Apolipoprotein A1 (ApoA1) is the primary structural protein of HDL particles. Each HDL particle contains ApoA1, which is essential for reverse cholesterol transport — collecting cholesterol from arterial walls and returning it to the liver for excretion.

ApoA1 directly represents the number and functional capacity of HDL particles, making it a more accurate measure of cardioprotective capacity than HDL cholesterol concentration alone. Counting actual HDL particles makes ApoA1 a stronger cardiovascular risk predictor than HDL-C in many patient groups, particularly those with metabolic syndrome.