Why Do My LDL-C and HDL-C Numbers Not Add Up to My Total Cholesterol?

When getting lab results back, you may have noticed that the number that represents your total cholesterol differs from simply adding your HDL-C and LDL-C.

So, why don’t they add up?

This is a very good question, and it can be answered with a simple math equation:

TC = HDL-C + LDL-C + IDL-C + VLDL-C + Lp(a)-C

…Well, it’s sort of simple. Let’s see what those letters represent:

  • TC = Total cholesterol
  • HDL = High-density lipoprotein cholesterol content
  • LDL = Low-density lipoprotein cholesterol content
  • IDL = Intermediate-density lipoprotein cholesterol content
  • VLDL = Very low-density lipoprotein cholesterol content
  • Lp(a) = Lipoprotein(a) cholesterol content

What Are Lipoproteins?

Lipoproteins are protein-wrapped vehicles that transport lipids like cholesterol and triglycerides through the bloodstream and have a vital function in the body. Let’s see what role some lipoproteins play:

  • Chylomicron: These are the largest and least dense. Formed from dietary fat, produced and secreted by the intestine into the lymph nodes as triglyceride (fat)-rich particles. Chylomicrons deliver fats and cholesterol from the intestines to the muscles, fat cells and the liver. While important, these do not play a role in determining total cholesterol
  • VLDL: These particles transport fats and cholesterol from the liver to tissues
  • IDL: These particles transport endogenous fats and cholesterol from the liver to tissues
  • LDL: These particles primarily contain cholesterol. LDL contributes to plaque formation by entering the walls of the arteries. This leads to an inflammatory response and can result in atherosclerosis
  • HDL: These have the highest protein to lipid ratio and are the densest. They’ve been associated with carrying cholesterol back to the liver to be broken down and passed or reused, but recent research has revealed their functions may be a little more intricate

Lipoprotein particles are classified by their density (least to most). To measure total cholesterol, each type of particle present in a deciliter (dL) of blood plasma is broken up, and the cholesterol content inside each one is measured. For example: The number that represents HDL-C on a lab test is the cholesterol content of all the HDL particles in a dL of blood plasma.

At Absolute Health, we send your specimen to a lab that provides us with a direct (more accurate) measurement of LDL-C than standard lab testing. The lab also utilizes nuclear magnetic resonance (NMR) technology to quantify the different classifications of lipoproteins and measure the number and size of the lipoprotein particles.

Each particle has a different life span in the body (e.g., chylomicrons are present only minutes after eating). When you are fasting, the majority of particles circulating are HDLs and LDLs. However, IDLs and VLDLs—even though their numbers are higher after eating—can be present in very small amounts during fasting because your body is constantly making them. In some patients, Lp(a) is also present.

All of that being said, it is technically true that TC = HDL-C + LDL-C. However, when those amounts do not add up, it is most likely due to IDL-C, VLDL-C and/or Lp(a)-C, which may also be present in the blood plasma.

Yet, it should be strongly noted that particle number is our priority and not cholesterol content. Yes, the cholesterol content of particles has served as a surrogate to assess at-risk patients because that is what could only be measured in the past. But for many years now, we can measure particle concentration, which is statistically and significantly related to atherosclerosis and other cardiovascular events.

For more information or to schedule an appointment with one of our specialists, contact Absolute Health today.