Apolipoprotein B (ApoB) is the primary structural protein of all atherogenic lipoprotein particles — LDL, VLDL, IDL, and Lp(a) each contain exactly one ApoB molecule. This makes ApoB measurement equivalent to counting the total number of atherogenic particles in the blood, regardless of how much cholesterol each particle carries.

ApoB is increasingly recognised as a superior marker of cardiovascular risk compared to LDL cholesterol. Because each ApoB-containing particle can cause arterial damage regardless of size, particle count (ApoB) is more predictive than the cholesterol mass (LDL-C). This is why some individuals with 'normal' LDL cholesterol still develop heart attacks — they may have a high number of small, cholesterol-depleted ApoB particles that standard tests undercount.