsulin
  • Insulin
  • Glucagon
  • Pancreatic polypeptide
  • C-peptide
  • Growth hormone levels

Medication and Blood Tests

Even if you are asked to fast for blood work, you should take your prescribed medications with water, unless specifically requested not to do so.

The exception to this is vitamins and supplements. These may affect certain lab tests, so they should be held the morning of a lab test.

Discuss what medications you are taking with your healthcare provider and clarify ahead of time if you have any questions on holding medications before blood work.

Pregnancy and Blood Tests

Most lab tests drawn in pregnancy do not require fasting, with the exception of the glucose challenge test. This test is performed to screen for a condition called gestational diabetes.

For this test, you will be asked to consume a special sugary beverage that contains a specific amount of glucose. Your blood glucose level will be tested at specific time intervals.

What to Do If You Accidentally Eat or Drink

If you accidentally ate or drank a sweetened beverage before your test, let your healthcare provider know. Depending on what the test was ordered for, you may be able to go ahead and have your blood drawn, and your healthcare provider will interpret it accordingly.

For example, if you are having a screening cholesterol panel and you ate breakfast before the test, it's not necessary to reschedule it. In fact, newer recommendations from the National Lipidology Association state that fasting for a screening lipid panel is optional.

While your breakfast will affect the triglyceride level, other important parts of the test, such as the total cholesterol and HDL (high-density lipoprotein, known as "good" cholesterol) will not be affected. LDL will only be affected if the triglyceride level is very elevated. If the triglyceride level is elevated, you may be asked to come back to repeat the test.

On the other hand, if a test was ordered specifically for blood sugar and you ate breakfast, the test may not be useful.

Pregnant patients undergoing the glucose challenge test will be asked to fast. If they do not, the test will be rescheduled.

Summary

Many lab tests do not require fasting. But for those that do, such as blood glucose tests, eating food can affect the results. Check with the healthcare provider who ordered the blood work to see if fasting is necessary, and if so, do not eat for to eight to 12 hours before the test. It's fine to take your prescribed medication and drink water before the test to stay hydrated.

(Video) Ask The Pharmacist – Ep 11 – Fasting and Bloodwork

A Word From Verywell

Blood tests can be a crucial part of monitoring health, and it's important that they be as accurate as possible. Make sure to confirm with your healthcare provider whether or not you are required to fast before your blood test. Remember that it's still important to stay hydrated by drinking water and to take your medications as prescribed.

Frequently Asked Questions

  • How long does it take to get blood test results?

    Depending on the test and how urgently your healthcare provider has indicated on the lab order form, blood test results can come back as soon as under an hour to several days. When the test is marked as "stat," it indicates to the lab that the test should be run and reported back as soon as possible, whereas "routine" means there is no rush for a result. The timing also depends on whether the test has to be transported to a special lab.

  • Why would I need to repeat a blood test?

    Your healthcare provider may ask that you have a repeat blood test when the results are invalid, to confirm unexpected results, or if not enough blood was provided to run all of the necessary tests. Blood is drawn in special tubes and transported to a lab for testing.

    The lab equipment requires a certain amount of blood to run the tests. Some lab tests are affected if the blood has sat in the tube for too long, if the tube was not maintained at the proper temperature, or if the blood underwent breakage (hemolysis) during the blood draw.

  • How do you book a blood test?

    Most blood tests require an order from a healthcare provider, such as a physician, nurse practitioner, or physician's assistant. Your healthcare provider's office may have a phlebotomist who can draw the labs right in the office, or you may be asked to go to a separate lab facility. Some facilities take walk-ins, while others require appointments.

    (Video) Vital Minute: The do’s and don’ts of fasting before routine bloodwork

5 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. MedlinePlus. Fasting for a blood test.

  2. Ashraf MM, Rea R. Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111

  3. Wilson PWF, Jacobson TA, Martin SS, et al. Lipid measurements in the management of cardiovascular diseases: Practical recommendations a scientific statement from the national lipid association writing group. J Clin Lipidol. 2021;15(5):629-648. doi:10.1016/j.jacl.2021.09.046

  4. Grundy Scott M, Stone Neil J, Bailey Alison L, et al. 2018 AHA/ACC/AACVPR/AAPA/ABC/ACPM/ADA/AGS/APHA/ASPC/NLA/PCNA guideline on the management of blood cholesterol.Journal of the American College of Cardiology. 2019;73(24):e285-e350. doi:10.1016/j.jacc.2018.11.003

  5. MedlinePlus. Glucose screening tests during pregnancy.

How Long Should You Fast Before a Blood Test? (2)

(Video) How to Fast Before a Triglycerides Blood Test

By Angela Ryan Lee, MD
Angela Ryan Lee, MD, is board-certified in cardiovascular diseases and internal medicine. She is a fellow of the American College of Cardiology and holds board certifications from the American Society of Nuclear Cardiology and the National Board of Echocardiography. She completed undergraduate studies at the University of Virginia with a B.S. in Biology, medical school at Jefferson Medical College, and internal medicine residency and cardiovascular diseases fellowship at the George Washington University Hospital. Her professional interests include preventive cardiology, medical journalism, and health policy.

