Why Is Fasting Required Before Blood Tests
Fasting before a blood test helps improve the accuracy and reliability of the test. Accurate test results are a critical component of diagnosing various health conditions and monitoring the effectiveness of treatment plans.
Your diet contains a range of nutrients, including carbohydrates, proteins, fats, minerals and vitamins. When you eat and drink normally, those nutrients are broken down and absorbed into your bloodstream. This can impact the levels of those substances in the blood and skew lab test results.
For example, certain methods of diabetes testing measure the patients baseline blood sugar levels after a period of fasting. Eating directly before the test will cause the patients blood sugar to spike, leading to an inaccurate test result. The patient could be misdiagnosed with prediabetes or diabetes, unless their doctor recommends that they complete a second test.
Fasting Before A Blood Test
In the hours after you eat, many chemicals in your blood undergo significant changes. Further changes can occur if you do not eat for an extended period e.g. more than 16 hours. Therefore, some blood tests need to be collected in the morning after an overnight fast.
Fasting means that you eat and drink nothing except water for 816 hours before your blood test. Fasting for 12 hours is optimal.
Many people find it more convenient to complete the fast overnight and present for blood collection in the morning after.
When you present for your blood test, our collector will ask when you last ate and drank.
What If I Accidentally Eat Or Drink Before My Blood Test
If you have been instructed to fast before your blood test and you accidentally eat or drink something during the fasting window, it is important to notify your healthcare provider or the person drawing your blood. If you dont, your results could be incorrectly interpreted.
It may be possible for your healthcare provider to interpret the non-fasted test results. Most likely, they will instruct you to reschedule the blood test at a time when you will be able to successfully complete the fast before the test.
Your physician will let you know the steps you need to take before your blood test, including whether or not fasting is required and for how long. By following all testing instructions and best practices , you can ensure a smooth testing process and the most accurate results.
*Post reviewed and approved by Dr. Sona Kirpekar, our in-house Medical Consultant.
Before you go
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Inpatient Vs Outpatient Blood Testing
Location can play a factor into how fast youll get your results back. For example, going to a place where theres an on-site laboratory may get you results more quickly than if your blood has to be sent to another laboratory. Specialty tests for rare conditions often need to be sent to specific laboratories.
According to Regional Medical Laboratory, most in-hospital results can be obtained within three to six hours after taking the blood. Sometimes blood drawn at other, non-hospital facilities can take several days to get results.
If youre hoping to receive blood test results as quickly as possible, some tips to do this can include:
- Ask to have blood drawn at a location where theres an on-site laboratory.
- Ask if there are quick test options for a particular test, such as an H and H for anemia.
- Ask if the results can be sent to you via a web portal.
- Ask if you can wait at the medical facility until results are available.
Sometimes, how quickly the blood tests take depends on how common the blood test is. Blood tests performed more often, such as a CBC or metabolic panel, are usually available more quickly than tests for rare conditions. Fewer laboratories may have the testing available for these conditions, which could slow results.
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 and produce ketones to have fuel for your brain and body functions.
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Why Some Blood Tests Require Fasting
Some blood tests require you to fast because the results can be easily influenced by the foods that you eat. The micronutrient, carbohydrate, protein, and fat levels in your food can all skew the results of some tests.
An example of a test that requires fasting is a blood glucose test, which measures your blood sugar levels. Eating foods high in carbohydrates can significantly raise your blood sugar levels within 15 minutes.
What To Know About Fasting Before Your Lab Test
With certain blood tests, you may be instructed to fast for up to eight hours before your appointment. Fasting before a blood draw means you dont eat or drink anything except water.
Dont wait until the day of your blood draw to ask if you should fast. That could cause your appointment to be rescheduled. If at any point youre unsure if fasting is required, contact your doctor.
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What To Expect During Tests
While fasting for blood tests is important, there are other things to know as well. Here is exactly what happens during different tests. Take cholesterol test for example:
A blood test ordered to check your cholesterol levels is usually done in the morning to get accurate results. A lab assistant will clean the puncture site, insert a needle, and draw some blood from a vein. They will also wrap an elastic band around your upper arm before inserting the needle this helps your veins to pop out a bit. After they collect the sample, they will cover the puncture site with a bandage. It usually takes a couple of minutes for the whole procedure to complete.
