A SCAI answered. 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.on-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.