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(Video) No Need to Fast Before Cholesterol Blood Test

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FAQs

Is 10 hours fasting enough for blood test? ›

Fasting is usually required for 10-12 hours before the test. Fasting blood glucose test: This test can be used to diagnose diabetes or prediabetes based on the measurement of glucose (sugar) in the blood after a period of not eating. Fasting is usually required for 8-10 hours before the test.

How soon should you drink water before a blood test? ›

Your results could come back wrong if you give in to temptation. Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand. If your appointment is at 8 a.m. and you're told to fast for 8 hours, only water is OK after midnight.

What are the rules for fasting blood work? ›

You can eat and drink as normal before some blood tests. But if you're having a "fasting blood test", you will be told not to eat or drink anything (other than water) beforehand. You may also be told not to smoke before your test.

Does fasting more than 12 hours affect blood test results? ›

IF YOU FAST LONGER THAN 16 HOURS - YOUR RESULTS MAY NOT BE ACCURATE, OR WE MAY NOT BE ABLE TO PERFORM SOME OF THE TESTS.

Can I brush my teeth before a fasting blood test? ›

Yes, brushing your teeth is permitted unless otherwise indicated by your physician, assuming you do not use large amounts of toothpaste and swallow the lather.

Does sleeping count as fasting? ›

While Intermittent Fasting, sleeping does count as fasting.

While you sleep, you're not consuming any food or drinks, so you're actually in a completely fasted state.

What should you not do before a blood test? ›

Preparing for a blood test

avoid eating or drinking anything (fasting) apart from water, for up to 12 hours – read more about eating and drinking before having a blood test. stop taking certain medicines.

What medications should you not take before a blood test? ›

Certain prescription and over-the-counter medications can impact the results of your blood test.
...
Examples of medicine that could skew your lab test results include:
  • Vitamins (for example, Biotin)
  • Antibiotics.
  • Antidepressants.
  • Steroids (like Prednisone)
  • Acetaminophen.
  • NSAIDs.
9 Aug 2022

Is 7 hours of fasting enough for blood 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.

What happens if you accidentally drink water while fasting? ›

Solid foods break your fast and cause your body to reenter the fed state, which lasts for several hours as your body breaks down and digests your food ( 1 ). However, water doesn't affect blood sugar or insulin levels. Therefore, you can safely drink it while intermittent fasting.

How long should I fast for cholesterol test? ›

You're generally required to fast, consuming no food or liquids other than water, for nine to 12 hours before the test. Some cholesterol tests don't require fasting, so follow your doctor's instructions.

Does drinking water affect blood sugar test? ›

Drinking water before a fasting blood sugar test can actually decrease blood sugar levels, or at least prevent levels from getting too high. Water allows more glucose to be flushed out of the blood. When you're dehydrated, it means your overall blood volume is lower than normal, but your sugars will be the same.

Does what you eat the night before affect blood test? ›

McKnight also mentioned the food or drinks you consume the day or night before a blood test does not impact your test results, unlike what you eat or drink the morning of your test. “It's recommended that you avoid coffee and other liquids during your fast,” McKnight said.

Why is my blood sugar high after fasting for 14 hours? ›

High levels of fasting blood sugar suggest the body was not able to lower blood sugar levels. This points to insulin resistance, inadequate insulin production, or in some cases, both. Very low blood sugar levels may indicate that diabetes medications are lowering the blood sugar too much.

What should blood sugar be after fasting for 14 hours? ›

The recommended blood glucose range after fasting in a person without diabetes is 70 to 130mg/dL. (The standard for measuring blood glucose is "mg/dL" which means milligrams per deciliter.)

What can affect blood test results? ›

Lots of things can affect certain lab test results, like:
  • Fasting vs nonfasting.
  • Being well hydrated vs dehydrated.
  • Medications.
  • Intense physical activity.
  • Some foods (like avocados, walnuts, and licorice)
  • Sunburn.
  • Colds or infections.
  • Having sex.
15 Nov 2021

Can I take blood pressure medication before a fasting blood test? ›

Often, it's advised to continue taking your regular medications, even before a fasting blood test.

What blood tests require 12 hour fasting? ›

This test analyzes one's iron levels, particularly to diagnose people with anemia. Fasting time: 12 hours.
...
Here are the blood tests that require fasting:
  • Blood Glucose Test. ...
  • Gamma Glutamyl Transferase (GGT). ...
  • Glucose Tolerance Test. ...
  • Liver Function Test. ...
  • Renal Function Test. ...
  • Vitamin B12 Test.

