I measured 1,000 lipid NMR and 1,000 apo B and now just order apo B

I used to put substantial weight on LDL subfractions.

The theory was compelling: small, dense LDL particles were believed to be more dangerous than large, buoyant LDL particles because they may circulate longer, penetrate the arterial wall more readily, bind less effectively to the LDL receptor, and be more susceptible to oxidative modification.

Early observational studies supported this idea. Studies such as the Québec Cardiovascular Study, prospective studies of myocardial infarction, EPIC-Norfolk, and later NMR-based cohorts repeatedly found that people with smaller LDL particles or higher concentrations of small LDL had more cardiovascular events.

But there was a problem: small, dense LDL rarely occurs in isolation.

It commonly appears as part of the insulin-resistant dyslipidemic phenotype:

• Elevated triglycerides• Low HDL-C• Increased VLDL production• More cholesterol-depleted LDL particles• A higher total number of ApoB-containing particles

Once researchers adjusted for triglycerides, insulin resistance, ApoB, or total LDL particle number, the independent predictive power of LDL size frequently weakened or disappeared. Small LDL was often identifying a patient with more particles, not proving that each small particle was uniquely dangerous.

This distinction matters.

A person may have predominantly large, buoyant LDL and still have an excessive number of LDL particles. Large LDL is not harmless. Any ApoB-containing particle that enters and becomes retained in the arterial wall can contribute to atherosclerosis.

Conversely, when LDL particles are small and cholesterol-depleted, LDL-C may look deceptively acceptable because each particle carries less cholesterol. The patient can therefore have a “normal” LDL-C while still carrying a high number of atherogenic particles.

That is exactly where ApoB becomes so useful.

Every atherogenic particle—VLDL, IDL, LDL, remnant particles and Lp(a)—carries one ApoB molecule. Therefore, ApoB provides a practical estimate of the total number of circulating atherogenic particles.

LDL-C measures the cholesterol mass inside the particles.ApoB measures how many particles are present.

The arterial wall is exposed to particles, not simply to a floating mass of cholesterol.

This is also why ApoB and NMR LDL particle number usually follow one another closely. In a clinical comparison of ApoB and NMR LDL-P, the two measurements were highly correlated, although some discordance remained.

Large prospective NMR studies have confirmed that lipoprotein particle concentrations predict cardiovascular risk. However, the additional clinical value of separating LDL into numerous size categories is much less consistent once the total atherogenic particle burden is known. A 2023 analysis examining ApoB together with estimated LDL particle size could not consistently demonstrate that cholesterol-depleted, smaller LDL particles were substantially more atherogenic than cholesterol-rich particles after accounting for ApoB. The authors concluded that the results supported routine ApoB measurement.

More recent large-scale genetic and observational work has strengthened this conclusion. A 2025 analysis found that ApoB particle concentration was strongly associated with coronary disease, while particle type and size added comparatively limited information after the overall ApoB burden was considered.

Discordance studies may provide the clearest clinical evidence. When LDL-C, non-HDL-C and ApoB disagree, cardiovascular risk generally follows ApoB—the particle count—more closely than it follows the amount of cholesterol carried inside those particles. A 2024 UK Biobank analysis found ApoB to be a more accurate marker of cardiovascular risk than LDL-C or non-HDL-C despite very high correlations among all three measurements.

The European Society of Cardiology and European Atherosclerosis Society recognized this in their guidelines.

They explain that every ApoB-containing particle carries one ApoB molecule, making ApoB a direct estimate of the concentration of circulating atherogenic particles. They also note that ApoB testing is standardized, automated, inexpensive, does not require fasting, and has better analytical performance than calculated LDL-C.

The guidelines state that ApoB is the preferred measurement to further refine ASCVD risk, particularly when LDL-C may underestimate particle burden—for example in patients with:

• Elevated triglycerides• Diabetes• Obesity or metabolic syndrome• Very low achieved LDL-C• Suspected discordance between LDL-C and particle number

They estimate that meaningful discordance between LDL-C and ApoB occurs in approximately 20% of patients.

The European ApoB secondary treatment goals are:

• Very high risk: ApoB below 65 mg/dL• High risk: ApoB below 80 mg/dL• Moderate risk: ApoB below 100 mg/dL

To be precise, the European guidelines still retain LDL-C as the primary therapeutic target. ApoB is currently listed as a secondary target and the preferred marker for clarifying the true atherogenic particle burden when standard cholesterol measurements may be misleading.

After comparing more than 1,000 NMR panels with ApoB in clinical practice, this is exactly what I observed.

The NMR usually provided a more elaborate description of the same biological problem that ApoB captured with one inexpensive, standardized number. Small LDL often increased when ApoB increased. LDL particle number generally followed ApoB closely. And when the results were discordant, ApoB was usually the measurement that better represented the patient’s total atherogenic burden.

LDL subfractions are scientifically interesting and may occasionally provide additional metabolic context. But for routine cardiovascular risk assessment and monitoring therapy, ApoB is simpler, more reproducible, more clinically actionable and more directly connected to the causal number of particles capable of entering the arterial wall.

Do not ask only whether the LDL particles are large or small.Ask how many atherogenic particles are circulating.