You do not need to take special care after your test. You can even drive and resume your normal activities immediately after the test. If you have been fasting for the last 12 hours, bring a snack with you to eat after the test. Your doctor will share the test results with you and explain what different values mean. If they find anything abnormal, they may ask you to have another test in a few weeks or months time.
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 youre done with the blood test.
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Fasting Before A Blood Test: Drinking Coffee Eating And More
Certain blood tests require fasting beforehand. If your healthcare provider has instructed you to fast before your upcoming test, it means that you should not eat or drink anything for the specified amount of time before having your blood drawn.
Understanding why fasting is important and how to properly fast before a blood test can be helpful for eliminating pre-test anxiety and simplifying the testing process. Below, learn how to fast for a blood test correctly to help make sure that your test results are accurate.
Is The A1c Test Used During Pregnancy
Health care professionals may use the A1C test early in pregnancy to see if a woman with risk factors had undiagnosed diabetes before becoming pregnant. Since the A1C test reflects your average blood glucose levels over the past 3 months, testing early in pregnancy may include values reflecting time before you were pregnant. The glucose challenge test or the oral glucose tolerance test are used to check for gestational diabetes, usually between 24 and 28 weeks of pregnancy. If you had gestational diabetes, you should be tested for diabetes no later than 12 weeks after your baby is born. If your blood glucose is still high, you may have type 2 diabetes. Even if your blood glucose is normal, you still have a greater chance of developing type 2 diabetes in the future and should get tested every 3 years.
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Common Blood Tests And How Long It Takes To Get Results
Some of the more common blood tests a doctor may order include:
- Complete blood count .This test measures the presence of 10 cell types in the white blood cells, red blood cells, and platelets. Examples of these results include hematocrit, hemoglobin, red blood cell count, and white blood cell count. CBC results are usually available to your doctor within 24 hours.
- Basic metabolic panel.This test measures common electrolytes in the blood as well as other compounds. Examples include calcium, glucose, sodium, potassium, carbon dioxide, chloride, blood urea nitrogen, and creatinine. You may be asked to fast for a certain period of time before having your blood drawn. These results are also typically sent to your doctor within 24 hours.
- Complete metabolic panel. This blood test measures all the factors mentioned in the test above as well as two protein tests, albumin and total protein, as well as four tests of liver function. These include ALP, ALT, AST, and bilirubin. A doctor may order this more comprehensive testing if they wish to understand more about your liver or kidney function. Theyll usually receive your results within one to three days.
- Lipid panel. Lipid panels measure the amount of cholesterol in the body. This includes high-density lipoprotein and low-density lipoprotein . Your doctor should receive results from the lab within 24 hours.
Can You Drink Alcohol If Youre Fasting Before A Blood Test
Some blood tests, such as those that assess liver health or triglyceride levels, may require you to not drink any alcohol for a full 24 hours. Trace amounts of alcohol can remain in your bloodstream for several days. If you have any concerns about alcohol consumption, discuss this with your doctor when you schedule your test.
Also ask your doctor if you can smoke cigarettes before the test, or if you should refrain from smoking during your fast.
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Can I Eat And Drink Before Having A Blood Test
It depends on the type of blood test you’re having. The healthcare professional arranging your test will tell you if you need to do anything to prepare for it.
You can eat and drink as normal before some blood tests. But if you’re having a “fasting blood test”, you will be told not to eat or drink anything beforehand. You may also be told not to smoke before your test.
Can I Chew Gum Or Brush My Teeth Before The Test
Chewing gum is not allowed because it determines your body to produce digestive fluids. At the same time, the sweetness of the gum can interfere with the blood sugar level. Dont chew gum, but you can brush your teeth, as long as you dont swallow anything. Swallowing toothpaste can start the production of hormones and enzymes in your body, so be mindful of that.
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How Long Does Intermittent Fasting Take To Work
Losing weight is among the top goals of people seeking a healthier lifestyle. Many are now turning to intermittent fasting to achieve a healthy weight. This diet involves a fasting period where you dont take any food and cyclic eating periods where you consume a healthy diet.
The excellent news is that there are no food restrictions with this diet, only rules on when is the right time to eat. Hence, this eating pattern moves away from the normal diet, but this isnt a free pass to eat junk food. Its better to stick with healthy foods during the eating periods.
When you choose to participate in intermittent fasting, you can fast from 16 to 24 hours. This isnt something new, as people have been engaging in fasting for centuries. Since it took time to find food, people could last for long periods without eating any food.