Can toothpaste break your fast? ›

Provided you aren't swallowing the toothpaste or mouthwash, it won't break your fast.

When does the body start burning fat during fasting? ›

Fat burning typically begins after approximately 12 hours of fasting and escalates between 16 and 24 hours of fasting.

What happens if you go to bed hungry? ›

Going to bed hungry can be safe as long as you're eating a well-balanced diet throughout the day. Avoiding late-night snacks or meals can actually help avoid weight gain and an increased BMI. If you're so hungry that you can't go to bed, you can eat foods that are easy to digest and promote sleep.

How do you get a good blood test result? ›

For the most accurate blood test results, your blood should be drawn when you're rested, Dr. Krajcik says. A workout before a fasting blood test can alter the results of cholesterol and glucose tests.

What are the three main blood tests? ›

A blood test is typically composed of three main tests: a complete blood count, a metabolic panel and a lipid panel.

Does lack of sleep affect blood test? ›

Sleep: Make sure you get enough sleep the night before the blood test. Exercise: Avoid any physical activity and stress prior to your blood test. It is recommended that you calm down and relax for 10-15 minutes prior to blood withdrawal.

What supplements affect blood tests? ›

The supplements that can cause these interactions include B vitamins such as biotin, niacin and riboflavin, as well as calcium supplements, St. John's wort, vitamin C, L-tryptophan, and 5-HTP, as well as certain foods and drinks.

Should I take my diabetes medication before a fasting blood test? ›

No. You should never take your insulin or antidiabetic medication prior to having a fasting blood test. The first reason is because the injected insulin will lower your glycemia and you could find yourself in a state of hypoglycemia.

What is the minimum time to fast? ›

The minimum amount of time it takes to make fasting efficacious hasn't been proven via study, but the prevailing notion is it's somewhere between 12 and 18 hours. But it can take a few days — sometimes weeks — of fasting regularly for your body to start burning fat for fuel.

What is a normal blood sugar level after a 12 hour fast? ›

A blood sample will be taken after you haven't eaten anything the night before (fast). A fasting blood sugar level less than 100 mg/dL (5.6 mmol/L) is normal. A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered prediabetes.

Can I drink water before a fasting blood test for cholesterol? ›

If a healthcare professional has asked a person to fast, they should not consume anything other than water the night before a cholesterol test. Fasting means that a person can only consume water for the hours before their test. If a person does not need to fast, they should be able to eat and drink normally.

Does drinking lemon water break a fast? ›

The truth about lemon water and intermittent fasting

Lemon water contains almost no calories and zero sugars, it doesn't raise insulin levels, which means it will not break your fast (1). So we can conclude that it is absolutely safe for fasting to drink lemon water.

What can I drink while fasting? ›

Generally speaking, while fasting with time-restricted intermittent eating, you don't eat any food at all and only drink beverages with very few calories, such as water or unsweetened coffee and tea without milk.

How much water should you drink while water fasting? ›

During a water fast, you are not allowed to eat or drink anything besides water. Most people drink two to three liters of water per day during a water fast ( 7 ). The water fast lasts for 24–72 hours. You should not water fast for longer than this without medical supervision because of health risks.

What can throw off a cholesterol test? ›

Improper fasting, medications, human error, and a variety of other factors can cause your test to produce false negative or false positive results. Testing both your HDL and LDL levels typically produces more accurate results than checking your LDL alone.

What happens if you dont fast before cholesterol test? ›

Do you need to fast? The truth is, your cholesterol can be tested without fasting. In the past, experts believed fasting ahead of time produces the most accurate results. This is because your low-density lipoproteins (LDL) — also known as “bad” cholesterol — may be affected by what you've recently eaten.

What is a healthy cholesterol level by age? ›

Cholesterol levels and age
Type of cholesterolAnyone 19 or youngerWomen aged 20 or over
total cholesterolless than 170 mg/dl125–200 mg/dl
non-HDLless than 120 mg/dlless than 130 mg/dl
LDLless than 100 mg/dlless than 100 mg/dl
HDLmore than 45 mg/dl50 mg/dl or higher

What is the normal blood sugar level for a 70 year old? ›

Normal ranges of blood sugar levels are between 70 and 130 mg/dL before eating meals. The American Diabetes Association recommends seniors have blood glucose levels of less than 180 mg/dL two hours after eating. Not every senior has the same care needs, which means they don't all need the same type of at-home care.

What is the fastest way to bring down your blood sugar? ›

When your blood sugar level gets too high — known as hyperglycemia or high blood glucose — the quickest way to reduce it is to take fast-acting insulin. Exercising is another fast, effective way to lower blood sugar. In some cases, you should go to the hospital instead of handling it at home.

