How Long Should You Fast Before a Blood Test?

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.

Woman getting blood drawn

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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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