Apolipoprotein B (ApoB) is a protein that is the main structural component of several types of cholesterol-carrying particles in the blood, including LDL (often called "bad" cholesterol), VLDL, and related particles. Because each of these particles contains one ApoB molecule, an ApoB measurement reflects the total number of these potentially artery-clogging particles in the blood, which can provide additional information beyond a standard cholesterol test. This test is used to help assess cardiovascular risk by measuring the number of atherogenic (artery-clogging) particles in the blood. It is sometimes ordered along with or instead of a standard lipid panel to give a more complete picture of heart disease risk, and it may be used to monitor how well lipid-lowering efforts are working. Results are interpreted by a healthcare provider in the context of other tests and individual risk factors.