Does walking lower blood sugar? ›

Physical activity can lower your blood sugar up to 24 hours or more after your workout by making your body more sensitive to insulin.

Is 7 hours of fasting enough for blood 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.

What is a normal blood sugar level after a 12 hour fast? ›

A blood sample will be taken after you haven't eaten anything the night before (fast). A fasting blood sugar level less than 100 mg/dL (5.6 mmol/L) is normal. A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered prediabetes.

Which blood tests require 12 hour fasting? ›

This test analyzes one's iron levels, particularly to diagnose people with anemia. Fasting time: 12 hours.
...
Here are the blood tests that require fasting:
  • Blood Glucose Test. ...
  • Gamma Glutamyl Transferase (GGT). ...
  • Glucose Tolerance Test. ...
  • Liver Function Test. ...
  • Renal Function Test. ...
  • Vitamin B12 Test.

How long should I fast before cholesterol test? ›

You're generally required to fast, consuming no food or liquids other than water, for nine to 12 hours before the test. Some cholesterol tests don't require fasting, so follow your doctor's instructions.

Does what you eat the night before affect blood test? ›

McKnight also mentioned the food or drinks you consume the day or night before a blood test does not impact your test results, unlike what you eat or drink the morning of your test. “It's recommended that you avoid coffee and other liquids during your fast,” McKnight said.

What should you not do before a blood test? ›

Preparing for a blood test

avoid eating or drinking anything (fasting) apart from water, for up to 12 hours – read more about eating and drinking before having a blood test. stop taking certain medicines.

Does drinking water affect blood sugar test? ›

Drinking water before a fasting blood sugar test can actually decrease blood sugar levels, or at least prevent levels from getting too high. Water allows more glucose to be flushed out of the blood. When you're dehydrated, it means your overall blood volume is lower than normal, but your sugars will be the same.

What drink lowers blood sugar? ›

Drinking water regularly may rehydrate the blood, lower blood sugar levels, and reduce diabetes risk ( 20 , 21 ). Keep in mind that water and other zero-calorie drinks are best.

Can your blood sugar go up from not eating? ›

Skipping breakfast—going without that morning meal can increase blood sugar after both lunch and dinner. Time of day—blood sugar can be harder to control the later it gets.

Why does my blood sugar go up without eating? ›

Physical or emotional stress triggers the release of hormones that can cause high blood sugar levels. Menstrual periods and menopause also cause changes in the hormones that affect blood sugar levels. Regular blood sugar testing can uncover patterns. This can help you and your health care team control your diabetes.

How can I lower my cholesterol before a blood test? ›

Foods to lower cholesterol
  1. barley.
  2. oats.
  3. okra.
  4. eggplant.
  5. nuts, such as almonds, peanuts, and walnuts.
  6. beans, such as kidney beans, garbanzo beans, and black-eyed peas.
  7. lentils.
  8. fruits, such as apples, citrus fruits, and strawberries.

Can I take blood pressure meds before blood work? ›

Often, it's advised to continue taking your regular medications, even before a fasting blood test.

Will one cup of coffee affect blood test? ›

For example, a 2005 study found that drinking one cup of coffee within an hour of a cholesterol test does not significantly impact the clinical test results. Specifically, black coffee did not significantly affect TC/HDL-C levels, and coffee with sugar and creamer did not significantly alter triglyceride levels.

What can throw off a cholesterol test? ›

Improper fasting, medications, human error, and a variety of other factors can cause your test to produce false negative or false positive results. Testing both your HDL and LDL levels typically produces more accurate results than checking your LDL alone.

What happens if you dont fast before cholesterol test? ›

Do you need to fast? The truth is, your cholesterol can be tested without fasting. In the past, experts believed fasting ahead of time produces the most accurate results. This is because your low-density lipoproteins (LDL) — also known as “bad” cholesterol — may be affected by what you've recently eaten.

Is 8 hours fasting enough for cholesterol test? ›

According to the CDC, most cholesterol tests (called “lipid panel” or “lipid profile”) require you to fast for 8 to 12 hours. [2] However, some tests don't require fasting, so it's best to check with your healthcare provider ahead of time.

Videos

1. Fasting for a Blood Test: Dos and Dont’s
(Health Insider)
2. Dr. Lilybeth Naguit tackles the importance of fasting before undergoing medical tests | Salamat Dok
(ABS-CBN News)
3. 6 things to do before a blood test
(Serra Medical Group)
4. Why You Don't Have to Fast for Blood Work
(365 Health)
5. Testosterone as a Fasting Blood Test?
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6. Hi9 | Blood Tests that Require Fasting | Dr R Vijaya Radhika | Pathologist
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