In short
ApoB (apolipoprotein B) is the main structural protein present on the surface of every LDL, VLDL, IDL, and lipoprotein(a) particle — the lipoproteins capable of depositing cholesterol into artery walls — with exactly one ApoB molecule per particle.
Because each atherogenic lipoprotein particle carries exactly one ApoB molecule, measuring ApoB directly counts the number of these particles circulating in the blood, rather than estimating the total cholesterol they carry, which is what standard LDL cholesterol measurements do. This distinction matters because particle number and cholesterol content don't always move together — a person can have a normal LDL cholesterol level but an elevated number of small, cholesterol-depleted, more atherogenic particles, a pattern common in insulin resistance and metabolic syndrome.
A substantial body of evidence, including a widely cited 2019 consensus statement from the European Atherosclerosis Society and analyses of major statin and PCSK9-inhibitor trials, supports ApoB (or particle number generally) as at least as strong a predictor of cardiovascular risk as LDL cholesterol, and a better predictor in populations with discordance between the two measures.
ApoB is a standard, inexpensive blood test, not an experimental biomarker, and current lipid guidelines (including from the European Society of Cardiology) already list it as an optional or preferred risk-assessment tool alongside standard lipid panels, particularly for people with diabetes, obesity, or high triglycerides where LDL-cholesterol alone may understate risk. It is not yet universally ordered as a first-line test in every country's standard primary-care lipid panel, which is a testing-guideline gap rather than a doubt about the underlying biology.