The Test Sample
What is being tested?
Low-density lipoproteins (LDL) are particles that transport lipids throughout the body. Each particle contains a combination of protein, cholesterol, triglyceride, and phospholipid molecules. Their composition changes as they circulate in the blood. Some molecules are removed and others are added, resulting in lipoprotein particles whose properties vary from large and fluffy to small and dense. LDL particle testing determines the relative amounts of particles of differing properties. This is often called subfraction testing.
Traditional lipid testing measures the amount of LDL cholesterol (LDL-C) present in the blood, but it does not evaluate the number of particles of LDL (LDL-P). Some studies have shown that increased numbers of small dense LDL particles are more likely to cause atherosclerosis than fewer light, fluffy LDL particles. Researchers think that the presence of an increased number of small, dense LDL could be one of the reasons that some people have heart attacks even though their total and LDL cholesterol concentrations are not particularly high.
However, the data are not clear on whether routine testing for LDL subfractions provides additional information about a person's cardiac risk or whether results from such testing should affect decisions about treatment. More clinical research is needed to determine the ultimate value in testing for LDL subfractions and how the results should be used. Recommendations on the use of LDL subfraction testing and LDL-P continue to evolve. This is illustrated with:
- A recommendation from the 2009 guidelines on Emerging Biomarkers of Cardiovascular Disease and Stroke from the National Academy of Clinical Biochemistry that states "Lipoprotein subclasses, especially the number or concentration of small, dense LDL particles, have been shown to be related to the development of initial CHD events, but the data analyses of existing studies are generally not adequate to show added benefit over standard risk assessment for primary prevention." (See Sources)
- A 2013 Assessment by the AACC Lipoprotein and Vascular Diseases Division Working Group on Best Practices, which compared the use of Apolipoprotein B (Apo B) with LDL-P as indicators of atherogenic particle numbers. The group concluded that both tests were nearly equivalent in their ability to assess cardiovascular disease (CVD) risk and that both were stronger than LDL-C. The group supported the adoption of either Apo B or LDL-P into CVD risk screening and treatment guidelines but expressed a current preference for Apo B because of its availability and several other factors. (See Sources) Note: Apo B is considered a potential substitute for LDL-P because a molecule of Apo B is present in each particle of LDL and very low-density lipoprotein (VLDL).
The number of small, dense LDL particles (sdLDL) a person has is partially genetically determined, partially due to sex (males tend to have more sdLDL than females), and partially due to lifestyle and a person's general state of health. Certain diseases and conditions, such as diabetes and hypertension, are associated with increased levels of sdLDL.
A variety of methods are used to determine lipoprotein subfractions. These include ultracentrifugation (separation by density), polyacrylamide gradient gel electrophoresis (separation by charge and size), and NMR (nuclear magnetic resonance) spectroscopy, which determines relative quantities of particles of different compositions.
It is also usually possible to predict whether a person has a high number of sdLDL particles by looking at the person's triglyceride and high-density lipoprotein cholesterol (HDL-C) levels. These tests are typically performed as part of a lipid profile. People who have high triglyceride and low HDL-C tend to have more sdLDL. Specifically, having a triglyceride level above 120 mg/dL and an HDL-C level lower than 40 mg/dL in men and lower than 50 mg/dL in women is associated with having more sdLDL.
Subfraction testing is also available for other lipoprotein particles, such as HDL and VLDL, but these tests are mostly used in research settings and are not addressed in this article.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Current standards recommend that lipid testing be done when you are fasting. For 9 to 12 hours before the test, only water is permitted. Follow any instructions you are given.