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How long to fast before a blood test

Updated March 23, 2017

Fasting before a blood test is important to obtain accurate readings. When you consume food, your body has to process the food's components as they travel through your bloodstream. Blood tests examine a variety of blood and sugar levels that can be compromised if foreign entities, such as food, contaminate blood, painting an inaccurate picture. Fasting beforehand allows the doctor to obtain a clear picture of your bodily functions for an accurate diagnosis.

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Tests that require fasting

Fasting before a blood test is typically required for fasting glucose, fasting lipid panel, fasting metabolic panel, fasting cholesterol, HDL or triglyceride tests. These tests look for cholesterol and glucose levels in your blood, and any food consumed before the test will provide an inaccurate reading.

How long should I fast?

Your doctor may recommend a certain time period for fasting; however, the standard amount of time is about eight to 12 hours of no food before having blood drawn. Schedule your blood test for early in the morning so you only need to fast overnight to meet requirements.

Fast for eight hours before a glucose test. The test is typically conducted in the morning while your body is still in a resting place for a more accurate reading.

You must fast at least 12 hours before taking a cholesterol blood test to get an accurate reading on triglycerides. The NHS warns against quick cholesterol tests you find in shopping centres and health fairs because fasting is imperative to obtaining an accurate result.

Fasting dos and don'ts

Stay hydrated and drink plenty of water. Take any medication that your doctor prescribed to you except for corticosteroids, oestrogen or androgens, oral contraceptives, some diuretics, antipsychotic medications, some antibiotics and niacin.

Do not smoke, drink any other liquid than water or exercise during your fast. Even chewing gum is off-limits. Any of these elements can adversely affect your test results.

Follow doctor's orders

Because some tests are ordered at the same time -- some require an eight-hour fast, others 12 and others none -- in an effort to be efficient, follow your doctor's orders regarding fasting and "nbo" status (nothing by mouth -- including water) before blood tests.

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About the Author

Gina Ragusa has made a career out of writing for the past 15 years, with an emphasis on financial institution writing. Ragusa has written for Consumer Lending News, Deposit and Loan Growth Strategies and Community Bank President. She earned her Bachelor of Arts from Michigan State University.

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