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Why Are You Supposed to Fast Before a Blood Test?

Medically reviewed on June 27, 2022 by Jordan Stachel, M.S., RDN, CPT. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.

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Blood tests are an important medical tool that offer a wide range of applications. They can evaluate current health, identify potential infections, and help to support diagnosis when used with other tests. However, depending on the blood test, your healthcare provider may ask that you go through a fasting period prior to getting blood drawn. Wondering, "Why fast before a blood test?" Read on to find out more.

Why Do You Need to Fast?

Fasting refers to avoiding all food and drinks, except for water, for a certain amount of time. When your healthcare provider asks you to fast, you cannot eat or drink anything other than water for several hours prior to getting your blood drawn [1].

Although your digestive system might seem entirely separate from your blood, the two are intrinsically connected. Everything you eat and drink gets broken down into its component parts. The vitamins and nutrients then get absorbed into the bloodstream via the digestive system. This could then affect certain types of blood tests [1].

The good news is that fasting for blood tests is becoming rarer. Most routine blood level tests, including thyroid tests, CBC tests, and tests for kidney and liver are not affected by fasting [2]. Additionally, some at-home blood tests from Everlywell, such as Nutritional & Digestive Health tests, do not require fasting but only a quick finger prick for sample collection.

Tests That Require Fasting

Glucose tests tend to be the most common tests that require fasting prior to getting blood drawn. They measure blood sugar levels, which can naturally fluctuate based on what you have eaten and how recently you’ve eaten. For a glucose tolerance test, you may need to fast for up to eight hours before the test. Once you arrive at the site, you will get your fasting blood tested, after which you will drink a liquid containing glucose. An hour later, you will get your blood drawn again. This may continue two hours later and three hours later for a more accurate result [1].

Lipid profile tests may also require fasting. Lipid profile tests measure cholesterol and a type of fat found in the body called “triglycerides.” Cholesterol comes in two forms, HDL and LDL. Generally, high levels of LDL cholesterol (considered the “bad” cholesterol) combined with high triglycerides can increase the risk of heart disease and other disorders [1].

The Rules of Fasting

If you are asked to fast prior to a blood test, your healthcare provider will typically give you more specific instructions. In general, you may not eat or drink anything other than water for eight to 12 hours before the test. That includes drinking alcohol, coffee, juice, and/or other beverages as they can all potentially alter test results. Your healthcare provider may also recommend that you avoid smoking, exercise, and chewing gum during your fast [1].

Thankfully, most tests that require fasting are scheduled in the morning, meaning that most of the fasting period happens overnight, while you are asleep. Once the blood has been drawn, you can return to eating and drinking normally, so you may want to bring a snack with you to eat right after your test is over [1].

Fasting is generally not common for most blood tests, but your healthcare provider may ask you to fast for certain blood tests. Make sure you follow instructions and ask for any clarification prior to your test.

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1. Fasting for a blood test. MedlinePlus. URL. Accessed June 27, 2022.

2. Ask the doctor: What blood tests require fasting? Harvard Health. URL. Accessed June 27, 2022.

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