Fasting before a blood test: What you need to know

Blood being taken for testing.

By Lana Burgess | Medical News Today

Fasting before a blood test is when people are asked not to eat or drink anything other than water before some blood tests. But which blood tests require fasting and how can people fast safely?

Fasting is not always necessary before a blood test, but when it is, it is only for a short time. Even so, the idea of not eating or drinking, even for a small amount of time, can seem daunting.

Understanding when and how to fast before a blood test can help to reduce unnecessary worry. This article explores the types of blood tests that require fasting, why fasting is needed, and how a person can do it safely.

When should you fast before a blood test?

Whether someone needs to fast or not before a blood test depends on the type of blood test they are having. Some blood tests require fasting to be effective, while others do not.

The types of blood test that require fasting are as follows:

Fasting blood glucose test

A fasting blood glucose test can diagnose diabetes.

Diabetes is a condition that can lead to there being too much sugar in the blood. A fasting blood glucose test measures levels of sugar in the blood to see if they are healthy.

It is important that a person has not had anything to eat or drink other than water for 8 to 10 hours before a fasting blood glucose test. Fasting helps ensure that the blood test records an accurate measure of fasting blood sugar levels. The results help a doctor to diagnose or rule out diabetes.

Iron blood test

Iron supplements.

An iron blood test measures the levels of the mineral iron in the blood. This test helps identify conditions that are caused by a lack of iron in the blood, such as anemia.

Iron is contained in some types of food and is absorbed very quickly from food into the blood. So, if a person eats food before the iron blood test, the results may show inflated levels of iron.

To ensure accurate results, people are asked not to eat anything on the morning of the test.

Some people may take iron supplements or multivitamin tablets that contain iron. These can also affect results. If a person regularly takes these supplements, they should not take them for 24 hours before the iron blood test.

Blood cholesterol tests

Cholesterol is a fatty substance in the blood. High cholesterol can lead to an increased risk of certain health conditions.

Blood cholesterol tests, also known as lipid profiles, assess the quantities of fats in the blood. The different fats tested for include:

  • HDL cholesterol, also known as “good cholesterol”

  • LDL cholesterol, also known as “bad cholesterol”

  • triglycerides

The amounts of these fats will increase if a person has recently eaten food. So, people are asked not to eat for 9 to 12 hours before the test, which helps to give an accurate profile of the amounts of these fats in the blood.

Some recent guidelines suggest that fasting is not necessary before all cholesterol and triglyceride tests. However, people who are having these tests should refrain from drinking alcohol for 24 hours before the test. It is always best for individuals to check with their doctor to see whether these new guidelines apply to them.

Gamma-glutamyl transferase test

A gamma-glutamyl transferase (GGT) test helps to diagnose liver disease. GGT is an enzyme in the liver that helps it to work effectively.

Eating does not affect GGT levels, but drinking alcohol and smoking can. People having this test are asked not to consume alcohol or smoke for 24 hours before the test.

Other blood tests that require fasting

People may be asked to fast for:

  • Basic or comprehensive metabolic tests: Tests for blood sugar, electrolyte balance, and kidney function. Typically, people will be asked to fast for 10 to 12 hours before having one of these tests.

  • Renal function panel: Tests to see how well the kidneys are working. Typically, people are asked to fast for 8 to 12 hours before these tests.

  • Vitamin B12 test: Tests for the levels of the vitamin B12 in a person’s blood. Typically, people are asked to fast for 6 to 8 hours for these tests. They must also let the doctor know what medications they take, as some can interfere with the test.

Why fasting is required

When people eat food and drink alcohol, the food and liquid gets broken down in their stomach and absorbed into the bloodstream, which can affect the levels of certain substances in the blood, such as:

  • sugar

  • minerals, such as iron

  • cholesterol and other fats

  • enzymes, such as GGT

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

  • diabetes

  • anaemia

  • 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 lead to a wrong diagnosis.

Other things to avoid

As well as food and drink, there are some other things to avoid when fasting for a blood test. These include:

  • Alcohol: Alcohol can also affect blood sugar and fat levels, giving inaccurate results to blood tests that require fasting. If a person is being asked to fast before a blood test, they should also refrain from drinking alcohol.

  • Smoking: Smoking can also affect blood test results. If a person has been asked to fast before a blood test, they should avoid smoking.

  • Coffee: Coffee affects digestion and can also affect the results of a blood test. As such, people should not drink coffee before a fasting blood test.

  • Chewing gum: Chewing gum, even if it is sugar-free, should be avoided when fasting for a blood test. This is because it can speed up digestion, which can affect results.

  • Exercise: Exercise can also speed up digestion and affect results, so people should avoid it for the recommended fasting period.

How to fast safely

There is a range of things that people can do when fasting for a blood test. These include:

  • Water: It is important to keep drinking plenty of water when fasting to stay hydrated. Water does not affect the results of a blood test and is acceptable to drink when asked to fast.

  • Timings: Whether a person has to fast for 8, 12, or 24 hours, it is a good idea to work out what is the latest time they can eat or drink before the test. For example, if a person is asked to fast for 12 hours before a blood test at 9 a.m., they should not eat anything after 9 p.m. the night before.

  • Medication: It is important for people to keep taking any regular medication while they are fasting unless they have been told by a doctor to do otherwise.

  • Pregnancy: It is usually safe for pregnant women to fast. However, it is a good idea to speak to a doctor before the test and get their advice on the best way to do this safely.


Following the steps above can help a person who has been asked to fast for a blood test to do so safely.

With the best will in the world, sometimes people do eat or drink during their fasting period because they were too hungry, or because they got the timings wrong. If this happens, it is a good idea to speak to the doctor and 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 is so important.

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