Fast before a Blood draw

for how long should I fast before a Blood draw?

By LabBee | 2022  5-minute read 

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).

How Long to Fast

Generally, you should fast for eight to 12 hours before lab work that requires it. 1 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.

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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. 2 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:

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 but check it first so you don’t need to reschedule the test

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

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

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