Fasting Before a Blood Test: Everything You Need to Know

When your doctor recommends you to fast before a blood test, many questions may run through your mind. You might want to know about things like how long you should fast, whether you should drink anything besides water, and more. Here is a helpful guide to fasting for blood tests.

Blood Tests

When and for How Long Should You Fast Before a Blood Test?

Not all blood tests require fasting. Standard blood tests that require fasting to be more effective are:

  • Fasting Blood Glucose Test

This test helps to diagnose diabetes in patients by measuring the level of glucose in their blood. For accurate test results, a patient needs to fast for 8 to 10 hours.

  • Iron Blood Test

It helps to diagnose the health conditions caused by lack of iron. This test measures the iron level in your blood. A patient should not consume food on the morning of the test to ensure accurate results.

  • Blood Cholesterol Test

It measures the level of cholesterols in the blood, including good cholesterol, bad cholesterol, and triglycerides.  Average fasting time required for this test is 9 to 12 hours.

  • Gamma-Glutamyl Transferase Test

This test helps to diagnose liver disease by measuring the level of GGT enzyme in your system. You should avoid smoking and alcohol intake for 24 hours before the test.

What Other Blood Tests Require Fasting?

Your doctor may also prescribe fasting for:

  • Renal Function Panel Test

This test is performed to check the health of your kidneys and how well they are working. A patient needs to fast for 8 to 12 hours before the test.

  • Basic or Comprehensive Metabolic Tests

This test measures your kidney function, blood sugar, and electrolyte balance. Standard fasting time for this test is 10 to 12 hours.

  • Vitamin B12 Test

It measures the levels of the vitamin B12 in your blood. 6 to 8 hours of fasting is required for this test.

Why Is Fasting Required in Blood Tests?

Fasting is essential in blood tests to ensure accurate results. When you take foods and drink alcohol, nutrients in food and drink gets broken down and occupied into your bloodstream, which may affect the levels of certain blood substances including:

  • Enzymes such as GGT
  • Sugar
  • Cholesterol and other fats
  • Minerals such as iron

The measurement of the levels of these substances is vital for diagnosing certain health conditions such as:

  • Liver disease
  • Diabetes
  • High cholesterol
  • Anemia

Tips for Fasting Before a Blood Test

Here are some tips for you to consider when fasting for a blood test:

  • Complete the recommended fasting timings
  • Consult your doctor if you’re pregnant
  • Drink more water and stay hydrated
  • Take your regular medications

Things You Should Avoid When Fasting for a Blood Test

You should avoid certain things when fasting for a blood test which include:

  • Exercise
  • Alcohol
  • Chewing gum
  • Coffee
  • Smoking

When Can I Eat or Drink Again?

You can eat or drink immediately after your blood test.


The above information can help a person safely fast for a blood test and get accurate results. Sometimes, you may be unable to complete the recommended timing for fasting. You should inform your doctor about this so that they can reschedule the test to another day.


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