Apolipoprotein A (Apo A) is the main protein in HDL cholesterol (good cholesterol). Apo A allows HDL to manage cholesterol and then allows it to be disposed of in the intestines through its transformation into biliary acid by the liver. This elimination through the intestines is the only mechanism for eliminating cholesterol from the human body. Levels of Apo A are expressed in grams per litre (g/L) and reference values are slightly higher in women (1.25 to 2.15 g/L) compared to men (1.10 to 2.05 g/L).
In general, levels of Apo A reflect levels of HDL cholesterol. The higher your HDL or Apo A, the better your protection from cardiovascular disease. However, there are different types of Apo A (Apo A-I, Apo A-II) and the protective effect of HDL may vary depending on the proportions of each subtype. An increase in Apo A-I is more closely associated with a reduction in the risk of a first heart attack or better statin protection than an increase in HDL cholesterol. Even when normal HDL levels are present, low Apo A-I levels better predict the presence of preclinical atherosclerosis (the onset of artery obstruction by cholesterol plaques).