Table of ContentsView AllTable of ContentsFasting TestsDurationWhy It’s NeededHow to FastWhat to Do if You EatEnd of FastingFAQ
Table of ContentsView All
View All
Table of Contents
Fasting Tests
Duration
Why It’s Needed
How to Fast
What to Do if You Eat
End of Fasting
FAQ
You may have to fast—avoiding anything but water—for eight to 12 hours before ablood test. This is because nutrients from foods and beverages are absorbed into your blood, which can cause inaccurate results.
Make sure you are clear about whether or not you need to fast for the test(s) you are scheduled for so you can avoid having to repeat them.
This article discusses why your healthcare provider might ask you to fast for blood work and how you can best prepare.
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What Blood Tests Require Fasting?
Mostblood testsactually do not require fasting, but some common ones do.
These include:
Most lab tests drawn duringpregnancydo not require fasting, with the exception of the glucose challenge test. This test is performed to screen for a condition calledgestational 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.
How Long You Need to Fast
Generally, you should fast for eight to 12 hours before lab work that requires it.
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.
Why Do You Need to Fast?
This can take hours, which is why you may be asked to fast for a period of time before certain blood tests.
For laboratory tests that don’t examine the levels of certain nutrients or other substances in your blood, eating and drinking should not affect the results. For those that do, however, anything you eat or drink in the hours prior may produce an inaccurate result.
For example, eating before ablood glucose testwill raise your blood sugar and lead to inaccurate test results, so fasting is required. However, thehemoglobin A1c test—also done for diabetes—does not require fasting. This is because it looks at a marker of blood sugar control over the past few months, rather than directly measuring blood sugar.
An inaccurate test result is problematic for several reasons:
How to Fast for Blood Work
Your healthcare provider will give you details on how long you need to fast before your blood test. Most of these types of tests are scheduled for first thing in the morning, so you can sleep during the fasting period.
During the fasting period, you shouldavoid all drinks (including coffee and tea) except for water. Drinking water is even encouraged before blood work because a 12-hour fast from drinking fluids can make you slightlydehydrated.This causes your veins to flatten and makes them harder to find for a venipuncture.
For some tests, you may also be asked to refrain from drinking alcohol for a longer period of time.This is because alcohol’s effects on the body can make the test results inaccurate.
Your healthcare provider may also ask you to avoid chewing gum and smoking. In some cases, you may also have to abstain from exercising.These activities can also affect certain test results.
What Does NPO After Midnight Mean?
Medication and Blood Tests
Even if you are asked to fast for blood work, you should take your prescribed medications with water, unless specifically requested not to do so.
The exception to this is vitamins and supplements. These may affect certain lab tests, so they should be held off the morning of a lab test.
Discuss what medications you are taking with your healthcare provider and clarify ahead of time if you have any questions about holding medications before blood work.
What to Do if You Accidentally Eat or Drink
If you accidentally ate or drank before your test, let your healthcare provider know. Depending on the reason the test was ordered, you may be able to go ahead and have yourblood drawn. The healthcare provider will just take this into account when interpreting your results.
For example, if you are having a screening cholesterol panel and you ate breakfast before the test, it’s not necessary to reschedule it. In fact, newer recommendations from the National Lipidology Association state that fasting for a screening lipid panel is optional.
While your breakfast will affect the triglyceride level, other important parts of the test, such as the total cholesterol andHDL(high-density lipoprotein, known as “good” cholesterol) will not be affected. LDL will only be affected if the triglyceride level is very elevated. If the triglyceride level is elevated, you may be asked to come back to repeat the test.
On the other hand, if a test was ordered specifically for blood sugar and you ate breakfast, the test may not be useful.
Pregnant people who do not fast before undergoing the glucose challenge will be asked to reschedule the test.
When Can I Eat or Drink After Fasting?
You can eat or drink as soon as the blood test is over. If you know you’ll be hungry after the test, bring food or drinks with you so you can eat right away.
Schedule fasting blood tests for early in the morning to avoid going too long without food.
Summary
Many lab tests do not require fasting. But for those that do, such as blood glucose tests, eating food can affect the results.
Check with the healthcare provider who ordered the blood work to see if fasting is necessary, and if so, do not eat for to eight to 12 hours before the test. It’s fine to take your prescribed medication and drink water before the test to stay hydrated.
Frequently Asked QuestionsDepending on the test and how urgently your healthcare provider has indicated on the lab order form, blood test results can come back as soon as under an hour to several days. When the test is marked as “stat,” it indicates to the lab that the test should be run and reported back as soon as possible, whereas “routine” means there is no rush for a result. The timing also depends on whether the test has to be transported to a special lab.Your healthcare provider may ask that you have a repeat blood test when the results are invalid, to confirm unexpected results, or if not enough blood was provided to run all of the necessary tests. Blood is drawn in special tubes and transported to a lab for testing.The lab equipment requires a certain amount of blood to run the tests. Some lab tests are affected if the blood has sat in the tube for too long, if the tube was not maintained at the proper temperature, or if the blood underwent breakage (hemolysis) during the blood draw.Most blood tests require an order from a healthcare provider, such as a physician, nurse practitioner, or physician’s assistant. Your healthcare provider’s office may have a phlebotomist who can draw the labs right in the office, or you may be asked to go to a separate lab facility. Some facilities take walk-ins, while others require appointments.
