ApoB, not LDL: why we changed the number we watch

LDL cholesterol measures the weight of cholesterol carried inside low-density lipoprotein particles. It has been the standard for decades because it was what laboratories could measure cheaply and reliably. It is a reasonable proxy for risk in most people, and a poor one in a substantial minority.

Apolipoprotein B measures something different: the number of atherogenic particles in circulation. Every LDL, VLDL, IDL and lipoprotein(a) particle carries exactly one ApoB molecule. Count the ApoB and you have counted the particles that can lodge in an arterial wall.

Why the distinction matters clinically

Consider two patients with an LDL cholesterol of 110 mg/dL. The first carries a modest number of large, cholesterol-rich particles. The second carries a much larger number of small, cholesterol-poor ones. Their LDL results are identical. Their particle counts, and their risk, are not.

This pattern of small dense particles clusters with insulin resistance, high triglycerides and low HDL, which is exactly the population in which a normal-looking LDL is most likely to be falsely reassuring. It is called discordance, and depending on the population studied it affects somewhere between one in five and one in three adults.

What we do with the number

We draw ApoB with the first fasting panel of a member's care and then at each review. The targets we work to are broadly these, though your physician will set yours based on your full risk picture:

• Under 90 mg/dL for adults at average risk
• Under 70 mg/dL where risk is elevated by family history, calcium score or diabetes
• Under 55 mg/dL after an established cardiovascular event

Lipoprotein(a), once, for everyone

While the blood is in the tube we also test lipoprotein(a). It is set genetically, it barely moves with diet or exercise, and roughly one in five people carries a level high enough to matter. Because it does not change, it is a once-in-a-lifetime test.

A high Lp(a) does not have a specific treatment in routine practice yet. What it does is change the aggressiveness with which we treat everything else, and it prompts screening of first-degree relatives, which is frequently how a family finds out.

What this does not mean

None of this makes a standard lipid panel useless. Triglycerides and HDL still carry information, and in someone with no metabolic risk factors and an unremarkable LDL, ApoB will usually tell the same story.

This article is general information and not medical advice. Whether these targets apply to you depends on your history. If you have chest pain, breathlessness or any symptom you think might be your heart, do not wait for a lipid panel. Call 911.