A test to measure the amount of apolipoprotein B100 (apoB100) in the blood. Lipoproteins are large complexes of molecules that transport lipids (primarily triglycerides and cholesterols) through the blood. Apolipoproteins are proteins on the surface of the lipoprotein complex that bind to specific enzymes or transport proteins on the cell membranes; this directs the lipoprotein to the proper site of metabolism . ApoB100 is in IDL (an intermediate form of lipoprotein) and LDL ( low density lipoprotein ).