If you have been prescribed blood tests, you may be wondering whether or not you should be fasting before the blood test. Commonly ordered blood tests that require fasting are blood glucose (blood sugar) testing, lipid (triglyceride and cholesterol) testing, and metabolic testing. Usually, your health care provider should let you know what’s allowed and what’s not before the prescribed blood test, but in this article, I’ll clear up some doubts about fasting and blood tests.
Why should you be fasting before a blood test?
Foods and drinks contain nutrients that enter the bloodstream and can affect your blood test result. For example, if you eat or drink before the blood sugar test, the results will be higher than normal and it may appear that your body has problems controlling blood sugar levels or that you have diabetes.
How long should you be fasting before a blood test?
Unless otherwise specified, you must fast for at least 8 hours and no more than 16 hours before the blood test. The optimal duration is about 10 to 12 hours. That’s why most fasting blood tests are scheduled for early in the morning, so you can eat dinner between 7 and 9 p.m., with most of the fasting taking place while you sleep, and have the blood test the next morning between 7 and 9 a.m. You can only drink water after dinner and until the blood test is completed.
What blood tests require fasting?
Only a few blood tests require fasting, such as the glucose test. Although the complete blood count (CBC), for example, does not require you to be fasting, if you have been prescribed a CBC and a blood sugar you will still need to be fasting. Here are some of the blood tests that require fasting.
- blood sugar test
- Glucose tolerance test
- Lipid profile test (triglycerides, total cholesterol, LDL, HDL)
- Metabolic panel (glycaemia, electrolytes, kidney function)
- Analysis of nutrients, such as iron, vitamin B12, amino acid profile…
- Gamma-glutamyl transferase (GGT) test
Other less commonly ordered blood tests may require you to be fasting, such as the adrenocorticotropic hormone (ACTH) test and the antidiuretic hormone (ADH) test, but you will receive instructions from your prescribing physician. Always follow the advice of your provider.
Can we drink water when fasting for a blood test?
Yes, absolutely. In fact, it is advisable to drink some water in the morning before the blood test to avoid dehydration which could also affect the test results. If you are 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 eat or drink anything other than water before a fasting blood test. This includes sodas, juices, coffee and tea. You should also not add flavorings to the water, or infuse the water with herbs, sliced fruits or vegetables. You should not eat sweets or chewing gum, even if they are sugar-free. You should also not smoke or exercise before the fasting blood test.
Other Frequently Asked Questions (FAQ) About Fasting Blood Testing
Can you drink coffee while fasting for a blood test?
No, you shouldn’t drink coffee or tea, even if you don’t add milk or sugar. Only water is authorized in the hours preceding a fasting blood test.
Can medication be taken before a fasting blood test?
Unless otherwise instructed by your doctor, you can continue to take the medicines prescribed for you. Certain tests, such as liver function tests, may require you to avoid certain medications. Some medications may need to wait until blood is drawn if you need to take them with food. Certain supplements, herbal remedies, and over-the-counter medications may also be best taken after the blood test. Ask your prescribing doctor for instructions tailored to your particular case.
Can you fast for too long before a blood test?
Yes it’s possible. The generally recommended fasting time for a blood test is between 8 and 16 hours, with the optimal time being 10 to 12 hours. After about 16 hours, some changes occur in your metabolism: for example, you begin to oxidize lipids (burn fat) and produce ketones to fuel your brain and body functions.
I accidentally ate before a fasting blood test, what should I do?
If you eat or drink anything other than water before a fasting blood test, you must tell the nurse who is taking your blood; the analysis may be postponed to another day or performed anyway, depending on the case. Also inform the doctor who will interpret the results that they may not be accurate because you were not fasting; your doctor can order the tests again or at least be aware of the possibility of abnormal results in the report caused by the fact that you were not fasting.
When can I eat after a fasting blood test?
You can eat and drink whatever you want right after the blood test. If you wish, you can bring drinks or snacks to consume immediately after the analysis.
If you are diabetic, do you have to be fasting before a blood test?
If you are diabetic and you have not received instructions on what to do before a fasting blood test, you should seek the advice of your doctor. Research shows that fasting before a blood test in diabetic patients leads to a high number of hypoglycemic episodes, with potentially dangerous blood sugar levels and consequences. Do NOT fast without medical advice.
Do you REALLY have to be fasting before a blood test?
According to new research and guidelines, for some common tests such as the lipid profile, which assesses cholesterol (LDL, HDL, total cholesterol) and triglyceride levels, there is no real negative implication if done without fasting, which actually simplifies the test for patients.
