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Fasting Before a Blood Test: Everything You Need to Know

Updated: 17 Jan 2024
Published: 15 Aug 2022
Fasting Before a Blood Test

What is a blood test?

A blood test is an examination of a blood sample used to detect various health issues. A blood test gives information about the organs and gives a diagnosis regarding it. Cells, chemicals and proteins are also examined based on a sample of blood. 

Why should you fast before a blood test?

A lot of food items and drinks hamper the blood test results. The nutrients from the items consumed can get into the bloodstreams which results in wrong derivations of the sample. 

A very basic and real life example of this is- if you consume sugar items such as carbonated drinks, sweets, or food rich in glucose, then the sugar levels will come up higher than normal. 

What types of blood tests require fasting? 

You must be wondering what types of blood tests require fasting? Not all tests require fasting before the test. A doctor will tell you and keep you informed before all your blood tests and whether you need to fast for it or not. However, when it comes to such blood tests, the doctor will definitely ask you to avoid all kinds of foods and drinks for at least 8-10 hours before the blood test. These blood tests are: 

  1. Glucose test: This test is done to check the sugar levels in the body. This test requires strict fasting as any amount of food or drink could possibly hamper the results. At least 8 hours of fasting is required for accurate results.
  2. Lipid Test: This test checks the levels of cholesterol and triglycerides, which is a type of fat found in the blood. Fasting in this test is not always required, but in some situations, your doctor may ask you to fast for at least 10-12 hours.
  3. Iron test: These tests are used to detect the levels of iron in your bloodstream. Low iron levels could lead to anemia. A lot of foods have high levels of iron in them, therefore your doctor may ask you to fast for a minimum period of 12 hours.
  4. Vitamin B12 test: This is the test to check the levels of vitamin B12 in the body. This test requires fasting of about 6-8 hours. Your doctor may also ask you for all the medications you take, as it can affect the results.

How long do you have to fast before a blood test?

Another important question is: how many hours of fasting before a test is required? All blood tests have different time frames. Each blood test requires fasting of different durations. Though, the average time period is about 8-10 hours, meaning overnight fasting.

What happens if you don’t fast before a blood test that requires fasting? 

If your doctor has suggested a fasting blood test, and under certain circumstances, you had to break your fast, then you must immediately contact your doctor for further guidance and information. But, it is advisable to follow your doctor’s instructions and avoid eating or drinking before the blood test. No kinds of foods or drinks such as soda, alcohol, juices should be consumed.

Will fasting before a blood test affect your health?

No, this kind of fasting before your blood test will not affect your health. Since this fasting is usually done overnight, it does not have any side effects. This way, the test is performed early in the morning, so that your routine is not disturbed.

When can you eat after the blood samples are taken?

You can eat right after your blood sample is taken. There is no restriction on eating or drinking right after your test is done. Your doctor may also ask you to carry glucose, or some fruit on your way in case you want to break your fast soon after the test is done.

For your blood tests, you can search “Blood test near me”, or you can also visit the nearest Pathkind labs for easy access. Remember to get tested on time, and follow your doctor’s instructions for best results.  

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