LDL/ApoB ratio

LDL cholesterol measures how much cholesterol sits in your LDL particles, not how many particles there are. ApoB does the reverse: every LDL particle carries exactly one molecule of ApoB, and so do the VLDL and IDL particles LDL arises from, so your ApoB is a count of every particle that can carry cholesterol into the vessel wall. Divide the cholesterol by the number of particles and you are left with how much cholesterol each particle holds on average, and that is a measure of how large the particles are. That is the idea behind the ratio. Large, buoyant LDL particles are full of cholesterol and give a high result; small, dense particles carry little cholesterol each and give a low one. A result of 1.2, calculated on mg/dl, corresponds to a mean particle diameter of 25.5 nm, the boundary between the pattern with large particles and the pattern with small, dense ones (Tani et al., 2017). Below that boundary you have many particles for little cholesterol, and that is exactly the combination in which a tidy LDL number says less than it seems. Your report does not print the ratio, and Dutch labs report LDL in mmol/l and ApoB in g/l while the boundary is published on mg/dl. That is why the calculator on this page converts first: LDL times 38.67 and ApoB times 100. Divide the Dutch units straight and you get a number around 3 that has nothing to do with the boundary.

Small, dense LDL particles are the particles that most easily pass through the vessel wall, linger there longer and oxidise faster, and therefore weigh more than the same cholesterol in large particles. In the Quebec Cardiovascular Study the number of heart events in people with particles of 25.5 nm or smaller rose as LDL rose, while in people with particles of 26.0 nm or larger it did not (Tani et al., 2017). The size of the particles therefore helps decide what an LDL number means. The second reason is the case in which LDL and ApoB contradict each other. An LDL inside the band with an ApoB above it means many small particles, and then the ApoB weighs more than the LDL. The ratio turns that contradiction into one number you can follow over the years. It falls with high triglycerides, belly fat and insulin resistance, because the liver then makes the small, dense variant, and in a Japanese cohort the ratio was lower in people with type 2 diabetes and in people with narrowed coronary arteries (Tani et al., 2017). In a German cohort the inverse ratio, ApoB per LDL cholesterol, went with more cardiovascular deaths and with the amount of small, dense LDL particles (Silbernagel et al., 2022). The honest limitation is that the ratio is an estimate and not a measurement of the particles themselves. The LDL in the numerator is, at most labs, a calculation from total cholesterol, HDL and triglycerides that comes out too low with high triglycerides and after a meal, and then the ratio falls along without your particles changing. That is why you always read it fasting, beside the ApoB and the LDL themselves, and beside the triglycerides.

A low ratio, below 1.2, means your LDL particles are on average small and dense: many particles for little cholesterol. The most common reason is that you were not drawn fasting, because the calculated LDL then comes out too low. Structurally it is the pattern of high triglycerides, belly fat and insulin resistance, in which the liver makes the small, dense variant, often years before the fasting glucose moves. A low LDL with a normal ApoB gives the same result, and then the ApoB is the value that counts. A high ratio means large, buoyant particles, and that is the favourable side: there is no upper limit that points to a condition. What does hold is that a high LDL with a high ratio is still a high LDL; the ratio says something about the shape of the particles, not about the amount. For the risk estimate Dutch doctors work with LDL, non-HDL and ApoB, and the ratio sits beside those, not in their place. What you do about a low ratio is what you do about high triglycerides: less sugar, soft drinks and alcohol, because the liver turns those into triglycerides and small particles, more fibre and unsaturated fat, vigorous exercise and losing weight if you carry belly fat. The ratio then moves on two sides: the triglycerides fall and the particles grow. Count on six to twelve weeks and then have blood drawn fasting, at the same lab as last time. The ratio belongs beside the ApoB, which counts the particles, beside the LDL itself, beside the triglycerides, which drive the small, dense variant, and beside non-HDL, the measure the risk estimate works on.