How Long do You Need to Fast Before a Blood Test?

A blood test is commonly used by your physician for monitoring your nutritional and health status, checking whether any medical ailments are present and assessing your risk of getting a particular disease. A blood test also helps your physician to know how well a treatment for a particular illness is working. The level of certain substances present in your blood may be affected significantly by drinking or eating during the time before performing the test. Hence, before getting any type of blood tests, make sure to ask your physician whether you are required to fast before the test and if yes then for how long.

How Long to Fast Before Blood Test?

The time you are required to fast depends on the type of test you are undergoing. For the majority of the tests, you will need to fast for at least 8 hours prior to the test. During this time you cannot drink or eat anything except pure water. For some tests you may need to fast for a time period of 12 hours.

Hence, if your blood test is scheduled at 9 a.m. and you have to fast for 8 hours, then don’t take anything except water after 1 a.m. If you have been told to fast for 12 hours, then don’t eat or drink anything after 9 p.m. on the previous night.

Tips and precautions:

You should avoid club soda and carbonated beverages and also any kind of tea.

Which Tests Require Fasting?

You may not require fasting prior to all types of blood tests. You will be told by your physician when you need to fast. After understanding how long to fast before blood test, let’s discuss which tests require fasting.

The following tests usually need fasting:

Why Is Fasting Required Before Blood Tests?

Nutrients present in drinks and food go into the blood and may change values of the things that are measured by the blood tests; thereby, skewing the results.

For example, if you drink or eat before you go for a fasting blood sugar test, your blood glucose value will come greater than if you had the test when fasting.

Can You Have Coffee Before Having a Blood Test?

Coffee, even if it is drunk black may interfere with the results of blood tests because it has caffeine, which skews the test results.

Furthermore, coffee is a diuretic, increasing your frequency of urination. This results in dehydration, making it difficult for the nurse to find your vein to withdraw blood. This may make the test more stressful and harder for you.

Can You Consume Alcohol Before Having a Blood Test?

Certain blood tests, including those measuring triglyceride levels or liver function tests, may require that you not consume any amount of alcohol for at least 24 hours before the test. Alcohol (trace amounts) may remain in the blood for many days after consumption. If there are any concerns regarding consumption of alcohol, discuss with your physician while scheduling the blood test.

What if You Eat or Drink Mistakenly Before a Test?

If you mistakenly drink or eat something before a test that requires fasting, the results may not come accurate. In such cases, make a call to your physician and enquire if you can still take the test. Certain tests may be analyzed with the notation that they aren’t fasting, but the results may be different. However, if fasting is absolutely required for best results, your physician may reschedule the blood tests. 

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