What You Should Know About Fasting Before a Blood Test

Less common tests that require fasting include:

How Long Should You Fast Before a Blood Test?

Dr. Angela Ryan Lee is board certified in cardiology and internal medicine. Her professional interests include preventive cardiology, medical journalism, and health policy.

Published on March 17, 2022

Michael Menna, DO, is a board-certified, active attending emergency medicine physician at White Plains Hospital in White Plains, New York.

Table of Contents
Table of Contents

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.

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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 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.

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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.

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.

What You Should Know About Fasting Before a Blood Test

So, your healthcare provider has asked you to fast before your blood test. Fasting means not eating and drinking for a period of time. You often need to do this before bloodwork. This is because what you eat and drink ends up in your bloodstream and can skew your results.

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But can you drink black coffee when fasting for bloodwork? How about taking your medications? Family medicine specialist Timothy Tramontana, MD, MS, answers your frequently asked questions.

Q: How long should you fast for a blood test?

A: Typically, you should fast before bloodwork anywhere between 10 and 12 hours. Usually, people choose to do blood tests first thing in the morning. This way, you can fast overnight, when you generally don’t eat anyway.

Types of fasted blood tests

Blood tests are a quick, effective way for doctors to check your health and to see if certain treatments are working. Blood tests you may need to fast for include:

• Basic metabolic panel: Measures overall body function.
• Blood glucose test: Measures glucose (sugar).
• Cholesterol test: Measures cholesterol and triglycerides, a type of fat. Also called lipid panel or lipid profile.
• Liver function test: Measures liver health by checking proteins, liver enzymes and bilirubin.
• Renal function panel: Measures kidney function.

Q: Can you drink water before a fasting blood test?

A: Yes. You can drink as much water as you want, but it should be plain. Avoid lemon-flavored and carbonated waters, as well as teas until after your test.

Q: Can you drink black coffee when fasting for bloodwork?

A: Don’t drink anything but water. Coffee contains caffeine. It’s also a diuretic, which removes water from your body. Many people think coffee counts toward your fluid requirements. However, it makes you urinate more, which can affect some of your lab results. So, get your blood test done, then get your cup of coffee!

Q: Can you take medication before a fasting blood test?

A: You can take over-the-counter and prescription medications unless your doctor advises you otherwise. On that note, it’s definitely a good idea to tell your doctor about all medications and supplements you’re taking to avoid something potentially skewing lab results.

There’s no indication that daily medications — such as blood thinners and blood pressure meds — will cause any problems with fasting bloodwork, so definitely continue taking them unless told otherwise. And if you have a headache, taking an ibuprofen or another over-the-counter pain reliever shouldn’t cause any issues.

Q: What if you forgot to fast or ate something by accident?

A: Let your doctor know so you don’t end up doing a test unnecessarily. If you’ve already gone for the test without fasting, and the results come back normal, then you’re most likely OK. But if results come back abnormal, that may mean you have to redo the bloodwork while fasting.

Q: What if you have more questions about your test?

A: Call or send a message to your primary care team. Someone should be able to field your questions so you can feel as prepared as possible.

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