Fasting in blood test: when and how should I do it?

It is common when a blood test is required for the doctor to indicate the need for a period of fasting prior to the extraction.

In this sense, there is numerous evidence on the impact of the food eaten and the time elapsed until the blood sample was obtained on the levels of the biochemical parameters analyzed.

In this way, the recommendations on the number of hours of fasting before carrying out a blood test vary according to the parameters to be analyzed, the most common being 10-12 hours of fasting before the blood is drawn, allowing only the intake of water and medicines prescribed by the doctor.

Fasting: In what types of analysis do I need it?

Depending on the parameters to be analyzed, a variable period of fasting may be necessary.

Fasting the blood count

The complete blood count is one of the most common laboratory tests included in a blood test. It consists of the count of the different types of cells that are present in the blood:

  • Red blood cells, red blood cells, or erythrocytes.
  • White blood cells or leukocytes.
  • Platelets

The hemogram also allows evaluating hemoglobin, a pigmented protein that gives color to red blood cells, responsible for transporting oxygen and carbon dioxide, as well as the hematocrit or ratio of red blood cells in relation to the total volume of the blood.

When the only test to perform is a CBC, no fasting is necessary.However, blood collection for a complete blood count is also often used to perform other tests that do require fasting, such as biochemistry to assess blood glucose or lipid profile.

Fasting on blood glucose

For blood glucose or glycemic tests, a fast of at least 8 hours should be observed.

In the case of the test of oral glucose tolerance It should be done in the morning, after 3 days of a diet without special restrictions and that includes at least 150 grams of carbohydrates per day. In the 8 hours preceding the test, the practice of intense physical exercises should be avoided.

In the case of people with diabetes, treatments with drugs to lower blood glucose or insulin, it should be the doctor who considers whether to continue with the treatment while fasting.

Fasting on the lipid profile

To properly analyze the lipid particles present in the blood, it is necessary to fast for 6 to 8 hours before the blood draw. This is to avoid that the values ​​obtained are not influenced by the absorption of fats from a recent meal. However, some studies show few differences between fasting and non-fasting lipid profiles, so various medical societies and guidelines from different countries support the analysis of the non-fasting lipid profile. In any case, the lipid profile within a blood test is usually accompanied by blood glucose, a parameter that clearly requires fasting.

In the case of cholesterol tests, fasting is not usually mandatory, although fasting for 10-12 hours is usually recommended.

If the patient has high triglycerides, the fast can be extended to 14 hours.

Fasting and hormone and protein levels

To know the levels of different hormones and proteins present in the blood, different degrees of fasting may be necessary.

In the case of TSH (thyroid stimulating hormone) and PSA (prostate specific antigen), a protein produced in the prostate that is used as a tumor marker, fasting must be at least 4 hours.

Frequent questions

How many hours do I have to fast before an analysis?

In general, the required fasting time is 8 to 12 hours before a test. Most tests that require fasting are scheduled early in the morning to facilitate completion.

Why is it important and what happens if I don’t?

The composition of the last food intake and the time elapsed until the blood sample was obtained can interfere with the biochemical parameters analyzed. In this way, the results of the analysis may appear distorted if the fast is not respected.

Can you drink water before the test?

During the fasting period you cannot ingest anything other than water. Coffee, juices, and other beverages can change the composition of your blood and interfere with test results. It is also not advisable to chew gum, smoke or exercise in the hours prior to performing the analysis.

Is fasting necessary for a urinalysis?

If you only need a urinalysis, you do not need to be fasting.

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