by Moe Zilla

Created on: August 06, 2009   Last Updated: October 18, 2009

Why is it important to fast before a blood test? That depends on the test! There's ways to test your cholesterol without fasting before a blood test - but unfortunately, the results won't be as accurate. And doctors may also order a blood test to check your glucose levels instead, where again, the fasting version of the test is far more accurate.

This makes it easier to understand why your doctor is requesting the fasting tests. If your cholesterol has tested high in the past, your doctor will want to monitor your cholesterol levels. This is especially important if you're taking a cholesterol medication - or if your family history places you at higher risk for heart disease. Unfortunately, there's several different kinds of cholesterol - good cholesterol (HDL), bad cholesterol (LDL) and very-low-density-lipoprotein (VLDL). And some cholesterol tests will be affected dramatically by what you've eaten before the test.

The measure of your total cholesterol isn't affected as much by your recent meals, according to Harvard Health Publications - and some versions of this test can even be performed at home! But doctors prefer to get a more detailed picture, which is why they perform tests which screen for all the components of your total cholesterol count. But even these tests don't measure your bad cholesterol directly. It's calculated by subtracting the good cholesterol and triglycerides from your total cholesterol level. Whatever's left is your bad cholesterol level!

And after a meal, your triglyceride level increases 20 to 30 percent, according to Harvard. So the extra triglycerides in your bloodstream would also get subtracted from the total cholesterol score - which would make bad cholesterol levels seem lower than they actually are. (And fasting isn't the only thing that affects the calculation. Psychological stress affects your HDL levels - and so does infection or injury!)

Blood tests are also ordered for diabetic patients, or to screen for diabetes. But in these cases, the doctor isn't testing the blood's cholesterol levels. Instead, a glucose test measures the amount of sugar in the bloodstream, to determine how the body is processing carbohydrates. The National Institute of Health notes that there's two kinds of glucose test, and only one of them requires fasting. A random glucose test can be performed at any time of day - even after a meal. But if the doctor wants to check your blood sugar levels after fasting, then you'll have to avoid eating before the test!

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