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Why do I need to fast before lab tests?

Last Modified: February 23, 2024

Family Medicine


This blog was written by Ann Vick, MT(ASCP), MLS(ASCP), laboratory chemistry specialist, Parkview Health. 

When your provider orders a blood test, you might be asked to fast beforehand. This direction often brings up questions like, “How long should I fast?” “Can I drink water?” “Should I take my medication?” To help bring clarity, we’ll go through some pre-lab fasting instructions and answer the most frequently asked questions.

Patient instructions for fasting specimens

Commonly, your direction before arriving for a fasted lab draw will look something like this:

1. Do not eat anything for 8-12 hours before having lab testing collected.  If you are going to have your lab testing collected in the morning, this would typically mean you should not eat anything after your evening meal.

2. Only drink water while you are fasting.  Do not drink juice, tea, coffee, soda, alcohol, or other beverages.

3. Do not fast for longer than 14 hours.

4. Do not smoke, chew gum, eat candy, or exercise while fasting.

Why do I need to fast?

Patients are commonly instructed to fast because the process your body undergoes while digesting food may change the values for some laboratory testing.

Do I need to fast for all lab testing?

No, many lab tests are unaffected by recent ingestion of food or drink.  Check with your provider to see if your lab testing requires fasting.

What tests do I need to fast for?

Common tests that you might be asked to fast for are:

But your physician may want a fasting sample for other testing, too.  Always check with your provider when lab testing is ordered to see if fasting is required. 

Should I take my medication while fasting?

Unless instructed otherwise by your provider, please take all medications on your normal schedule.  If you routinely take vitamins or supplements, please check with your physician to see if these should be skipped before lab testing.

Can I drink black coffee or tea if I don’t add milk or sugar?

No, you should only drink water while fasting.

Can I drink water before a fasted lab draw?

While it is not recommended to drink excessive amounts of water, your body needs to stay hydrated.  Hydrated veins are easier to locate and will give an easier collection experience.

Can I chew gum if it is sugarless?

No. Gum (even sugarless) stimulates your digestive system and may change your test results.

How soon after my lab testing is collected can I eat?

You can eat as soon as your blood collection is complete.  You are welcome to bring along a small snack to eat following your blood collection.

What happens if I forget to fast before my testing?

We want to provide you and your provider with the most accurate lab results possible.  If you are not fasting, you may be asked to return on a different day when you have fasted.

Who should I contact if I have questions about fasting for my lab work?

Please contact the provider who ordered your lab tests if you have any questions. If you need help finding a primary care provider or scheduling lab work, we can help! Call our Access Center any time, 24/7 at 877-PPG-TODAY or 877-774-8632 for assistance.

Learn more about our convenient Lab & Diagnostic Imaging options by visiting us here.



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