Why do you need to fast for some blood tests?

It’s not uncommon to be required to fast for 8 to 12 hours prior to getting bloodwork. But many people don’t know the reason behind this guidance or which blood tests require fasting.

“Fasting means not eating or drinking anything other than water for a given period of time,” explains Dr. Jonathan Katz, a family medicine physician at Aurora Health Care. “The reason fasting is so important is because it gives a baseline measurement for many different levels of substances in the blood.”

Dr. Katz says when the body absorbs nutrients after eating and drinking, components of the food or beverage enter the bloodstream, ultimately impacting the levels of certain substances in your blood.

“Not fasting can significantly impact the results of blood tests and even has the potential to lead to incorrect diagnoses,” he adds. “That’s why if you forget to fast before a blood test, you will most likely be asked to reschedule.”

Some of the most frequently ordered bloodwork that requires fasting include blood glucose tests, lipid panels (cholesterol levels and blood fats), liver function, kidney function and iron tests.

In most cases, bloodwork that requires fasting is scheduled in the morning so the fasting period occurs primarily while you sleep, making it as simple as possible.

“Even if you are fasting before a blood test, do still drink some water prior to your arrival,” Dr. Katz recommends. “Being well hydrated makes it easier to get a blood sample because there is more fluid in your veins.”

If you are unsure if fasting is required before your blood test, check in with your health care provider.

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