Fasting for Blood Tests

It's the morning of your bloodwork and your doctor said to fast before the test. But your stomach is growling and you have serious caffeine withdrawal hours before you roll up your sleeve. A bite of toast and a few gulps of coffee won't really make a difference, right?

Not so fast. Your results could come back wrong if you give in to temptation.

Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand.

So, if your appointment is at 8 a.m. and you're told to fast for 8 hours, only water is okay after midnight. If it's a 12-hour fast, avoid food and drink after 8 p.m. the night before.

You also shouldn't smoke, chew gum (even sugarless), or exercise. These things can rev up your digestion, and that can affect your results.

Take your prescription medications unless your doctor tells you to skip them. But ask your doctor before you take any over-the-counter drugs.

What Tests Do I Fast For?

Blood tests help doctors check for certain health problems and find out how well your body is working. Doctors also use them to figure out how well treatments are working. You don't need to fast before all blood tests. Your doctor will tell you if you need to.

These tests typically require fasting:

  • Fasting blood glucose measures the amount of glucose (sugar) in your blood to test for diabetes or prediabetes.
    Typical fasting time: At least 8 hours
  • Lipid profile is used to check the level of cholesterol and other blood fats. High levels put you at risk for developing heart disease or having a stroke.
    Typical fasting time: 9-12 hours
  • Basic or comprehensive metabolic panel is often part of a routine physical. The tests check your blood sugar, electrolyte and fluid balance, and kidney function. The comprehensive test checks your liver function, too.
    Typical fasting time: 10-12 hours
  • Renal function panel is used to gauge the health of your kidneys and how well they're working.
    Typical fasting time: 8-12 hours
  • Vitamin B12 test measures how much of the vitamin is in your blood. It can help diagnosis a specific type of anemia and other problems. Some medications can interfere with this test. Tell your doctor about all the drugs you take.
    Typical fasting time: 6-8 hours
  • Iron tests are used to see if iron levels in your system are too low or too high.
    Typical fasting time: 12 hours
  • Gamma-glutamyl transferase (GGT) shows the level of the GGT enzyme in your system. A high reading may indicate liver disease, bile duct problems, or alcohol abuse.
    Your doctor may ask you to fast for at least 8 hours beforehand. You also may need to avoid alcohol and some prescription drugs the day before the test because they can affect GGT levels. Talk to your doctor before stopping any prescribed medicines.

 

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Why Do I Have to Fast?

Nutrients in food and drinks go into your bloodstream and can change things measured by the tests, skewing your results.

For instance, if you eat or drink before a fasting blood glucose test, your blood sugar probably will be higher than if you hadn't had anything. When you're fasting, doctors get a baseline result so tests can be compared to give a true picture of your sugar levels over time.

What If I Slip Up?

If you make a mistake and eat or drink anything besides water, tell the person taking your blood. Your doctor will want to know so she can interpret your tests correctly. For the best results, she may ask you to reschedule.

When Can I Eat or Drink Again?

As soon as your blood is taken, your fast is over. You might want to bring a snack and a drink with you so you can eat as soon as possible after the test.

WebMD Medical Reference Reviewed by William Blahd, MD on July 06, 2016

Sources

SOURCES:

American Academy for Clinical Chemistry: "Basic Metabolic Panel," "Comprehensive Metabolic Panel," "GGT," "Glucose Tests," "Iron Tests," "Lipid Profile," "Renal Function Panel," "Vitamin B12 & Folate."

British National Health Service: "Can I Eat and Drink Before Having a Blood Test?"

Crystal Moore, MD, PhD, FCAP, anatomical and clinical pathologist, Chesapeake, VA.

National Heart Lung and Blood Institute: "What Are Blood Tests?"

Nemours Foundation: "Blood Test: Comprehensive Metabolic Panel (CMP)."

Providence Laboratory Services (Oregon): "Fasting Instructions."

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