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What is the recommended fasting time before doing a blood test

It is important to respect the fasting periods before doing blood tests when they are necessary, as food or water intake may interfere with the results of some tests, especially when it is necessary to assess the amount of a certain substance that can be altered by consuming food, such as cholesterol or sugar levels, for example.

The fasting time in hours depends on the blood test that will be performed, here are some examples below:

  • Glucose: It is recommended that adults fast for 8 hours and children 3 hours, before doing the test;
  • Cholesterol: Although it is no longer mandatory, it is recommended to fast for up to 12 hours to obtain more reliable results of a persons general health;
  • TSH levels: It is recommended to fast for at least 4 hours;
  • PSA levels: It is indicated to fast for at least 4 hours;
  • Hemogram: It is not necessary to fast, because the components evaluated are not altered by foods, like the number of red blood cells, leucocytes or platelets.  See the purpose of a complete blood count.

People who have diabetes need to take various blood glucose measurements several times a day. These measurements should be guided by the doctor, who will indicate at what times they should be taken and how long after meals should they be done. 

In addition, the fasting time may vary according to the laboratory in which the test will be performed, as well as which tests will be performed on the same day, and therefore it is important to seek medical or laboratory guidance about the fasting time required.

What is the recommended fasting time before doing a blood test

Is water allowed during fasting?

During the fasting period you are allowed to drink water, however, only enough to quench your thirst should be ingested, as the excess may alter the test results.

However, other types of beverages, such as soft drinks, teas or alcohol, should be avoided as they can cause changes in blood components.

Other cautions to follow before doing the test

While preparing for a glycemia or cholesterol blood test it is important not to perform physical activities 2 to 3 days before the test. When a PSA blood test is going to be done, sexual activity should be avoided in the 3 days prior to the test, in addition to situations that may increase PSA levels, such as riding a bicycle and taking some medications, for example. See more about the PSA exam.

In all cases, the day before the blood test, smoking and drinking alcohol should be avoided, as they influence the results of the analysis, especially in blood tests that measure glycemia and triglycerides levels. In addition, some medications, such as antibiotics, anti-inflammatory drugs, or aspirin, influence blood test results. Therefore, it is important to tell the doctor which medication you are taking so he can give advice on it's suspension in the days prior to the testing or to take them into consideration while interpreting the results.

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