The measurement of ApoB in blood. Apolipoprotein B is a major protein constituent of chylomicrons (apo B-48), LDL (apo B-100) and VLDL (apo B-100). Apo B-100 functions as a recognition signal for the cellular binding and internalization of LDL particles by the apoB/E receptor, and is upregulated in response to enterovirus 71. The trait is Experimental Factor Ontology term EFO_0004615 (apolipoprotein B measurement).