You may have to fast—avoiding anything but water—for eight to 12 hours before a blood 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.
Tests That Require Fasting
Most blood tests actually do not require fasting, but some common ones do.
These include:
- Basic metabolic panel
- Blood glucose test
- Cholesterol test
- Triglyceride level test
- Lipoprotein panel
- High-density lipoprotein (HDL) level test
- Low-density lipoprotein (LDL) level test
- Renal function panel
- Liver function tests
- Iron level test
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.
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?
Everything you eat or drink is broken down by your digestive system and absorbed into your bloodstream. The nutrients from foods and beverages circulate the body for a period of time so they can be “delivered” throughout the body.
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 a blood glucose test will raise your blood sugar and lead to inaccurate test results, so fasting is required. However, the hemoglobin 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:
- It may cause you unnecessary stress if your results are outside the range of normal.
- It may prompt your healthcare provider to alter a medication dose.
- It may prompt additional testing that is unnecessary and costly.
- It may be compared to results from previous tests in which you did fast.
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 should avoid 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 slightly dehydrated. This causes your veins to flatten and makes them harder to find for a venipuncture.
Your healthcare provider may also ask you to avoid chewing gum and smoking. In some cases, you may also have to abstain from exercising.
What Does NPO After Midnight Mean?
“NPO after midnight” means “nil per os,” which is Latin for “nothing by mouth”—including water. This is used before procedures and is not the same type of fasting required for blood work.
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 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 on 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 your blood 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 and HDL (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.
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 QUESTIONS
- How long does it take to get blood test results?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.
- Why would I need to repeat a blood test?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.
- How do you book a blood test?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.
- When can I eat normally after a blood test?You can go back to eating and drinking as you normally would once your blood has been drawn. You may want to bring a snack along with you so you can have something to eat as soon as you are done with your appointment.