Fasting before a blood test means that an individual should not eat or drink anything other than water before undergoing certain blood tests. But which blood tests require fasting, and how can people fast safely?

A person does not always need to fast before a blood test. When a blood test requires fasting, it is typically needed for a short time only.

Even so, the idea of not eating or drinking, even for a small amount of time, may seem daunting. Understanding when and how to fast before a blood test can help reduce unnecessary worry.

This article explores the types of blood tests that require fasting, why fasting is necessary, and how an individual can do it safely.

Whether someone needs to fast or not before a blood test depends on the type of blood test they need to undergo. Some blood tests require fasting to provide accurate results, while others do not.

The types of blood tests that require fasting are:

Fasting blood glucose test

A fasting blood glucose test can help diagnose diabetes.

Diabetes is a condition that can lead to excessive amounts of sugar in the blood. A fasting blood glucose test measures levels of sugar in the blood to see whether they are healthy.

It is important that an individual has not had anything to eat or drink other than water for 8–10 hours before a fasting blood glucose test. A person will typically fast overnight and do the test early in the morning.

Fasting helps ensure that the blood test records an accurate measure of fasting blood sugar levels. The results help a doctor diagnose or rule out diabetes.

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:

  • high-density lipoprotein cholesterol, also known as “good” cholesterol
  • low-density lipoprotein cholesterol, also known as “bad” cholesterol
  • triglycerides

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

Research has suggested that fasting may not be 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 a doctor to see whether these new guidelines apply to them.

Gamma-glutamyl transferase test

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

A person may need to avoid all eating and drinking for 8 hours before the test, as well as avoid drinking alcohol and taking some prescription medications.

Iron blood test

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. Therefore, if a person eats food before the iron blood test, the results may show inflated levels of iron.

To ensure accurate results, a person should fast from midnight the night before the test until after the test is done.

Other blood tests that require fasting

People may be asked to fast for:

  • Blood glucose test: This tests the sugar in a person’s blood and can help confirm or rule out diabetes.
  • Liver function test: This looks at how well a person’s liver is working.
  • Serum electrolyte and kidney function tests: These tests look at the function of the kidney to assess for chronic kidney disease.
  • Vitamin B12 test: This tests for the levels of vitamin B12 in a person’s blood. Typically, people are asked to fast for 12 hours before the test. They must also let the doctor know whether they take any medications, as some can interfere with the test.

When people eat food and drink alcohol, the food and liquid get broken down in their stomach 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.

There is a range of things that individuals can do when fasting for a blood test, such as:

  • 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 a person needs 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.

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

  • Alcohol: Alcohol can 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 refrain from smoking.
  • Chewing gum: A person should avoid chewing gum, even if it does not contain sugar, when they are fasting for a blood test. This is because chewing gum 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.

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 the test does require fasting, following the steps above can help an individual who has been asked to fast for a blood test to do so safely.

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 is so important.