Not everyone needs to fast before a blood test. Only certain types of blood test require this. Each test has different guidelines on how long a person should fast beforehand.
Fasting before a blood test means that a person should not eat or drink anything other than water. Typically, if a blood test requires fasting, doctors schedule it for the morning. This way, a person can fast overnight and then take the test before eating the next day.
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 test. Some blood tests require fasting to provide accurate results, while others do not.
Some types of blood test that require fasting include:
Fasting blood glucose test
A fasting blood glucose test can help diagnose diabetes.
Diabetes is a condition that causes high amounts of sugar in the blood. A fasting blood glucose test measures levels of sugar in the blood when a person is fasting to determine if their baseline is within a typical range.
For the test, a person will typically fast
Blood cholesterol tests
Cholesterol is a fatty substance that the body needs to function. However, high cholesterol levels increase the risk of certain medical 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 doctors may ask some people to fast for 9 to 12 hours, often overnight, before this test.
Some 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
It is always best to check with a doctor to avoid any mistakes.
Liver function tests
A liver function test can measure substances that the liver produces to see if it is healthy or whether it has signs of damage. It may include tests for:
A person may need to avoid all eating and drinking for 10 to 12 hours before the test, as well as avoid drinking alcohol and taking some prescription medications.
People taking medications should discuss this before their test to make sure it is okay for them to continue taking them.
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 enters the bloodstream quickly. Therefore, if a person eats food before the blood test, the results may show inflated levels of iron.
Some doctors may ask people to fast for 12 hours before their iron test to prevent this from obscuring any potential deficiency.
Kidney function tests
Some tests that assess the kidneys may also require a few hours of fasting beforehand.
For example, doctors may ask people to fast before a glomerular filtration rate test, which can tell them how well the kidneys are working to filter the blood.
When people eat or drink, the food or liquid breaks down in their stomach and small intestine, before substances from those foods enter the bloodstream.
This, in turn, could alter or obscure atypical levels of these substances, preventing doctors from getting a clear picture of a person’s health. In some cases, it could lead to an incorrect diagnosis.
The tests that require fasting can potentially help doctors diagnose conditions that require treatment, such as:
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 water when fasting to stay hydrated. Water does not affect the results of a blood test.
- 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 has a blood test at 9 a.m. in the morning, they cannot eat or drink anything except water after 9 p.m. the night before.
- Medication: It is important for people to keep taking any regular medication while they are fasting unless a doctor tells them otherwise. Ask a doctor about this when booking the blood test.
As well as food and drink other than water, 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 may 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 when they are fasting for a blood test.
- Exercise: Exercise can affect blood sugar levels, among other measurements. Avoid strenuous exercise during the fasting period.
- New medications: Try to avoid taking new medications or supplements in the run-up to the blood test, unless a health professional recommends it.
Some blood tests require fasting, which means a person needs to avoid eating or drinking anything other than water. Usually, people need to fast for a set period of time. This may be for a few hours, overnight, or longer.
However, not all blood tests require fasting beforehand. People can find out if they need to fast by speaking with a doctor or medical professional before their test.
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.
If fasting becomes too difficult and a person breaks the fast, they should contact the doctor to reschedule the blood test appointment.