The Test Sample
What is being tested?
Very low-density lipoprotein (VLDL) is one of the four major lipoprotein particles. The other three are high-density lipoprotein (HDL), low-density lipoprotein (LDL), and chylomicrons. Each particle contains a mixture of cholesterol, triglyceride, and protein, but in varying amounts unique to each type of particle. LDL contains the highest amount of cholesterol. HDL contains the highest amount of protein. VLDL and chylomicrons contain the highest amount of triglyceride.
VLDL particles are released into the blood by the liver and circulate in the bloodstream, ultimately being converted into LDL as they lose triglyceride, having carried it to other parts of the body. According to the National Heart, Lung and Blood Institute's National Cholesterol Education Program Guidelines ATP III, there is growing evidence that VLDL plays an important role in atherogenesis, in which plaques form on the interior walls of arteries, narrowing these passageways and restricting blood flow, which can lead to heart disease and increase the risk of stroke.
At present there is no simple, direct way of measuring VLDL cholesterol. However, since VLDL-C contains most of the circulating triglyceride (if a person is fasting) and since the composition of the different particles is relatively constant, it is possible to estimate the amount of VLDL-C based on the triglyceride value.
To estimate VLDL-C, divide the triglyceride value by 5 if the value is in mg/dL or divide by 2.2 if the value is in mmol/L. This estimate is used in most settings. The calculation is not valid, however, when the triglyceride level is greater than 400 mg/dl (4.5 mmol/L) because other lipoproteins are usually present. Triglycerides testing most often requires that the patient fast before sample collection; otherwise these calculations may be invalid.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. Sometimes a drop of blood is collected by puncturing the skin on a fingertip.
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?
Since VLDL-C is usually calculated from the triglyceride level, preparation is the same as for the triglyceride test. Current recommendations for this testing include fasting (having nothing to eat or drink except water) for 9 to 12 hours beforehand and avoiding alcohol consumption for at least 24 hours before the test. Follow any instructions you are given.