Instructions for fasting before a blood test

Updated July 19, 2017

Certain blood tests are sensitive to any type of food or drink. When these tests are performed, it is necessary for the patient to fast for several hours before blood is drawn. Some tests require a 12-hour fast and others only for eight hours. It's important to know which is required before blood is drawn to assure the accuracy of the results.

Types of Tests

The main tests that require fasting are blood glucose, triglycerides and cholesterol levels. These tests are very sensitive and require the patient to not have eaten or had anything to drink before blood is drawn. Fasting is required for some vitamin tests, such as vitamins A and E, and also for certain specialised tests.

How Long?

When fasting is required, it is important to find out how long a fast is needed. For example, triglyceride and cholesterol screenings typically require at least a 12-hour fast. Other tests, such as glucose and vitamin levels call for a fast of only eight hours. Following these guidelines is imperative.

What is Allowed

When fasting before a blood test, water is allowed. Frequently it is allowable to take your regular dosages of medication, but some medications can affect blood test results. Always check with your physician first. For some tests, black coffee or tea with no sugar is also sometimes allowed. Caffeine can alter certain test results as well.

What is Not Allowed

Gum chewing is not allowed, especially gum with sugar in it. Liquids other than water should not be consumed unless you have permission from your doctor. People frequently think that clear liquids are acceptable to consume, however they are not.

What Can Happen

If you do not fast before a blood test, it is most likely that your results will be inaccurate. This can lead to being treated for a disease you do not have, such as diabetes or high cholesterol. It will also mat lead to a another test. If you have not fasted as you were supposed to, it is necessary to admit that to your physician when your results come in.

Keep in Mind

Fasting is not a torture tactic for patients. Always ask if a test needs to be fasting or not prior to having your blood drawn. Call ahead to the lab to find out which days they usually have the fewest patients so you can be sure you will taken care of quickly. Let the lab know as soon as you get there that you are fasting.

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About the Author

Beth Celli is a native New Yorker who is now based in Delaware. She has a varied background in both health care and education, having worked in several different hospitals and medical centers prior to teaching allied health classes. She attended New York University and her writing has appeared on various online publications.