Chances are very good your doctor will ask you to fast before you have a cholesterol test. Technically, you do not need to fast if you are only being tested for total cholesterol, but you will need to fast for at least 12 hours before having your blood drawn if you are having a test that measures cholesterol fractions (HDL and LDL cholesterol) or if the blood test order includes tests for other substances. Ask your doctor or nurse if you are uncertain. Generally, it is best to fast before a blood test if you are uncertain or unless given other instructions.
Other Restrictions
You should not drink any alcoholic beverages for 24 hours before having your blood drawn for cholesterol fraction tests. Avoid fatty foods such as milk shakes, peanut butter, steaks, french fries and other deep fried foods the night before the test. Avoid exercise right before the test. Ask your doctor or nurse whether you should take your medications, vitamins or supplements before the test. If you are having a venipuncture, the usual blood drawing process, instead of a finger stick, avoid strenuous exercise or heavy lifting for a day after the blood draw or you may bruise severely at the site of the venipuncture.
What is Cholesterol?
Cholesterol is a type of fat your body needs to make skin, membranes, hormones, vitamins and digestive aids. Cholesterol is very insoluble in water. Skin and membranes have a high cholesterol content. It helps make your skin watertight. Your body uses cholesterol as a building block for vitamin D, hormones and bile salts which your digestive system uses to dissolve fats. Cholesterol and other fats are also called lipids.
Where Does Cholesterol Come From?
Cholesterol comes from two major sources. Animal fats are the major dietary source of cholesterol. Your body also makes cholesterol. Reducing the cholesterol in your diet can lower blood cholesterol. Some people's bodies make too much cholesterol. This is usually hereditary and is called familial hypercholesterolemia. Reducing the cholesterol in the diet may not be enough to lower blood cholesterol to healthy levels for people with familial hypercholesterolemia.
What are Normal Cholesterol Levels?
Doctors recommend that total fasting cholesterol levels be below 200 mg per deciliter, abbreviated mg/dL, with the lower end of normal being 120 mg/dL. Cholesterol is further divided into HDL or high density lipoprotein cholesterol and LDL or low density lipoprotein cholesterol. LDL cholesterol, sometimes referred to as "bad" cholesterol, is cholesterol coming in to be processed. LDL cholesterol should be below 100 mg/dL. HDL cholesterol, sometimes called "good" cholesterol, is cholesterol on its way out of the body. Normal for HDL is 40 to 60 mg/dL. Triglycerides are usually measured along with cholesterol. LDL levels are generally estimated from the total cholesterol and triglycerides using the Friedwald equation, which is:
LDL cholesterol = total cholesterol - HDL cholesterol / 5.
The Friedwald equation only works for triglycerides below 200 mg/dL.
Why Do I Need to Fast?
Triglyceride levels increase after eating, affecting the LDL results. You need at least 12 hours after eating for the fat levels to return to equilibrium values before you give blood for a cholesterol fractions test. Alcohol also affects triglycerides levels. Measurements of the cholesterol fractions HDL and LDL provide a more complete picture of health than total cholesterol alone. Fasting is important to obtain accurate results.
