The do’s and don’ts of fasting before bloodwork - Cape Cod Healthcare

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Published on March 30, 2021

The do’s and don’ts of fasting before bloodwork

Blood Fasting

If you’re used to starting the day with a big breakfast, a pre-bloodwork fast overnight and into the morning can leave your belly grumbling.

But, the sacrifice is worth it to ensure you’re getting the most accurate results from many blood tests, said Whitney Thomas, a clinical laboratory supervisor at Cape Cod Healthcare’s Corelab. Below, in her words are answers to a few questions about the process.

What kinds of things are doctors looking for when they send a patient for routine bloodwork?

Routine blood work is often ordered for yearly physicals. Some of the most common profiles ordered are a complete metabolic profile, a lipid profile and a complete blood count. This set of testing will check for diabetes, liver function, cholesterol levels and anemia.

Why is it important to fast? How would eating breakfast affect the results?

For the best quality of sample, a patient should fast.  If the patient has just eaten, some results could be falsely elevated and the sample itself could be ‘cloudy,’ which interferes with some of the individual testing process.

When should the patient stop eating and drinking?

Depending on the test, fasting should occur between 8 and12 hours before the blood is drawn. The physician will guide the patient.

Does that include water, tea and coffee? Does it include medications?

Drinking water is good because it hydrates your veins and makes them easier to find. Your physician will instruct you whether you should take your medications that morning. If it’s yes, you can take them with water, black tea or black coffee.

Can you think of any myths people have about this or are there any mistakes that people make?

A common perception is when a cholesterol result is elevated, the patient thinks it was due to what they ate yesterday. Cholesterol is a lipid that builds slowly over time.

Anything else?

Ask your doctor if you have any questions before you go for bloodwork. And when you’re there, be sure to tell the phlebotomist if you’ve had anything to eat or drink or taken any medications during the fasting period. If you forgot and had a little nibble, you might need to reschedule.