Fasting before Blood Test: Importance, Guidelines - Healthians

What Is Fasting Before A Blood Test?

Importance of fasting before test

Contributed by – Dr. Snehal Singh

Have you ever wondered about the significance of fasting before the blood tests? Why fasting is required only for some particular blood tests?

Blood tests are really important part of the healthcare world. It helps diagnose the health problems and help us become much more aware about our health status. A lot of these blood tests demand few hours of fasting before the sample collection. The fasting before tests holds a lot of significance as particular tests results can be influenced due to even the slightest food intake.

Let us try to understand the importance of fasting before blood test and why is it advised.

Why Is It Important To Fast Before Few Blood Tests?

Fasting simply means not eating anything for certain hour before going for blood tests. This helps to ensure that the test results are accurate and are not influenced by the food or the drinks taken. For example, eating almost anything can increase blood sugar levels. This will give inappropriate results and will not get an accurate diagnosis nor proper treatment.


Which Tests Need Fasting Before Blood Test?

Some common tests that need fasting before the test include:

Apart from these, there may be some specialised tests that need fasting; hence it is necessary to follow medical advice. Ideally, fasting before blood test means not having any food as any food substance can interfere with the blood test results. However, water does not affect the test results, hence only plain water is normally allowed.


Can You Drink Black Coffee Before A Blood Test?

A cup of black coffee can seem harmless as it is just coffee brewed in water. But, the fact is that various compounds in coffee including caffeine can interfere with blood tests. Hence, black coffee before a blood test is best avoided. Also, coffee is a diuretic and you do not want to get dehydrated before a blood test.


Can You Drink Before A Blood Test?

Just like food and caffeine, alcohol too interferes with most blood tests. For example, tests done to assess the liver function or cholesterol may be easily affected with presence of alcohol in blood. It is advised to avoid alcohol at least 24 hours before such tests are performed. Also, as traces of alcohol can remain in the blood for many days, it is best if you can avoid alcohol for few days to a week before the test. You can drink only plain water and even lemon water, juices or herbal teas should not be consumed.


How Long Do You Have To Fast Before A Blood Test?

The time duration of fasting and the need for fasting before few blood tests may vary. The ideal fasting time, for blood glucose tests is about eight hours. However, certain tests like cholesterol and lipids need 12 hours of fasting.


Fasting before blood test - Healthians


Why Over Fasting Before Blood test Is To Be Avoided?

Fasting too long before blood test is not advisable as that too can affect the tests and will not give accurate results. It can also cause dehydration and create imbalances of salts, ions and blood sugar, thus affecting your test results. Normally, eight to 12 hours of fasting is suggested for accurate results, so it is best to avoid fasting for longer than that, unless you are advised to do so.


What Happens If You Don’t Fast Before A Blood Test? 

If you don’t fast before a blood test that needs fasting, your food intake will interfere with the test and give inappropriate results. For example, if you eat before a fasting blood glucose, your results with surely show higher blood glucose levels which is not the correct diagnosis and also affect the treatment procedure. So, tests without fasting can lead to inappropriate results, wrong diagnosis and  inaccurate treatment. Hence, you need to be sure that the tests are performed in the most accurate manner and only as advised.


What To Do If You Accidentally Eat Before A Fasting Blood Test?

As you now know that food interferes with blood tests, you must have found a new found appreciation for fasting before blood test. So, if you accidentally eat before a fasting blood test, it is best to inform the lab or your doctor about what you ate. If possible, the lab will perform the test considering the variability of food intake and review the results accordingly. However, if it is not possible to consider the food intake variable and fasting is the only way out, you may have to reschedule the test appointment. The best way out is to be candid and inform the concerned team about any food intake, to ensure most accurate results based on all required parameters.


Do You Have To Fast For A Thyroid Blood Test?

As thyroid hormones are regulated by the brain as needed, they are not affected by food. Thyroid tests can be performed any time during the day, without any interference of food. However, if thyroid tests are ordered with other tests that need fasting like blood glucose test, you will have to fast.


What Are The Dos And Don’ts Before Fasting Blood Test?

Now that you know all about the important role that fasting plays, it is time to discuss the major do’s and don’ts related with it.


  • Follow medical advice regarding fasting blood tests.
  • Enquire if your blood tests need fasting and for how many hours.
  • Sip only plain water during your fasting hours
  • Inform about any food that you have accidentally eaten.
  • Take your regular medications as advised



  • Avoid eating or drinking anything except water during your fasting time
  • Avoid fasting for longer than eight to 12 hours, unless advised to do so.
  • Avoid rigorous exercises before a fasting test
  • Refrain from alcohol, coffee, smoking and sugar free chewing gum or other products before a fasting test.


Stay aware and seek doctors advice when in doubt. These tests help you know your health better and tackle the health issues wisely. So, it is extremely crucial to follow the required steps to get the correct diagnosis.


Know Your Health Better

This post has already been read 5798 times!


  • Parbodh Chander Bali

    In UK you do not need fastening before tests. Moreover fastening is abnormal stage of body as in routine we eat food at regular intervals and body always maintain that balance which is after food. As such it is true that blood samples be taken any time of the day irrespective of fastening.

Leave a Reply

Your email address will not be published. Required fields are marked *

Talk to our Health Advisor