Cholesterol is a lipid needed to form cell membranes and a component of the materials that render the skin waterproof. It also helps form bile salts, adrenal corticosteroids, estrogen, and androgens. Cholesterol is obtained from the diet (exogenous cholesterol) and also synthesized in the body (endogenous cholesterol). Although most body cells can form some cholesterol, it is produced mainly by the liver and intestinal mucosa. Cholesterol is an integral component in cell membrane maintenance and hormone production. Very low cholesterol values, as are sometimes seen in critically ill patients, can be as life-threatening as very high levels.

According to the National Cholesterol Education Program, maintaining cholesterol levels less than 200 mg/dL significantly reduces the risk of coronary heart disease. Beyond the total cholesterol and high-density lipoprotein cholesterol (HDLC) values, other important risk factors must be considered. The Framingham algorithm can assist in estimating the risk of developing coronary artery disease (CAD) within a 10-yr period. Many myocardial infarctions occur even in patients whose cholesterol levels are considered to be within acceptable limits or who are in a moderate-risk category. The combination of risk factors and lipid values helps identify individuals at risk so that appropriate interventions can be taken. If the cholesterol level is greater than 200 mg/dL, repeat testing after a 12- to 24-hr fast is recommended. Another predictor of CAD is lipoprotein (a) or Lp(a). Lp(a) is considered an independent risk factor for CAD and cerebral infarction at levels greater than 30 mg/dL.