Low Density Lipoprotein (LDL) cholesterol is a key factor in the pathogenesis of atherosclerosis and Coronary Artery Disease (CAD), while HDL cholesterol has often been observed to have a protective effect. Even within the normal range of total cholesterol concentrations, an increase in LDL cholesterol can produce an associated increased risk for CAD. LDL cholesterol binds to receptor sites on macrophages in blood vessel walls inciting several changes to the blood wall which enhance atherosclerotic plaque development.
Monitoring and maintaining healthy levels of lipids is important for staying healthy. Eating too much of foods that are high in saturated fats and trans unsaturated fats (trans fats) or having an inherited predisposition can result in a high level of cholesterol in the blood. The extra cholesterol may be deposited in plaques on the walls of blood vessels. Plaques can narrow or eventually block the opening of blood vessels, leading to hardening of the arteries (atherosclerosis) and increased risk of numerous health problems, including heart disease and stroke.
LDL-C is considered to be undesirable and is often called "bad" cholesterol because it deposits excess cholesterol in blood vessel walls and contributes to hardening of the arteries and heart disease. This is in contrast to high-density lipoproteins (HDL) that tend to transport cholesterol from the arteries to the liver. HDL is thought to protect against heart disease and so it is often called "good" cholesterol.
The LDL-C test can help determine an individual's risk of heart disease and help guide decisions about what treatment may be best if the person is at borderline or high risk. The results are considered along with other known risk factors of heart disease to develop a plan of treatment and follow up. Treatment options may involve lifestyle changes such as diet and exercise or lipid-lowering medications such as statins.
The results of a standard lipid profile, which consists of total cholesterol, HDL-C, and triglycerides, are usually used to calculate the amount of LDL-C in the blood. The results are entered into a formula that calculates the amount of cholesterol present in LDL (see below). In most cases, the formula provides a good estimate of the LDL-C, but it becomes less accurate with increased triglyceride levels when, for example, a person has not fasted before having blood drawn. In this situation, the only way to accurately determine LDL-C is to measure it directly. Direct measurement of LDL-C is less affected by triglycerides and can be used when an individual is not fasting or has significantly elevated triglycerides (above 400 mg/dL).
If a Direct LDL-cholesterol measurement is to be performed along with triglycerides, but not part of a lipid panel, then the patient should be fasting 9-12 hours prior to collection. If the Direct LDL cholesterol is ordered along with a lipid panel, then a fasting sample is not required.