LDL particle number: what it adds to a cholesterol test

LDL cholesterol (LDL-C) measures the weight of cholesterol riding on LDL particles. LDL particle number measures how many such particles there are. The distinction matters because particles vary: Lab Tests Online UK explains that LDL and other lipoprotein molecules vary in size and density, and that people with a larger percentage of small dense LDL molecules are believed to be at greater risk of atherosclerosis than those with more large LDL.

Each particle can enter the artery wall, so in principle a higher count means more chances for cholesterol to be deposited, even if the total cholesterol riding on them looks acceptable.

Apolipoprotein B-100 is the protein wrapped around each VLDL, intermediate-density and LDL particle, so one molecule corresponds to one particle. Lab Tests Online UK says apo B concentrations tend to mirror LDL-C and that many experts think apo B may eventually prove a better indicator of cardiovascular risk than LDL-C, although its clinical use, like that of apo A-I, Lp(a) and hs-CRP, is not fully established.

The same source describes apo B as a non-standard test mostly limited to hospital specialists, used when someone has a personal or family history of heart disease or high blood lipids, especially with significantly raised triglycerides, which prevent accurate calculation of LDL cholesterol. It may also monitor treatment as an alternative to non-HDL cholesterol.

Particles can be separated by density using ultracentrifugation, by size and electrical charge using electrophoresis, and by other techniques. Lab Tests Online UK says the clinical usefulness of these tests is still to be proven. Such techniques are complex and are carried out mainly in specialist or research laboratories, which is why LDL-P is not a routine NHS test and why private providers sometimes sell it as part of advanced lipid panels.

For the same reason, interpretation depends on the method, and numbers from different laboratories are not interchangeable.

Specialists may consider apo B or particle testing when triglycerides are high, when there is a discordant picture such as a strong family history despite a modest LDL-C, in suspected inherited lipid disorders, or in people with metabolic syndrome and diabetes whose LDL-C understates risk. For most people the routine profile is enough to guide treatment.

The NHS notes that cholesterol treatment decisions are based on overall cardiovascular risk, which comes from age, sex, ethnicity, blood pressure, smoking and cholesterol measures, not one number.

A blood sample from a vein is needed. No special preparation is required for apo B, though your doctor might recommend fasting for other lipid tests on the same sample. Illness, surgery, pregnancy and sudden weight change can alter lipid values, so tests are best done when well.

Whatever the particle result, the practical steps are the same: lower overall risk with healthy eating, activity, stopping smoking, treating blood pressure and, where advised, taking statins or other lipid-lowering medicines. Chest pain or stroke symptoms are emergencies; call 999.