Lipids are transported in serum in the form of micelles, known as lipoproteins. Lipoproteins are complex macromolecules made up of protein, cholesterol and phospholipids as external structure and esters of cholesterol and triglycerides as internal structure. Lipoproteins are classified according to their increasing density: chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL).
Apolipoprotein A1 (Apo A1) is the protein found mostly in HDL. It is recognised that the assay of apolipoproteins is a better indicator than the assay of HDL and LDL-cholesterol in the assessment of the risk of coronary heart disease, and that it enables better discernment in the identification of a patient with atherosclerosis.