The primary protein in high-density lipoprotein (HDL, the good cholesterol) is a protein called apoprotein A-I (apo A-I), which has specialised roles in the transport and metabolism of lipids. The apo A-I level in the blood is determined by this test. Lipids are like oil that floats on water; they cannot disintegrate on their own in the blood. In order to create the lipoprotein particles that can carry lipids throughout the bloodstream, apolipoproteins must be combined with lipids. Apolipoproteins give lipoproteins structural stability and protect the water-repellent (hydrophobic) lipids at their core from external contaminants.