Frequently Asked Questions
Depending on the test and how urgently your healthcare provider has indicated on the lab order form, blood test results can come back as soon as under an hour to several days. When the test is marked as “stat,” it indicates to the lab that the test should be run and reported back as soon as possible, whereas “routine” means there is no rush for a result. The timing also depends on whether the test has to be transported to a special lab.
Your healthcare provider may ask that you have a repeat blood test when the results are invalid, to confirm unexpected results, or if not enough blood was provided to run all of the necessary tests. Blood is drawn in special tubes and transported to a lab for testing.The lab equipment requires a certain amount of blood to run the tests. Some lab tests are affected if the blood has sat in the tube for too long, if the tube was not maintained at the proper temperature, or if the blood underwent breakage (hemolysis) during the blood draw.
Your healthcare provider may ask that you have a repeat blood test when the results are invalid, to confirm unexpected results, or if not enough blood was provided to run all of the necessary tests. Blood is drawn in special tubes and transported to a lab for testing.
The lab equipment requires a certain amount of blood to run the tests. Some lab tests are affected if the blood has sat in the tube for too long, if the tube was not maintained at the proper temperature, or if the blood underwent breakage (hemolysis) during the blood draw.
Most blood tests require an order from a healthcare provider, such as a physician, nurse practitioner, or physician’s assistant. Your healthcare provider’s office may have a phlebotomist who can draw the labs right in the office, or you may be asked to go to a separate lab facility. Some facilities take walk-ins, while others require appointments.
15 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.MedlinePlus.Fasting for a blood test.MedlinePlus.Basic metabolic panel (BMP).MedlinePlus.Blood glucose test.MedlinePlus.Cholesterol levels.MedlinePlus.Triglycerides test.MedlinePlus.Lipoprotein (a) blood test.MedlinePlus.Glomerular filtration rate (GFR) test.MedlinePlus.Creatinine test.MedlinePlus.Liver function tests.MedlinePlus.Alkaline phosphatase.MedlinePlus.Iron tests.MedlinePlus.Vitamin B test.MedlinePlus.Glucose screening tests during pregnancy.Ashraf MM, Rea R.Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111Wilson PWF, Jacobson TA, Martin SS, et al.Lipid measurements in the management of cardiovascular diseases: Practical recommendations a scientific statement from the national lipid association writing group.J Clin Lipidol. 2021;15(5):629-648. doi:10.1016/j.jacl.2021.09.046
15 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.MedlinePlus.Fasting for a blood test.MedlinePlus.Basic metabolic panel (BMP).MedlinePlus.Blood glucose test.MedlinePlus.Cholesterol levels.MedlinePlus.Triglycerides test.MedlinePlus.Lipoprotein (a) blood test.MedlinePlus.Glomerular filtration rate (GFR) test.MedlinePlus.Creatinine test.MedlinePlus.Liver function tests.MedlinePlus.Alkaline phosphatase.MedlinePlus.Iron tests.MedlinePlus.Vitamin B test.MedlinePlus.Glucose screening tests during pregnancy.Ashraf MM, Rea R.Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111Wilson PWF, Jacobson TA, Martin SS, et al.Lipid measurements in the management of cardiovascular diseases: Practical recommendations a scientific statement from the national lipid association writing group.J Clin Lipidol. 2021;15(5):629-648. doi:10.1016/j.jacl.2021.09.046
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
MedlinePlus.Fasting for a blood test.MedlinePlus.Basic metabolic panel (BMP).MedlinePlus.Blood glucose test.MedlinePlus.Cholesterol levels.MedlinePlus.Triglycerides test.MedlinePlus.Lipoprotein (a) blood test.MedlinePlus.Glomerular filtration rate (GFR) test.MedlinePlus.Creatinine test.MedlinePlus.Liver function tests.MedlinePlus.Alkaline phosphatase.MedlinePlus.Iron tests.MedlinePlus.Vitamin B test.MedlinePlus.Glucose screening tests during pregnancy.Ashraf MM, Rea R.Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111Wilson PWF, Jacobson TA, Martin SS, et al.Lipid measurements in the management of cardiovascular diseases: Practical recommendations a scientific statement from the national lipid association writing group.J Clin Lipidol. 2021;15(5):629-648. doi:10.1016/j.jacl.2021.09.046
MedlinePlus.Fasting for a blood test.
MedlinePlus.Basic metabolic panel (BMP).
MedlinePlus.Blood glucose test.
MedlinePlus.Cholesterol levels.
MedlinePlus.Triglycerides test.
MedlinePlus.Lipoprotein (a) blood test.
MedlinePlus.Glomerular filtration rate (GFR) test.
MedlinePlus.Creatinine test.
MedlinePlus.Liver function tests.
MedlinePlus.Alkaline phosphatase.
MedlinePlus.Iron tests.
MedlinePlus.Vitamin B test.
MedlinePlus.Glucose screening tests during pregnancy.
Ashraf MM, Rea R.Effect of dehydration on blood tests.Pract Diab. 2017;34(5):169-171. doi:10.1002/pdi.2111
Wilson PWF, Jacobson TA, Martin SS, et al.Lipid measurements in the management of cardiovascular diseases: Practical recommendations a scientific statement from the national lipid association writing group.J Clin Lipidol. 2021;15(5):629-648. doi:10.1016/j.jacl.2021.09.046
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