LDL particle number (LDL-P)

A standard lipid panel measures the amount of cholesterol carried inside LDL particles (LDL cholesterol). LDL particle number instead counts how many LDL particles there are. This matters because it is the particles themselves — each capable of entering and lodging in the artery wall — that drive atherosclerosis, the furring of the arteries behind heart attacks and strokes. Usually the cholesterol amount and the particle number agree, but in some people they diverge: particularly in insulin resistance, type 2 diabetes, metabolic syndrome and high triglycerides, LDL particles become small and cholesterol-poor, so a person can have a 'normal-looking' LDL cholesterol yet a high particle number — and a higher risk than the cholesterol alone suggests. This 'discordance' is the reason the test exists. A closely related and more widely available marker, apolipoprotein B (apoB), measures essentially the same thing (one apoB per atherogenic particle) and is often preferred for this reason. LDL-P and apoB are used to refine cardiovascular risk assessment, especially when triglycerides are high or diabetes is present, so treatment can be matched to the true particle burden.

• A high burden of atherogenic particles — greater cardiovascular risk, sometimes higher than the LDL cholesterol alone suggests.
• Insulin resistance, type 2 diabetes or metabolic syndrome — commonly cause a high particle number with small, dense LDL.
• High triglycerides — often accompany a raised particle number.
• Discordance — a high LDL-P with an unremarkable LDL cholesterol is the key pattern the test detects.

• A low particle number is favourable — fewer atherogenic particles and lower cardiovascular risk.
• A good response to treatment — a falling particle number with lipid-lowering therapy.