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Why fast before a blood test?

Why fast before a blood test?

The blood test is one of the most used medical tests, since it provides a lot of information about a person's health status. So much so that it is considered a routine and basic test for almost any medical procedure.

Blood tests can be used to analyze different parameters. They are often ordered based on the suspected diagnosis, although in most cases certain values ​​are usually included. For example, blood glucose level or blood cell count.

This test is preferably done first thing in the morning. In addition, it is recommended to go fasting, that is, without having eaten or drunk anything in the last eight hours.

What can a blood test be used for?

A blood test involves taking blood from the patient and analyzing it in a laboratory. Through this test, many different aspects of the body can be studied to obtain information about the overall health status or that of a particular organ.

An analysis includes a blood count. It is the concentration of each of the types of cells in the blood; that is, red blood cells, white blood cells and platelets. In addition, it allows us to observe the shape of these cells to check if they are normal.

Similarly, numerous biochemical values ​​can be studied, such as blood glucose concentration or iron. This group includes cholesterol, triglycerides, thyroid enzymes, urea, etc.

All these parameters are essential to be able to detect certain pathologies. For example, thyroid enzyme values ​​can reveal thyroid disease. Meanwhile, iron and other related elements help assess anemia.

The blood test can even be used to suspect certain types of tumors. Some of them work by releasing specific substances that can be found in the bloodstream.

Why is fasting before a blood test important?

When a blood test is planned, you are asked to go to it having fasted for at least 8 hours. That is, it is recommended not to eat or drink anything (except water) the night before. This is because foods and drinks are absorbed into the blood and can alter certain parameters.

However, this fasting is relative and depends on what is going to be analyzed. For example, if the only thing you want to observe is the blood count, fasting does not have to be necessary. However, in the case of glucose, it is required to be 8 hours in adults and at least 3 hours in children.

On the other hand, in the case of assessing the thyroid, a 4-hour fast is sufficient. For cholesterol, fasting for around 12 hours can even be recommended. Additionally, another aspect that must be considered is medications.

Certain medications, like foods, can alter blood values. Normally, the doctor who orders the analysis is aware of this and knows whether it is necessary to pause the medication or not.

What else should be taken into account?

Healthcare personnel usually make other recommendations so that the blood test goes as well as possible. First of all, in addition to fasting, you should avoid smoking just before the test. It seems that tobacco can also alter blood glucose.

You can drink water, but in moderation, to quench thirst. What is totally contraindicated is drinking alcoholic beverages before the analysis. It is also not recommended to exercise the day before, as certain markers can be confused.

In any case, the ideal is for the doctor to provide individualized advice. As we have mentioned before, each patient may need to pause the medication or not, or may adapt the fast depending on the aspects they wish to study.


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