How to Fast for a Blood Test 

Certain blood tests require fasting beforehand. If your healthcare provider has instructed you to fast before your upcoming test, it means that you should not eat or drink anything (besides water) for the specified amount of time before having your blood drawn.

Understanding why fasting is important and how to properly fast before a blood test can be helpful for eliminating pre-test anxiety and simplifying the testing process. Below, learn how to fast for a blood test correctly to help make sure that your test results are accurate. 

Why Is Fasting Required Before Blood Tests?

Fasting before a blood test helps improve the accuracy and reliability of the test. Accurate test results are a critical component of diagnosing various health conditions and monitoring the effectiveness of treatment plans. 

Your diet contains a range of nutrients, including carbohydrates, proteins, fats, minerals and vitamins. When you eat and drink normally, those nutrients are broken down and absorbed into your bloodstream. This can impact the levels of those substances in the blood and skew lab test results.

For example, certain methods of diabetes testing measure the patient’s baseline blood sugar levels after a period of fasting. Eating directly before the test will cause the patient’s blood sugar to spike, leading to an inaccurate test result. The patient could be misdiagnosed with prediabetes or diabetes, unless their doctor recommends that they complete a second test.

Which Blood Tests Require Fasting?

Not all lab tests require fasting beforehand, but many of the most common blood tests do. Tests that typically require fasting include:

  • Basic or comprehensive metabolic test: Commonly part of a routine physical, this test measures the body’s blood sugar, kidney function, and other vital organ function. Fasting is usually required for 10-12 hours before the test.
  • Fasting blood glucose test: This test can be used to diagnose diabetes or prediabetes based on the measurement of glucose (sugar) in the blood after a period of not eating. Fasting is usually required for 8-10 hours before the test.
  • Cholesterol test: Also known as a lipid profile, this test measures the amount of cholesterol and other fats in the blood. Fasting is usually required for 9-12 hours before the test.
  • Gamma-glutamyl transferase (GGT) test: This test measures the level of GGT enzyme in your blood, which can indicate liver disease. Avoiding alcohol for up to 24 hours beforehand is usually required; additionally, you may or may not be required to avoid food for up to 8 hours before the test.
  • Other nutritional tests: Blood tests that measure vitamins and minerals (such as iron) often require fasting beforehand. Depending on the type of nutritional testing, you may be asked to fast for between 6-12 hours before the test. 

Can I Drink Water Before a Blood Test?

Yes, you can drink water while fasting before a blood test—in fact, drinking plenty of water can help ensure that you receive accurate test results. Dehydration can affect certain blood tests such as cholesterol, electrolyte and BUN tests. Be sure to stay hydrated before your test by drinking the recommended amount of water for your weight and activity level.

Can I Drink Coffee / Alcohol / Juice / Soda / Tea While Fasting?

No. Any beverage other than water should not be consumed before a blood test, unless approved by your healthcare provider. Alcohol, soda, and juice contain high quantities of sugar, which can interfere with many common test results. 

Black coffee, tea, and other caffeinated beverages are diuretics, which can have a dehydrating effect and cause test results to be inaccurate. For best results, avoid drinking all non-water beverages for the recommended amount of time before your test.

What If I Accidentally Eat or Drink Before My Blood Test?

If you have been instructed to fast before your blood test and you accidentally eat or drink something during the fasting window, it is important to notify your healthcare provider or the person drawing your blood. If you don’t, your results could be incorrectly interpreted.

It may be possible for your healthcare provider to interpret the non-fasted test results. Most likely, they will instruct you to reschedule the blood test at a time when you will be able to successfully complete the fast before the test.

Your physician will let you know the steps you need to take before your blood test, including whether or not fasting is required and for how long. By following all testing instructions and best practices (including how to fast), you can ensure a smooth testing process and the most accurate results.

Are you a patient in need of lab testing? Find a convenient testing location and schedule an appointment near you.

*Post reviewed and approved by Dr. Sona Kirpekar, our in-house Medical Consultant.