Why do I need to fast before a blood test?

Fasting for blood tests

Fasting for blood tests-If your doctor or health professional has instructed you to fast before a blood test, it means that you should not eat or drink anything, except water, for several hours before the test. When you eat and drink, food and drinks are absorbed into the bloodstream. That can affect the results of some types of blood tests.


For what types of blood tests is it necessary to fast?

Some of the most common types of tests that require fasting are:

  • Glucose tests, which measure blood sugar. One of them is the glucose tolerance test. Before this test, you have to fast for 8 hours. When you arrive at the laboratory or medical center:
  • They do a blood test
  • You drink a special liquid that has glucose
  • Then, the blood test is repeated one, two and sometimes three hours later

Glucose tests are used to diagnose diabetes.

  • Lipid tests, which measure triglycerides (a type of fat found in the bloodstream) and cholesterol (a waxy, fat-like substance found in the blood and in all cells of the body). High levels of triglycerides or a type of cholesterol called LDL can increase your risk of heart disease.


How long do You have fast before the test?

Usually, you have to fast from 8 to 12 hours before a test. Most tests that require fasting are scheduled early in the morning. In that way, most fasting occurs at night.

When I fast, can I drink anything other than water?

No. Juice, coffee, sodas, and other beverages can enter the bloodstream and influence the results. Also, you should not:

  • Chew gum
  • Smoke
  • Exercise
These activities can also influence the results.

But you can drink water. In fact, it is advisable to drink water before a blood test. It helps to have more fluid in the veins, and that can facilitate the extraction of blood.

Can I continue taking my medications during the fast?

Ask your doctor or health professional. You can usually take your usual medications, but sometimes you have to avoid some, especially if they have to be taken with food.

What happens if during my fast, by mistake, I eat or drink something other than water?
Tell your doctor or health professional before the test. I may give you a new appointment when you can fully complete your fast.


When will I be able to eat and drink normally again?

You will be able to eat and drink again as soon as the test is over. Bring a snack so you can eat it right away.

Is there anything else I need to know about fasting before a blood test?

If you have questions or concerns about fasting, check with your doctor or health professional.

It is important that you talk with your doctor or health professional before any laboratory tests. For most tests, do not fast or make any special preparations. For other tests, you may have to avoid certain foods, medications, or activities. Following the indications before a test helps the results to be correct.