High-density lipoproteins (HDL) are one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80-100 proteins per particle (organized by one, two or three ApoA; more as the particles enlarge picking up and carrying more fat molecules) and transporting up to hundreds of fat molecules per particle. Unlike the larger lipoprotein particles which deliver fat molecules to cells, HDL particles remove fat molecules from cells which need to export fat molecules. The fats carried include cholesterol, phospholipids, and triglycerides; amounts of each are quite variable.

Increasing concentrations of HDL particles are strongly associated with decreasing accumulation of atherosclerosis within the walls of arteries. This is important because atherosclerosis eventually results in sudden plaque ruptures, cardiovascular disease, stroke and other vascular diseases. HDL particles are sometimes referred to as “good cholesterol” because they can transport fat molecules out of artery walls, reduce macrophage accumulation, and thus help prevent or even regress atherosclerosis. However, studies have shown that HDL-lacking mice still have the ability to transport cholesterol to bile, suggesting that there are alternative mechanisms for cholesterol removal.

Guidelines published by the American Heart Association (AHA) and the National Cholesterol Education Program (NCEP), which is sponsored by the National Heart, Lung and Blood Institute (NHLBI), recommend that physicians determine HDL-C levels together with the other tests in a standard lipid profile prior to administering dietary or drug therapies for CHD. The NCEP guidelines state that patients with high cholesterol or borderline high cholesterol with risk factors (e.g. HDL-C less than 40 mg/dL, hypertension, smoking, family history, etc.) be tested 2-3 times for HDL-C within 1-8 weeks. Subsequently, HDL-C and other lipid parameters should be measured 3-4 times per year to monitor the progress of therapy. Because of its protective effect, the NCEP has designated high HDL-C at or above 60 mg/dL as a negative risk factor.