How to Fast Before a Blood Test

You have to fast for a certain period of time before certain blood tests, especially ones which are done to measure glucose and cholesterol levels. If there is food in your digestive system, you may not be able to get accurate results, which will create problems for your doctor in assessing your medical condition. Before you go for a blood test, your doctor is likely to advise you to fast anywhere from 2 to 12 hours.

Most of the patients however, do not know the rules of fasting before a blood test and make mistakes.


  • 1


    Some patients think that they cannot even drink water while fasting before a blood test; however, this is not true. You are allowed to drink pure water before a test, in fact, the doctors advise their patients to drink a lot of water so that they remain hydrated.

    However, you must not drink any juice or add any flavour, such as lemon, to the water. Just drink plain water, avoiding beverages like coffee, tea or soft drinks.

  • 2

    Solid food

    You are advised not to take any solid food during fasting before the blood test. This is precisely the reason why most of the blood tests are scheduled early in the mornings. Have an early dinner and do not eat anything before going to bed. Wake up early morning and go for the blood test after drinking a glass or two of plain water.

    Chewing gum, eat breath mints or any other form of food substitutes is also not allowed during the fast before the blood test.

  • 3


    If you are on medication, let the doctor know when he is advising a blood test and go with what he/she recommends.  In order to make any changes in your medication schedule, you must follow the physician’s directions.

  • 4

    Food supplements or herbs

    There are some herbs, food supplements and vitamins which can interfere with blood tests. Do not forget to tell your doctor about your daily diet or any food supplements that you are taking. Follow your doctor’s advice because he/she would know which supplements or herbs you can take during a fast before the blood test.

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