The formation of a clot (or thrombus) in the region of this plaque can block the flow of blood to part of the heart muscle and cause a heart attack. If a clot blocks the flow of blood to part of the brain, the result is a stroke. A high level of LDL cholesterol reflects an increased risk of heart disease. That is why LDL cholesterol is often called "bad" cholesterol.

Your doctor can judge your risk of heart attack more accurately by determining the amount of cholesterol carried by your LDL's. If your LDL cholesterol is more than 160 milligrams per deciliter of blood (mg/dL), it's high risk if you have 2 or more risk factors or if you have heart disease. Ideally, your LDL cholesterol is below 130. Total blood cholesterol can indicate your level of risk. If it's 200 mg/dL or over, your doctor will probably measure your LDL cholesterol level, which is a more accurate indicator of heart disease. To learn more about cholesterol levels, click here to visit What Do Your Cholesterol Numbers Mean? in this section.

High-Density Lipoproteins (HDL's)
About one-third to one-fourth of blood cholesterol is carried by high density lipoprotein or HDL. Medical experts think HDL tends to carry cholesterol away from the arteries and back to the liver, where it's passed from the body. Some experts believe HDL helps remove excess cholesterol from atherosclerotic plaques and thus slows their growth. HDL is known as "good" cholesterol because a high level of HDL helps to protect against heart attack.

The opposite is also true: a low HDL level indicates a greater risk. Low HDL cholesterol levels can be caused by many factors. The most common are cigarette smoking, obesity and physical inactivity. Fortunately, all can be corrected, resulting in an increased HDL level and a reduced risk of heart attack.