Why It's Important to Fast Before Blood Tests

In general, patients should fast before blood draws for at least 8 hours. However, this will depend on the specific type of test. Today, we'll tell you why.
Why It's Important to Fast Before Blood Tests
Leonardo Biolatto

Written and verified by the doctor Leonardo Biolatto.

Last update: 27 May, 2022

Blood tests are one of the most common types of medical tests since they provide a lot of information about a person’s health. In fact, they’re considered a routine and basic prodecure when it comes to any medical intervention. In general, patients must fast before blood tests for at least eight hours and, if possible, have their blood drawn in the morning.

Blood tests can help analyze different parameters. Often, doctors order specific tests to confirm or rule out a suspected diagnosis. However, many values are more of a question of routine. For example, there are blood sugar levels or blood counts.

In the article below, we want to walk about why patients should fast before blood tests. In other words, why is it so important to obtain from eating and drinking for at least eight hours before the procedure?

We’ll answer this question and more.

What are blood tests for?

Blood tests consist of extracting blood from the patient and analyzing the sample in a laboratory. They allow doctors to study many different aspects of the body to obtain information regarding the overall health of patients. At the same time, the information can shed light on the state of a specific organ.

In general, blood tests include a hemogram. This refers to the concentration of each one of the types of cells that there are in the blood. In other words, red blood cells, white blood cells, and platelets. What’s more, this allows for the observation of the shape of these cells to make sure that they are normal.

In the same way, blood tests allow for the analysis of several biochemical values. For example, the levels of glucose or iron in a person’s blood. Within this group of values, we can also mention cholesterol, triglycerides, thyroid enzymes, urea, etc.

All of these parameters are essential in the detection of certain diseases. For example, thyroid enzyme values help to reveal a thyroid illness. At the same time, iron and other values can help doctors determine the presence and gravity of anemia.

What’s more, blood tests can even help in the case of the possibility of certain types of tumors. These tumors act by releasing certain substances that can be found in the bloodstream.

A doctor analyzing blood tests.

Discover more: How Often You Should Get Blood Tests?

Why is it important to fast before blood tests?

When the day comes that you need to have a blood test, it’s important to fast for at least eight hours beforehand. In other words, don’t consume any foods or liquids (except water) the night before. That’s because the blood absorbs foods and drinks and this can alter certain parameters.

However, whether or not a patient needs to fast, and for how long, actually depends on what tests will be taking place. For example, if your doctor only ordered a hemogram, then fasting may not be necessary. However, in the case of glucose levels, adults need to fast for at least eight hours, and children for at least three hours.

At the same time, in the case of analyzing thyroid function, a fast of just four hours should suffice. When it comes to cholesterol levels, doctors may recommend fasting for up to 12 hours.

Another aspect to consider is medication. Certain medications, just like foods, can alter values in the blood. Doctors are generally aware of this and will be able to tell if you should pause from taking medications or not.

The importance of fasting.
Fasting before blood tests will depend on the specific tests that will be taking place. Not every type of blood test requires fasting beforehand.

You may also want to read: What to Eat to Have Normal Hemoglobin Levels

What else should you keep in mind?

Health professionals tend to make other recommendations for blood tests to go as well as possible. First of all, besides fasting, you should avoid smoking just before having blood tests. This is because tobacco can alter glucose levels.

You can drink water moderately to quench your thirst. However, drinking alcoholic beverages before blood tests is strictly prohibited. Also, you should avoid exercising the day before, as this can alter certain markers.

In any case, you should always consult your doctor and request specific instructions for your particular case. As we already mentioned, some patients may need to take a break from medications while others will not. At the same time, the time a patient needs to fast before blood tests can also vary. Therefore, it’s best to discuss this personally with your doctor.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Quinn JG, Tansey EA, Johnson CD, Roe SM, Montgomery LE. Blood: tests used to assess the physiological and immunological properties of blood. Adv Physiol Educ. 2016;40(2):165-175. doi:10.1152/advan.00079.2015
  • Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Table 1, Complete blood count. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2263/table/ch1.T1/
  • Ortiz Sánchez, Y., García Tase, M. M., Rosales Arias, K. K., Vázquez Belizón, Y., & Fonseca Olivares, E. (2005). Interferencias de medicamentos con pruebas de laboratorios. Revista Cubana de Farmacia39(3).
  • McMillin JM. Blood Glucose. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 141. Available from: https://www.ncbi.nlm.nih.gov/books/NBK248/
  • Kackov S, Simundic AM, Gatti-Drnic A. Are patients well informed about the fasting requirements for laboratory blood testing?. Biochem Med (Zagreb). 2013;23(3):326-331. doi:10.11613/bm.2013.040

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.