Lipid Analysis for Important Clinical Conditions

Abstract

Lipid analyses are primarily used for determining the risk of diseases resulting from atherosclerosis, including coronary heart disease (CHD), ischemic cerebrovascular disease, and peripheral arterial disease. These disorders are often marked by hyperlipidemia, with increased serum low‐density lipoprotein cholesterol (LDL‐C) and apolipoprotein (Apo) B, but decreased high‐density lipoprotein cholesterol (HDL‐C) and Apo A‐I. For use in risk assessment, serum cholesterol, LDL‐C and HDL‐C cut‐points have been derived from epidemiological data from several nations. The acceptance of these cut‐points, coupled with the remarkable heterogeneity of lipoproteins, has made standardization of cholesterol analytical procedures essential. An international network of laboratories (the Cholesterol Reference Method Laboratory Network (CRMLN)) coordinated by the US Centers for Disease Control (CDC) was formed to address this issue. Thus, several reference methods have been adopted and are used to standardize and certify the commercial reagents for lipid assays for use in the routine clinical laboratory. Other standardization efforts include those by the International Federation of Clinical Chemistry, World Health Organization (WHO) and CDC on Apos or lipoprotein(a) (Lp(a)). In addition to analytical standardization, it is important to standardize preanalytical procedures, and to be aware of potential biological variability. In the 1980s and 1990s, significant advances have been made in the procedures for routine analysis of lipids and lipoproteins. The analyses have become much more precise and efficient, frequently performed in automated analyzers using small sample volumes. A variety of enzymatic methods for cholesterol and triglyceride (TG) measurements are in current use. HDL‐C is generally determined after precipitation of low‐density lipoprotein (LDL) and very low‐density lipoprotein (VLDL). LDL‐C is calculated from the difference between total cholesterol (TC), and the sum of HDL‐C and a total TG‐derived estimate of the very low‐density lipoprotein cholesterol (VLDL‐C) (the Friedewald equation). Several direct methods for measurement of LDL‐C and HDL‐C have been introduced with improvement in performance and labor efficiency, Apo B which is associated with LDL in equimolar ratio and Apo A‐I which is associated with high‐density lipoprotein (HDL) may be measured in conjunction with LDL‐C and HDL‐C or used in their place for clinical assessment. Other important lipid measurements include the C and E Apos, Lp(a), TG‐rich remnant lipoproteins, lipoprotein particle size and markers of oxidative stress.