How Long to Fast Before Blood Work

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.

What Blood Tests Require Fasting?

Most blood tests actually do not require fasting, but some common ones do.

These include:

• Basic metabolic panel: This test measures eight different substances that reveal information about the body's chemical balance and metabolism. Typically, you'll fast for eight hours before the test.
• Blood glucose test: By measuring a sugar in your blood called glucose, a blood glucose test can see whether you may have diabetes and certain other medical conditions. Some blood glucose tests don't require you to fast, but others do. If you're asked to fast, it will usually be for at least eight hours before the test.
  
• Lipid panel: Also called a cholesterol test or lipid profile, this test measures cholesterol and certain fats in your blood. A typical fast for cholesterol levels is nine to 12 hours before the test.
• High-density lipoprotein (HDL) level test: Sometimes called "good" cholesterol, HDL cholesterol helps get rid of LDL cholesterol. It is usually measured during a lipid panel. The fasting period is typically nine to 12 hours beforehand.
• Low-density lipoprotein (LDL) level test: Aka "bad" cholesterol, LDL cholesterol is what mainly causes blockages in arteries. It's also a component of the lipid panel. The standard fasting period is nine to 12 hours prior to the test.
• Triglyceride level test: Triglyceride levels are usually measured during a lipid panel. Triglycerides are a type of fat in the blood and can help determine your risk for heart attack or stroke. You'll likely be asked to fast for nine to 12 hours beforehand.
• Lipoprotein (a) blood test: Lipoprotein (a) is a particle made up of protein and fats (lipids). It's what carries cholesterol through your blood stream and can estimate your risk of heart disease and stroke. You'll probably need to fast for nine to 12 hours beforehand.
  
• Renal function panel: Also called kidney function tests, this panel is used to see how well your kidneys are working. It includes blood, urine, and other diagnostic tests. If you're getting your glomerular filtration rate (GFR) checked, you may need to fast for several hours before the test. If you're getting a waste product called creatinine tested (as part of a comprehensive or basic metabolic panel), you may be asked to fast for up to 12 hours before the test.
• Liver function tests: The purpose of liver function tests is to check how well your liver is working by measuring several substances made by your liver. Expect to fast for 10 to 12 hours before the test, and tell your healthcare provider about all medicines you're taking because some can affect the test results.
• Iron level test: Iron is important for making the protein hemoglobin, which carries oxygen in the blood to all parts of the body. Iron is also important for healthy growth and development. Iron tests usually require you to be fasting for 12 hours beforehand.
• Vitamin B12 test: You may need a vitamin B12 test if you're experiencing fatigue, irregular heartbeats (arrhythmia), unexplained weight loss, or tingly or numb hands and feet. For this test, you'll be asked to come in fasting for several hours.

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 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.

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 tests results.

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.

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.