Measurements of low density lipoprotein cholesterol (LDL-C) are helpful in assessing a patient's risk for heart disease and in following therapy to lower cholesterol. A standard lipid profile consists of total cholesterol, high-density lipoprotein cholesterol (HDL-C), and triglycerides. By applying a formula, the amount of cholesterol present in low-density lipoprotein can be determined, and this calculated value is typically reported as well. When triglycerides are high, the formula gives an incorrect result. In this situation, the only way to accurately determine LDL-C is to measure it directly.

High triglycerides may be due to a metabolic disorder affecting lipids, some medications  and other illness e.g. acute pancreatitis, especially after a fatty meal or alcohol. In these situations, the direct LDL-C test can determine the amount of LDL in a patient's blood.

Direct LDL-C may be specifically ordered when calculation of LDL cholesterol will not be accurate because the patient's triglycerides are significantly elevated. In some laboratories, this test will automatically be run on the same sample if the triglycerides are too high. This saves the doctor an extra step and speeds up the test turn-around time. Some laboratories may choose to use this method for all LDL-C testing.

Elevated levels of LDL, as measured with the direct LDL-C test, indicate a greater risk of developing heart disease. Decreasing levels show a response to lipid-lowering lifestyle changes and/or drug therapies and indicate a decreased risk of heart disease.

Low levels of LDL are not generally a concern and are not monitored. They may be seen in patients with an inherited lipoprotein deficiency and in patients with hyperthyroidism, infection and inflammation.

The direct LDL-C, like calculated LDL-C, should be measured when you are healthy and 'metabolically stable'. Illness, surgery, trauma, a heart attack, sudden weight loss or gain, and pregnancy can all temporarily affect LDL levels.

Calculated LDL-C is about as accurate as direct LDL-C when triglyceride levels are normal. It can be done at no additional cost to the client when a lipid profile is performed. Some laboratories choose to use this test for all LDL-C testing because of the high frequency of patients with elevated triglycerides in the population they service.

Yes, LDL particles vary in size and density. People with high levels of small but dense LDL particles are believed to be at greater risk for atherosclerosis, than those with high levels of large LDL particles.

However, particle size is not routinely tested for in Australia because experts now believe that the actual number of LDL particles in the bloodstream is more important than the particle size. LDL particle number can be assessed by another blood test, the apolipoprotein B test, which is readily available in Australia. Each LDL particle has only one apolipoprotein B molecule, so your apolipoprotein B level indicates the total number of LDL particles in your bloodstream. In addition, the apolipoprotein B level does give an indication of particle size - for example, if you have a relatively low LDL-cholesterol level but a high apolipoprotein B level, this suggests you have a lot of small LDL particles.