Apolipoprotein A-1 (Apo A-1) is the major protein component of high-density lipoprotein (HDL) cholesterol, comprising 60-70% of HDL protein content. This test measures Apo A-1 levels in the blood to evaluate cardiovascular risk and HDL function. Apo A-1 serves as an activator of lecithin-cholesterol acyltransferase (LCAT), which is essential for reverse cholesterol transport - the process of moving cholesterol from peripheral tissues back to the liver for elimination. Low Apo A-1 levels are associated with increased risk of coronary artery disease, myocardial infarction, and peripheral vascular disease. Apo A-1 measurements may provide better cardiovascular risk assessment than HDL cholesterol alone, as they directly measure the protein responsible for HDL's protective effects. This test is particularly valuable for evaluating patients with cardiovascular disease risk factors and monitoring response to lipid-lowering therapies.