It's known that high levels of blood cholesterol increase the risk of developing blocked arteries, which can lead to heart attack and stroke. Because of this, physicians routinely perform cholesterol tests, usually by drawing a sample of blood. In addition to measuring your total cholesterol level, the test usually shows how much of that reading is low density lipoprotein, or LDL, (L-D-L) and how much is high density lipoprotein, or HDL (H-D-L). LDL is considered the 'bad' cholesterol because it builds up over time, clogs the arteries that lead to the heart, and restricts the flow of blood. HDL is known as the 'good' cholesterol because it's thought to carry cholesterol out of the bloodstream. Prior to your test, you'll be instructed not to eat anything for about twelve hours before your visit. You'll also need to avoid all beverages except water. Cholesterol is measured in milligrams per deciliter (DESS-ih-lee-ter). A desirable cholesterol reading is below 200; moderate to high cholesterol is between 200 and 240; and very high cholesterol is over 240. Also important is the ratio of 'good' to 'bad' cholesterol. For more information about cholesterol testing, talk to a doctor.