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Why do I have to fast before a cholesterol blood test?

In order to accurately and consistently measure a cholesterol panel, fasting is required before a blood draw. When someone's cholesterol is measured, usually more than just the total cholesterol is checked. Cholesterol is broken down into "good" (HDL) and "bad" (LDL) cholesterol. A related substance, called triglycerides, is also measured. Of these, the triglycerides and the LDL can be affected by recent calorie intake.

Fasting for 9 to 12 hours before a cholesterol blood test ensures accuracy in the test. It helps make sure your body does not have extra sources of fat—say, from the ice cream you had for dessert—that could alter the test results.

Fasting before a blood test means you will not have any food or drink other than water. Tests that require fasting usually are called “lipid panels.” They measure levels of various kinds of fatty substances circulating naturally in your blood that are reliable indicators of heart disease, or risk for heart disease.

After you have a fasting cholesterol test, you will know your levels of total cholesterol, both the “good” (HDL) cholesterol and the “bad” (LDL) cholesterol, and triglycerides. Triglycerides are a form of blood fat found in arteries.

 

While some components of the Lipid Profile (Total Cholesterol and HDL Cholesterol) are not impacted by food, others, particularly the triglyceride level, can be. The LDL cholesterol is a value often obtained by a calculated method that relies on the accurate measurement of triglycerides. For this reason, a non-fasting sample can raise triglycerides and yield an inaccurate LDL cholesterol calculation. Non-HDL cholesterol, another calculated measure, does not rely on triglycerides, however, and can be done as a non-fasting sample. For these reasons, if one desires an accurate triglyceride and calculated LDL cholesterol level, it is ideal to fast for 8-12 hours prior to the test. However, if this is not the case, total cholesterol, HDL cholesterol and non-HDL cholesterol values may still be obtained accurately, and will be useful.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.