Blood tests are highly sensitive tests that you doctor can order for a variety of reasons. To ensure accuracy, some blood tests require that you fast before testing. Prior to having a blood test, your doctor will provide you with specific instructions to follow.
Blood Tests That Require Fasting
Blood tests used to check your cholesterol levels, glucose levels and triglyceride levels typically require fasting. If you having a blood test to check for vitamin and mineral levels, fasting is also a likely requirement. The vitamin, minerals and sugars in the foods you consume can interfere for the accuracy of your blood test. A comprehensive metabolic panel, which tests for the presence of liver and kidney disease as well as diabetes requires fasting. High levels of proteins in your body can interfere with test results.
Blood Tests That Do Not Require Fasting
There are blood tests that do not require any special preparation. For example, you will not need to fast prior to undergoing a blood test that determines if you are pregnant. Fasting is also not likely required for a blood test that determines your blood counts -- such as a complete blood count, or CBC. The number of blood cells in your body stays the same regardless of whether or not you eat anything.
Fasting
If you are required to fast prior to your blood test, your doctor's office will provide you with specific instructions. Most cholesterol and triglyceride blood tests require that you fast for 12 hours prior to your test. If you are having your glucose levels or vitamin levels checked, you will need to fast for at least eight hours prior to your test.
Fasting Requirements
You are permitted to have water even when you fast for a blood test. Depending on the nature of your test, your doctor may also allow you to drink black coffee or tea prior to your test, otherwise you should not consume any other form of liquid besides water. If you are required to fast, you will not be permitted to have any food -- this includes gum with sugar content. Talk to your doctor about taking medications prior to your blood test.
Considerations
If you do eat when instructed not to eat, it can drastically change the results of your blood test. For example, you could have abnormal test results when they may have been normal if you fasted. When asked if you have had anything to eat prior to having your blood test, be honest with the individual administering your test. Failure to be honest may result in your having to have a second blood test.
References
- "Phlebotomy Handbook"; Diana Garza and Kathleen Becan-McBride; March 6, 2009
- "Phlebotomy Essentials"; Ruth E. McCall and Cathee M. Tankersley; February 14, 2011
