The primary protein component of lipoproteins like very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL, the bad cholesterol) is apolipoprotein B-100, commonly known as apolipoprotein B or apo B. It plays a role in the metabolism of lipids. The apo B level in the blood is determined by this test. Lipids are carried throughout the bloodstream by apolipoproteins, which bind to them. Apolipoproteins give lipoproteins structural stability and protect the water-repellent (hydrophobic) lipids at their core from external contaminants. The majority of lipoproteins are triglyceride- or cholesterol-rich and transport lipids throughout the body for cellular absorption.