July 8, 2022

Facts About Fasting Before Blood Tests

Key Takeaways

  • Not every blood test requires you to fast, and your doctor will likely notify you if fasting is not required. 
  • It’s recommended that you fast anywhere between 8 to 12 hours before a blood test. Certain blood tests require longer or shorter fasting times, in which case, your doctor will inform you about the appropriate amount of time.
  • You’re allowed to drink water while fasting before a blood test. In fact, drinking plenty of water before a blood test is encouraged as it can help keep you hydrated.
  • Coffee, juice, tea, soda, and alcohol can all interfere with lab results and should be avoided while fasting before a blood test.

When you’re tired, you sleep. When you’re thirsty, you drink. And when you’re hungry, you eat. Unless, that is, your doctor recommends fasting before a blood test. In which case, what should you do?

We’re so accustomed to responding to hunger by immediately eating food that the simple request to fast for a period of time can send us spiraling. How long should I fast? Is a little snack okay? Can I still have my morning cup of coffee? What if I take prescription medicine?

With so many questions, we wanted to provide some facts about fasting before blood tests, why it’s important, how long you should fast, and some explanations about why eating before a blood test can impact results.

Let’s dive in.

How Long Do I Need to Fast Before a Blood Test?

It’s recommended that you fast anywhere between 8 to 12 hours before a blood test. Certain blood tests require longer or shorter fasting times, in which case, your doctor will inform you about the appropriate amount of time. Fortunately, most blood tests are scheduled for early in the morning, so the majority of fasting will take place while you sleep.

Why Is Fasting Before a Blood Test Important?

Blood tests help doctors understand how your body is functioning and check for certain health problems. To provide the most accurate picture of your overall health, it’s important to establish a baseline measurement of your results. Because everything you eat and drink contains nutrients that can impact different measurements, abstaining from food and drink is critically important to receiving accurate results.

For example, if you eat an entire sleeve of Oreo cookies before a blood glucose test, your blood sugar levels will be much higher than if you hadn’t eaten anything at all. This could alert your doctor to medical conditions that aren’t actually present by skewing the test results far outside their normal range.

What Blood Tests Require Fasting?

Not every blood test requires you to fast, and your doctor will likely notify you if fasting is not required. Here are a few blood tests that you should fast for:

  • Blood glucose test - Measures the amount of glucose (sugar) in your blood and is useful to test for diabetes or prediabetes.
  • Liver function test - Helps diagnose and monitor liver disease or damage by measuring certain proteins and enzymes in your blood.
  • Cholesterol test - Cholesterol is a waxy substance that your body needs to build cells and make vitamins and other hormones. There are two types of cholesterol: LDL cholesterol (the bad kind) and HDL cholesterol (the good kind). Because cholesterol circulates in your bloodstream, too much LDL cholesterol can start to build up in the inner walls of your arteries and pose a serious risk to your health.
  • High-density lipoprotein (HDL) level test - Measures the levels of good cholesterol in your body. HDL circulates throughout your bloodstream and collects LDL cholesterol, triglycerides, and harmful fats and brings them to the liver to be broken down and removed from your body.
  • Low-density lipoprotein (LDL) level test - Measures the levels of bad cholesterol in your body. Too much LDL can clog your arteries with a buildup of plaque.
  • Triglyceride level test - Measures the levels of triglycerides, a type of fat, found in your body. Too much triglyceride could indicate risk for developing heart disease.
  • Basic or comprehensive metabolic panel - This is often part of any routine physical and measures your blood sugar levels, electrolytes, fluid balance, kidney function, and liver function.
  • Renal function panel - Renal relates to your kidneys. A renal function panel is a collection of measurements to test the health of your kidneys.
  • Iron test - Iron is a major component of hemoglobin, a type of protein in red blood cells that carries oxygen from your lungs to the rest of your body. An iron test measures whether your iron levels are too high or low.
  • Lipoprotein panel - Measures the amount of certain fat molecules, or lipids, in your blood. A lipoprotein panel can determine if you are at risk for cardiovascular disorders.
  • Vitamin B12 test - Measures the amount of the vitamin you have in your bloodstream. This test can help diagnose anemia and other disorders.

Can I Drink Water While Fasting Before a Blood Test?

Yes! You’re allowed to drink water while fasting before a blood test. In fact, drinking plenty of water before a blood test is encouraged as it can help keep you hydrated. Dehydration can impact the results of certain blood tests, so ensuring you have plenty of water in your system will improve the accuracy of your blood test.

Can I Drink Coffee Before a Blood Test?

No, even black coffee can interfere with the results of your blood test. Coffee contains caffeine and is also a diuretic, which means it removes water from your body. Save that cup of Joe for after the blood test.

Is There Anything I Can Drink Besides Water While Fasting Before a Blood Test?

Not really. Juice, soda, and alcohol all contain high-levels of sugar and will interfere with your test results.

Tea is similar to coffee in that it contains caffeine and is a diuretic and removes water from your body. It’s best to avoid drinking all non-water beverages before taking a blood test.

What Else Should I Avoid While Fasting Before a Blood Test?

Besides not eating or drinking any non-water beverages, you should also avoid exercising, smoking, or chewing gum. These can activate your digestive system and affect results.

Can I Still Take my Prescription Medicine Before a Blood Test?

It’s okay to continue taking your prescription drugs, unless your doctor asks you to skip them. However, you should ask your doctor before taking any over-the-counter medication as it may skew results.

When Can I Eat and Drink Again After a Blood Test?

Once the blood draw is complete and the phlebotomist collects all the samples they need, you’re free to eat and drink whatever you want.

Receive a Blood Test At Home

Fasting before a blood test can be uncomfortable, but it’s important to follow these guidelines to ensure your test results provide an accurate picture of your health. Fortunately, if you’d like to quickly break your fast as soon as the blood draw is complete, Getlabs can help.

Getlabs provides the perfect solution for patients who prefer to take blood tests in the comfort of their homes. With over 5,000 different lab tests available, you can book a nearby phlebotomist for many of the tests your doctor may recommend.

If you want to learn more about Getlabs, visit www.getlabs.com/faq to learn more.

This page is provided for educational and informational purposes only and does not constitute the provision of medical advice or professional services. The information provided should not be used for diagnosing or treating a health problem or disease, and those seeking personal medical advice regarding any of the tests and conditions referenced above are advised to consult with a licensed clinician. Always seek the advice of your qualified health provider regarding a medical condition and do not disregard professional medical advice or delay in seeking it because of any information on this page. If you think you may have a medical emergency, immediately call 911 or go to the nearest urgent care center or hospital.

Book an at-home lab collection

Getlabs sends a nearby medical specialist to you to collect your labs for Labcorp and Quest. Available same-day, nationwide.

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