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.
:max_bytes(150000):strip_icc()/GettyImages-4831245201-dea84a29aded4d56a97deb3c9c52dcb6.jpg "Fasting Before a Blood Test: How Long, Facts, and More (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.
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.
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.
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.
MedlinePlus. Fasting for a blood test.
Ashraf MM, Rea R. Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111
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
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
MedlinePlus. Glucose screening tests during pregnancy.
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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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FAQs
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.
Is it OK to fast longer than 12 hours before blood test? ›Patients should not fast for longer than 12 hours. While fasting is critical to these blood tests' reliability and validity, over fasting can result in dehydration or other side effects. When fasting, remind patients that sleep also counts as fasting.
How many hours is over fasting for blood test? ›A fasting blood test is usually done in the morning after you have fasted for 8 to 16 hours.
What are the rules for fasting blood work? ›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. If it's a 12-hour fast, avoid food and drink after 8 p.m. the night before.
How much water should you drink before a blood test? ›Ideally, start drinking more fluids the day before your blood draw, and continue to drink water before you have your blood drawn. Excessive amounts aren't necessary; most sources ecommend that an adult drink 64 ounces of water per day for good health, which is more than adequate for having your blood drawn.
Can fasting too long affect blood sugar levels? ›And if you fast for more than a day or so, your body may not get enough of the nutrients it needs without supplements. But the biggest danger of fasting if you have diabetes is that your blood sugar levels could go dangerously low (this is called hypoglycemia).
Can you brush teeth before 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.
Is fasting blood sugar 8 or 12 hours? ›Common fasting blood tests
a fasting blood glucose test (used to test for diabetes) – you may be asked to fast for 8 to 10 hours before the test. an iron blood test (used to diagnose conditions such as iron deficiency anaemia) – you may be asked to fast for 12 hours before the 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. When you eat and drink normally, those foods and beverages are absorbed into your bloodstream.
Does sleeping count as fasting? ›Does sleeping count as fasting? A. Yes, while following intermittent fasting, sleeping is considered a fasting period. Therefore, one does not consume food or drinks during this state.
Why is my blood sugar high after fasting for 16 hours? ›
When fasting the hormone glucagon is stimulated and this increases plasma glucose levels in the body. If a patient doesn't have diabetes, their body will produce insulin to rebalance the increased glucose levels.
How long is too long to fast? ›If you want to increase your fasting period to more than 72 hours, you should seek medical supervision. Summary Longer periods of fasting increase your risk of side effects, such as dehydration, dizziness and fainting. To reduce your risk, keep your fasting periods short.
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 medications should you not take before a blood test? ›...
Examples of medicine that could skew your lab test results include:
- Vitamins (for example, Biotin)
- Antibiotics.
- Antidepressants.
- Steroids (like Prednisone)
- Acetaminophen.
- NSAIDs.
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.
It can take just 5 minutes to begin rehydrating your body. On the other hand, if you drink water while eating, your body will prioritize digesting food before water. This often takes up to 120 minutes to digest water and rehydrate your body.
Does drinking water affect blood test? ›Water does not affect the results of a blood test and is acceptable to drink when a person needs to fast. Timings: Whether a person has to fast for 8, 12, or 24 hours, it is a good idea to work out what is the latest time they can eat or drink before the test.
How long does it take for water to get to your veins? ›Liquids are rapidly absorbed into the bloodstream, and fluids in excess of the body's needs are eliminated via the kidneys as urine, much faster. Water absorption can occur as soon as 5 minutes after ingestion and peaks around 20 minutes after ingestion.
Will drinking water lower fasting blood sugar? ›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 time of day is blood sugar highest? ›The dawn phenomenon, also called the dawn effect, is the term used to describe an abnormal early-morning increase in blood sugar (glucose) — usually between 2 a.m. and 8 a.m. — in people with diabetes.
How can I lower my blood sugar before a blood test? ›
It's a simple, safe and common way to diagnose prediabetes, diabetes or gestational diabetes. A healthcare provider will prick your finger or use a needle to draw blood from a vein in your arm. Don't eat or drink anything (except water) for 8 to 12 hours before the test.
What can affect blood test results? ›- 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.
Often, it's advised to continue taking your regular medications, even before a fasting blood test.
What should you not do after a blood test? ›- Avoid tight or restrictive clothing on the arm.
- If bleeding occurs, apply firm pressure with your fingers directly over the needle site.
- Avoid strenuous use of the arm eg sports or heavy lifting.
- Changing the timing or type of your diabetes medications.
- Eating a lighter breakfast.
- Increasing your morning dose of diabetes medication.
- If you take insulin, switching to an insulin pump and programming it to release additional insulin in the morning.
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.
How long does it take to get sugar out of your system for a blood test? ›For people without diabetes, their blood sugar returns to near normal range about 1-2 hours after eating as a result of the effects of insulin.
Is 10 hours long enough to fast for blood work? ›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.
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.
Can I drink coffee before a fasting blood test? ›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.
› articles ›
Fasting blood sugar levels and testing
Fasting Blood Sugar Levels
Fasting Blood Sugar: Normal, High, & Low + Ways to Improve
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 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.
Is 14 hours of fasting before blood test? ›A fasting blood glucose test usually requires an overnight fast that lasts for about 8 to 10 hours. Blood cholesterol test. Some types of cholesterol tests don't require fasting. Some like a direct LDL cholesterol test may require up to a 14-hour fast.
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.)
Does drinking water increase fasting blood sugar? ›Water will not raise blood glucose levels, which is why it is so beneficial to drink when people with diabetes have high blood sugar, as it enables more glucose to be flushed out of the blood.
What time of day is blood sugar highest? ›The dawn phenomenon, also called the dawn effect, is the term used to describe an abnormal early-morning increase in blood sugar (glucose) — usually between 2 a.m. and 8 a.m. — in people with diabetes.
How do I get my fasting blood sugar down? ›- Exercise regularly. ...
- Manage your carb intake. ...
- Eat more fiber. ...
- Drink water and stay hydrated. ...
- Implement portion control. ...
- Choose foods with a low glycemic index. ...
- Try to manage your stress levels. ...
- Monitor your blood sugar levels.
Avoiding specific foods and drinks such as cooked meats, herbal tea, or alcohol. Making sure not to overeat the day before a test. Not smoking. Avoiding specific behaviors such as strenuous exercise or sexual activity.
Can blood sugar go up without 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. Dawn phenomenon—people have a surge in hormones early in the morning whether they have diabetes or not.
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.