Fasting For a Blood Test: Why Fasting is Required?

Some blood tests will require you to fast beforehand. In these cases, your doctor will instruct you not to eat or drink anything, except water, in the hours leading up to the test.

Fasting before certain blood tests is important to help make sure that your test results are accurate. The vitamins, minerals, fats, carbohydrates, and proteins that make up all food and beverages can impact blood-level readings, clouding the results of your test.

What types of blood tests require fasting?

The most common types of tests that require fasting include:

  • Glucose tests

This test measures blood sugar. One type of glucose test is called a glucose tolerance test. For this test, you will need to fast for 8 hours before the test. When you arrive at the lab or health care facility, you will:

  • Have your blood tested
  • Drink a special liquid containing glucose
  • Have your blood re-tested one hour later, two hours later, and possibly three hours later

Glucose tests are used to diagnose diabetes.

  • Lipid tests

This test measures triglyceride, a type of fat found in the bloodstream, and cholesterol, a waxy, fat-like substance found in your blood and every cell of your body. High levels of triglycerides and a type of cholesterol, called LDL can put you at risk for heart disease.

Other blood tests that you will likely need to fast for include:

  • Liver function test
  • Cholesterol test
  • High-density lipoprotein (HDL) level test
  • Low-density lipoprotein (LDL) level test
  • Basic metabolic panel
  • Renal function panel
  • Lipoprotein panel

If your doctor has prescribed a new blood test for you or doesn’t mention whether or not you should fast or for how long, ask them if fasting is required. Some tests, such as a fecal occult blood test, don’t require fasting but do limit certain foods. Red meats, broccoli, and even some medications may cause a false positive test. Always follow your doctor’s advice when preparing for a test.

Why fasting is required

When people eat food and drink alcohol, the food and liquid get broken down in their stomachs and absorbed into the bloodstream. This can affect the levels of certain substances in the blood, such as blood glucose or cholesterol.

Measuring the levels of these substances is crucial to diagnose certain conditions, such as:

  • diabetes
  • anemia
  • high cholesterol
  • liver disease

For correct diagnosis of these conditions, it is important that a person fasts. Eating or drinking before the test may raise the levels of a particular substance in the blood, leading to inaccurate results. Incorrect results could in turn lead to an incorrect diagnosis.


Before a person decides to fast, they should speak with a doctor to find out whether they should fast, and if so, for how long.

If fasting becomes too difficult, and a person breaks the fast due to extreme hunger or thirst, they should contact the doctor to reschedule the blood test appointment.

People can receive the wrong diagnosis if their blood test results are inaccurate, leading to further health complications. This is why following best practices around fasting before blood tests are so important.

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