ApoB (apolipoprotein B)

One protein per particle

Cholesterol and triglycerides can't travel through your blood on their own, so they ride inside particles called lipoproteins. LDL, VLDL, IDL, chylomicrons and lipoprotein(a) each carry exactly one apoB molecule. HDL, the so-called good cholesterol, doesn't carry apoB at all.

Because the ratio is one to one, an apoB result tells you how many potentially plaque-forming particles are in circulation. A standard LDL cholesterol number tells you how much cholesterol those particles hold, which isn't quite the same thing.

Two forms

There are two versions of the protein, both made from the same gene. ApoB-100 is produced in the liver and is the form on LDL and VLDL; it's also what lets LDL dock with receptors that clear it from the blood. ApoB-48 is made in the small intestine and is needed to package fat from food. Blood tests for apoB mainly reflect apoB-100.

Why clinicians order it

Many studies have found that apoB predicts cardiovascular risk at least as well as, and often better than, traditional cholesterol measures. It can be especially informative when the numbers on a standard panel don't line up, for example:

• High triglycerides, where LDL cholesterol can understate the number of particles.
• Insulin resistance, type 2 diabetes or metabolic syndrome, which often come with many small, cholesterol-poor particles.
• A family history of early heart disease despite an unremarkable LDL result.

Reading the number

The 2018 AHA/ACC cholesterol guideline treats a persistently elevated apoB of 130 mg/dL or higher as a 'risk-enhancing factor,' roughly equivalent to an LDL cholesterol of 160 mg/dL or more. Risk enhancers can tip the decision toward starting or intensifying treatment when your overall risk is borderline or intermediate.

There's no universally agreed treatment target for apoB yet, and a single result is interpreted alongside your blood pressure, smoking status, diabetes, kidney function, family history and age. ApoB can be measured without fasting.

If your apoB is high, the next steps are usually the familiar ones: a review of diet, activity, weight, blood pressure and smoking, and a conversation about whether a cholesterol-lowering medicine makes sense for your overall risk. Repeating the test after changes gives you a sense of direction.

ApoB in midlife

Cholesterol often rises in the years around menopause, and cardiovascular risk climbs with age in both men and women. If you're considering hormone therapy, testosterone or a weight-loss medication, a fuller picture of your lipids, sometimes including apoB and lipoprotein(a), can help you and your clinician make decisions with more information.

ApoB is one input, not a verdict. A slightly high value in someone with no other risk factors means something different from the same value in someone with diabetes and a strong family history.