Fasting Before your Blood Test—Tips and Advice

A woman drinking water while fasting in preparation for a blood test

When your doctor says you need to fast before a blood test, what exactly does this mean? Fasting requires that you do not consume any food or liquids other than water for a specific amount of time leading up to your test. Water, in most cases, is allowed during your fast. Thankfully, it does not affect the results of the majority of blood tests. Anything else that you consume including food and other beverages will affect the outcome of specific tests, and—if you’re taking a test that requires fasting—can render the results inaccurate and difficult for your doctor to interpret.

Why do you have to fast before a blood test?

Certain blood tests require that you fast before having blood drawn, as nutrients in food and beverages are broken down and absorbed into your bloodstream. When this happens, it causes changes in the amount of minerals, sugar, certain enzymes, fats, and other materials in your blood. By fasting before the test and keeping your blood levels unaffected by food and other ingestible items, you have the best chance of getting the most accurate results.

Do you need to fast before a blood test?

Although many tests do not require any type of fasting beforehand, your doctor will always let you know if you do need to fast before a test and for how long. If you have been asked to fast by your doctor before a blood test, then you do need to properly complete the fast or else the test results will be negatively affected.

How long do you have to fast for before a blood test?

Depending on the type of blood test you are taking, you may need to fast for at least 6-12 hours beforehand. On occasion, you may also be asked to abstain from drinking alcohol for longer than your fast (for example, 24 hours). Your doctor will confirm the exact amount of time required to ensure accurate results, so if you’re unclear, it’s best to confirm.

Can you exercise while fasting?

Exercise can speed up digestion and affect results, so you should not exercise during your fast hours leading up to your blood test.

Can you drink coffee or alcohol while fasting?

You cannot drink anything other than water while you fast. Alcohol can affect blood sugar and fat levels, while coffee affects digestion and can also cause inaccurate results. So, unfortunately, not even black coffee is ok during the fasting period.

Can you smoke or chew gum while fasting?

Since chewing gum can affect digestion (even the sugarless variety) your should avoid it. Smoking can also affect certain tests, so be sure to refrain from both when asked to fast unless your doctor has specifically told you otherwise.

What happens if you don’t fast?

It’s important that you communicate to your healthcare provider if you were unable to fast before your blood test. If this is the case, it may need to reschedule for a later date. If you were supposed to fast, didn’t, and still take the test, there is a good chance your results will come back erroneous and could lead to a misdiagnosis.

Which blood tests require fasting?

Relatively few blood tests require fasting, but many that do happen to be among the most common. Fasting is often paired with: BMP or basic metabolic tests; tests that measure levels of glucose, cholesterol, triglyceride, HDL (high density lipoprotein), and LDL (low-density lipoprotein); tests that measure glucose or lactose tolerance; and tests that function to assess liver or renal function.

Do children need to fast before a blood test?

Unfortunately, children also need to fast before certain blood tests. Your pediatrician may indicate that your child needs to fast before a test. In an effort to keep them comfortable during the fast, many parents schedule the blood test for first thing in the morning. This way, all or most of the fasting hours can take place overnight.

Can you fast while pregnant?

Always consult with your doctor if you are pregnant and need a blood test that requires fasting. They can best advise on if and how you can do this safely. Generally speaking, if you do not have a high-risk pregnancy or any other health considerations, you can set up a safe fasting plan in advance of your blood test. Your doctor will be able to help with other considerations too. For instance, what to do if you experience heartburn while pregnant and fasting for a blood test, which can be common.

When can I stop fasting?

You only fast leading up to the time of your blood test. When your test is over, you can eat and drink as normal. Keep in mind, it’s a good idea to bring a snack and drink to your appointment. You may feel faint following the test and, depending on how long you’ve been fasting for, you could be hungry.

Can I keep taking my regular medications while fasting?

This depends, as some medications can negatively affect the results of your test and make them difficult to interpret. Always discuss your regular medication schedule with your doctor in advance of fasting and your blood test. You also should not take any over-the-counter medications or vitamins while fasting without first discussing with your doctor. These too may affect results of certain tests.

Tips for fasting before a blood test

If possible, try to schedule your blood test in the morning. This way, most of your fasting time can occur while you are sleeping the night before. Unless instructed otherwise, drink plenty of water during your fast. Staying hydrated keeps your veins plump which helps your phlebotomist locate one more easily. This usually results in a shorter appointment time and smoother experience at your lab. Make sure you’re clear on the timeline and guidelines specific to your fast and blood test. If you’re not sure about anything, reach out to your doctor or healthcare professional to clarify.


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