Start with what a standard panel reports. LDL cholesterol is the amount of cholesterol carried inside LDL particles. It weighs the cargo. It does not count the particles, and on most reports it is calculated from the rest of the panel, not measured, so a high triglyceride can throw it off.
ApoB is the count. Every particle that can carry cholesterol into an artery wall has one ApoB molecule on it, so measuring ApoB counts those particles. This explains why two people with the same LDL cholesterol can carry different risk. In large studies ApoB predicted heart disease better than LDL cholesterol, and risk fell as ApoB fell, with no floor found. Guidelines set goals by a person's overall risk. No trial has assigned healthy adults to an ApoB target, so how low to aim is a judgment to make with your doctor.
If ApoB has not been measured, there is a free stand-in. Non-HDL cholesterol is total cholesterol minus HDL cholesterol. It is on every standard panel and needs no fasting.
The second marker is lipoprotein(a), written Lp(a). It is an LDL-like particle with an extra protein attached. Its level is more than 90 percent inherited, it is reached in early childhood, and it stays put for life, so one measurement usually settles it. About one adult in five has a high level, and most do not know, because it is not on a standard panel.
Here is the honest limit. The evidence that high Lp(a) contributes to heart attack, stroke and narrowing of the aortic valve is strong. Diet and exercise barely move it, and the evidence that lowering it helps does not exist yet. A high result is not a verdict. It is a reason to take the rest of the picture more seriously, and because it is inherited, it is worth sharing with close relatives.