Fasting for blood work: what blood tests require fasting?

If you have been prescribed some blood work, you may be wondering whether you need to fast or not before having your blood drawn. Commonly prescribed blood tests that require fasting are the blood glucose test (blood sugar), the lipid panel (triglycerides and cholesterol) and the metabolic panel. In general, your healthcare provider should instruct you on what is allowed and what not before the prescribed blood test, but in this article I will clear some of the doubts around fasting and blood tests.

fasting for blood test

Why do you have to fast before blood work?

Foods and drinks have nutrients that enter the blood stream and that can affect the result of your blood test. For instance, if you eat or drink before the blood glucose test, the results will be higher than normal and it may look like your organism has some issues in controlling blood sugar levels or that you have diabetes.

How long do you have to fast before blood work?

Unless otherwise directed, you need to fast between for at least 8 and no more than 16 hours before your blood is drawn. The optimal amount of time is about 10-12 hours. This is why most fasting blood work is scheduled early in the morning: so you can have dinner around 7-9 pm, with most part of the fasting happening in your sleep, while having the blood test the next morning between 7-9 am. You can only drink water after dinner and until you’re done with the blood test.

Which blood tests require fasting?

Only a few blood tests require fasting, like the glucose test. Although the complete blood count (CBC) for example does not require fasting, if you have been prescribed a CBC plus blood glucose, you’ll have to fast nonetheless. The following are some of the blood tests that require fasting.

Other less commonly prescribed blood tests may require you to fast, such as the adrenocorticotropic hormone (ACTH) test and the antidiuretic hormone (ADH) test, but you will be instructed by your prescribing physician. Always follow the advice from your provider.

Can you drink water when fasting for blood test?

Yes, absolutely. It is actually advised to drink some water in the morning before the blood test to avoid dehydration which could also affect the test results. If you’re dehydrated, it may also be more difficult to draw blood from your veins.

What should you not do before a fasting blood test?

You should not drink or eat anything but water before a fasting blood test. This includes soda, juices, coffee, and tea. You should also not add water flavorings to the water, nor infuse the water with herbs, fruit or vegetable slices. You should not eat candies or chew gums, even if sugar-free. You should also not smoke nor exercise before the fasting blood test.

Other Frequently Asked Questions (FAQs) on fasting and blood work

Can you drink coffee while fasting for a blood test?

No, you should not drink coffee, nor tea, even if you don’t put milk or sugar in it. Only water is allowed in the hours preceding a fasting blood test.

Can you take medication before a fasting blood test?

Unless directed otherwise by your physician, you can still take your prescribed medications. Some tests, like the liver panel, may require you to avoid some medications. Some medications may have to wait until the blood is drawn if you need to take them with food. Some supplements, herbal remedies and OTC medication may also be better taken after the blood is drawn. Ask your prescribing doctor for directions tailored to your specific case.

Can you fast for too long before a blood test?

Yes, it is possible. The generally recommended time you should fast for a blood test is between 8 and 16 hours, with 10-12 hours being the optimal range. After about 16 hours, there are some changes in your metabolism: for example, you start to oxidize lipids (burn fat) and produce ketones to have fuel for your brain and body functions.

I accidentally ate before a fasting blood test, what to do?

If you eat or drink anything but water before a fasting blood test, you should tell the nurse drawing your blood; you may have your blood work rescheduled to another day, or they may proceed anyway depending on the case. Also let the doctor who will interpret the results that they may not be accurate because you were not fasting; your physician may prescribe the tests again or at least he will be mindful of the possible abnormal results in the report caused by not fasting.

When can I eat after a fasting blood test?

You can eat and drink anything you want right after the blood is drawn. If you wish, you can bring drinks or snacks to consume right after the test.

If you have diabetes, should you fast before a blood test?

If you have diabetes and haven’t been instructed on what to do before a fasting blood test, you should ask your doctor for medical advice. Research shows that fasting prior to blood test in diabetic patients results in a high number of hypoglycemic episodes, with potentially dangerous blood glucose levels and consequences. Do NOT fast without prior medical advice.

Do you REALLY have to fast before a blood test?

According to new research and guidelines, for some common tests like the lipid profile assessing cholesterol levels (LDL, HDL, Total Cholesterol) and triglycerides there is no real negative implication if they are performed in a non-fasting state, which actually simplifies the test for the patients.



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