The Apolipoprotein B (Apo B) test measures the level of apolipoprotein B in the blood. Apo B is a key protein found on the surface of atherogenic lipoproteins, including LDL (bad cholesterol), VLDL, IDL, and lipoprotein(a). Each of these particles contains one Apo B molecule, making Apo B a reliable indicator of the total number of cholesterol-carrying particles that can contribute to plaque formation in arteries.