With that in mind, how long does intermittent fasting take to work? Truthfully, it takes time to experience fat loss, so give yourself about ten weeks. During this period, certain changes occur, making it easier for your body to burn fat.
Can The A1c Test Result In A Different Diagnosis Than The Blood Glucose Tests
Yes. In some people, a blood glucose test may show diabetes when an A1C test does not. The reverse can also occuran A1C test may indicate diabetes even though a blood glucose test does not. Because of these differences in test results, health care professionals repeat tests before making a diagnosis.
People with differing test results may be in an early stage of the disease, when blood glucose levels have not risen high enough to show up on every test. In this case, health care professionals may choose to follow the person closely and repeat the test in several months.
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How Long To Fast
Generally, you should fast for eight to 12 hours before lab work that requires it. Fasting for a blood sugar test, which is included in the basic metabolic panel, is generally eight to 12 hours.
You can always clarify how long to fast with your healthcare provider. If you are unsure, aim for 12 hours of fasting. For example, if you schedule your test for first thing in the morning, you should generally not eat anything after dinnertime the night before.
Pregnancy And Blood Tests
Most lab tests drawn in pregnancy do not require fasting, with the exception of the glucose challenge test. This test is performed to screen for a condition called gestational diabetes.
For this test, you will be asked to consume a special sugary beverage that contains a specific amount of glucose. Your blood glucose level will be tested at specific time intervals.
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When Will I Get The Test Results
Most fasting lab results are available for your doctor in about 24 hours. However, some tests may take longer to complete. The lab will provide a report to your doctor, and your doctor will use the report to help with next steps in your diagnosis and treatment.
Learn more about the lab services we offer at Bon Secours.
What Can I Eat & Drink While Fasting Before A Blood Test
A common question from patients is, Can you drink coffee before a blood test?. The answerunless your doctor specifically tells you otherwiseis no.
Other activities your doctor willlikely have you refrain from include chewing gum, smoking and exercising. Allcan accelerate your digestion, which can adversely affect your results.
Important note: You should continue to take your prescribed medication while fasting unless your doctor specifically directs you not to.
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How Does Blood Glucose Work
Blood sugar, or glucose, is the main sugar found in your blood. When you eat or drink, your body breaks down carbohydrates in the food or liquid into sugar for energy.
Then your pancreas produces a hormone called insulin, which helps blood sugar enter your bodys cells. It also helps your body store blood sugar for later use. As the cells use the blood sugar, glucose and insulin levels in the blood decrease.
But with diabetes, your body has trouble regulating glucose. Either your pancreas doesnt make enough insulin, or your body doesnt process insulin effectively. Therefore, the amount of sugar in your blood is higher than normal. And a lot of glucose in your blood is bad for your body.
A fasting blood glucose test measures the amount of glucose in your blood when it should be at its lowest. That happens in the morning and when you havent had anything to eat or drink for a while.
Get Your First Blood Test Today
Did you know that you can get fasting blood tests directly from the internet these days? These tests are called direct-to-consumer labs and can be bought at affordable prices. You dont need a doctors appointment or prescription and you get your results back within a few business days.
Visit DiscountedLabs.com and check out the popular blood tests listed there. Some of these blood tests might require fasting, and all of them will help you gauge the state of your health.
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Q: Can You Drink Black Coffee When Fasting For Bloodwork
A: Dont drink anything but water. Coffee contains caffeine. Its also a diuretic, which removes water from your body. Many people think coffee counts toward your fluid requirements. However, it makes you urinate more, which can affect some of your lab results. So, get your blood test done, then get your cup of coffee!
When Can I Eat Or Drink Again
As soon as your blood is taken, your fast is over. You might want to bring a snack and a drink with you so you can eat as soon as possible after the test.
Show Sources
American Academy for Clinical Chemistry: “Basic Metabolic Panel,” “Comprehensive Metabolic Panel,” “GGT,” “Glucose Tests,” “Iron Tests,” “Lipid Profile,” “Renal Function Panel,” “Vitamin B12 & Folate.”
British National Health Service: “Can I Eat and Drink Before Having a Blood Test?”
Crystal Moore, MD, PhD, FCAP, anatomical and clinical pathologist, Chesapeake, VA.
National Heart Lung and Blood Institute: “What Are Blood Tests?”
Nemours Foundation: “Blood Test: Comprehensive Metabolic Panel .”
Providence Laboratory Services : “Fasting Instructions.”
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