Why to Fast before Blood Tests?

medlabz - 31 May 2017
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Why to Fast before Blood Tests?

Blood tests are often done to understand the underlying causes of symptoms you may display when you are sick or check an anomaly that may not be discovered under normal circumstances. This is why these tests are known as diagnostic tests, where they help the medical practitioner read and diagnose correctly.


Why is fasting before blood tests important?

  • The nutrients and other components present in the food and drinks are absorbed by the blood, which could affect the factors that are measured in some of the tests.
  • blood undergoes changes after eating which affect the levels of glucose in our body along with lipids and iron.
  • Fasting result of any blood test will be true without being influenced by what you ate before.

What would a fast mean?

Well, a fast before a blood test means you don’t eat or drink anything except water for 10-12 hours before the test.

Can you drink water when fasting for a blood test?If yes Why?


Yes, you can drink water when fasting for a blood test when you feel thirsty.We need water to keep our veins hydrated, which makes them easier to find and draw blood from during the test. So one should ideally drink plenty of water before a blood test.

Can you drink coffee or tea when fasting for a blood test?


No, other than water you can’t have any other beverages like tea or coffee.

Can you smoke when fasting for a blood test?


Smoking may alter the test results, which means you cannot smoke while fasting.

Can you exercise while fasting for a test?

woman exercise

No, exercising may also bring changes in your blood that may affect the test results.

How many hours before should I stop eating?

Most often you need to fast for a minimum of 8 hours for most lab tests. However, you should check with your doctor for instructions with regards to the test including the fast period before the test.

Are medications allowed during the fast?


You can take your normal medications as scheduled if the blood test is one you have done before. If you are still unsure, then do check with your physician before taking your medication.

If I don’t fast before a test?


Fasting improves the accuracy of your blood test for a number of diseases. Not following the instructions of the doctor would mean a false positive report, which could lead to a wrong diagnosis.

What blood tests require fasting?

There are a number of blood tests that require fasting. The most common one is glucose testing, which checks for levels of sugar in your body. There are others such as the complete metabolic profile, the normal metabolic profile, renal function test, lipid profile, cholesterol range, as well as Vitamin B12 levels. If you have trouble fasting, then you may want to schedule an early morning appointment for the blood test.

When can I eat after the blood test?

You can drink and eat a snack as soon as your blood test is done.

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