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Lipoproteins are the "packages" in which cholesterol and triglycerides travel throughout the body. Measuring the amount of cholesterol carried by each type of lipoprotein helps determine a person's risk for cardiovascular disease (disease that affects the heart and blood vessels, also called CVD).
Cholesterol and triglycerides are fat-like substances called lipids. Cholesterol is used to build cell membranes and hormones. The body makes cholesterol and gets it from food. Triglycerides provide a major source of energy to the body tissues. Both cholesterol and triglycerides are vital to body function, but an excess of either one, especially cholesterol, puts a person at risk of cardiovascular disease.
Because cholesterol and triglycerides can't dissolve in watery liquid, they must be transported by something that can dissolve in blood serum. Lipoproteins contain cholesterol and triglycerides at the core and an outer layer of protein, called apolipoprotein.
There are four major classes of lipoproteins: chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). There also are less commonly measured classes such as lipoprotein(a) and subtypes of the main classes. Each lipoprotein has characteristics that make the cholesterol it carries a greater or lesser risk. Measuring each type of lipoprotein helps determine a person's risk for cardiovascular disease more accurately than cholesterol measurement alone. When a person is discovered to be at risk, treatment by diet or medication can be started and his or her response to treatment monitored by repeated testing.
Chylomicrons are made in the intestines from the triglycerides in food. They contain very little cholesterol. Chylomicrons circulate in the blood, getting smaller as they deposit the triglycerides in fatty tissue. Twelve hours after a meal, they are gone from circulation. Serum collected from a person directly after eating will form a creamy layer on the top if left undisturbed and refrigerated overnight. This creamy layer is the chylomicrons.
Very low-density lipoproteins (vldl)
VLDL are formed in the liver by the combination of cholesterol, triglycerides formed from circulating fatty acids, and apolipoprotein. This lipoprotein particle is smaller than a chylomicron, and contains less triglyceride but more cholesterol (10-15% of a person's total cholesterol). As the VLDL circulates in the blood, triglycerides are deposited and the particle gets smaller, eventually becoming a low-density lipoprotein (LDL). Serum from a person with a large amount of VLDL will be cloudy.
Low-density lipoproteins (ldl)
LDL, often called "bad" cholesterol, is formed primarily by the breakdown of VLDL. LDL contains little triglycerides and a large amount of cholesterol (60-70% of a person's total cholesterol). Although the particles are much smaller than chylomicrons and VLDL, LDL particles can vary in size and chemical structure. These variations represent subclasses within the LDL class. Serum from a person with a large amount of LDL will be clear.
LDL carries cholesterol in the blood and deposits it in body tissues and in the walls of blood vessels, a condition known as atherosclerosis. The amount of LDL in a person's blood is directly related to his or her risk of cardiovascular disease. The higher the LDL level, the greater the risk. LDL is the lipoprotein class most used to trigger and monitor cholesterol lowering therapy.
High-density lipoproteins (hdl)
HDL is often called "good" cholesterol. HDL removes excess cholesterol from tissues and vessel walls and carries it to the liver, where it is removed from the blood and discarded. The amount of HDL in a person's blood is inversely related to his or her risk of cardiovascular disease. The lower the HDL level, the greater the risk; the higher the level, the lower the risk. The smallest lipoprotein, it contains 20-30% of a person's total cholesterol and can be separated into two major subclasses.
Lipoprotein(a) is found in lower concentrations than other lipoproteins, yet it carries a unique and significant risk for cardiovascular disease. Because of its similarity to LDL, test methods often don't measure it separately, but include it within the LDL class. Testing specifically for this class may uncover why a person is not responding to standard cholesterol-lowering treatment. High lipoprotein(a) levels may not respond to treatment aimed at high LDL.
The Expert Panel of the National Cholesterol Education Program (NCEP) sponsored by the National Institutes of Health has published guidelines for the detection of high cholesterol in adults. The NCEP panel recommends that adults over the age of 20 be tested for cholesterol and HDL every five years. If the cholesterol is high, the HDL is low (below 35 mg/dl), or other risk factors are present, a complete lipoprotein profile that includes total cholesterol, triglycerides, HDL, and calculated LDL should be done.
There are a variety of methods to measure the lipoprotein classes. All require separation of the classes before they can be measured. One way to separate them is by spinning serum (the yellow, watery liquid that separates from the cells when blood clots) for a long time in a high-speed centrifuge (called ultracentrifugation). The most dense classes will settle toward the bottom, the least dense toward the top. Following centrifugation, the most complete measurement of all the lipoprotein classes is done using electrophoresis. This procedure measures the quantity of each lipoprotein class based on its movement in an electrical field.
In 2003, a new test called the vertical auto profile or VAP, was developed that provides detailed measurements of cholesterol subclasses. These subclasses play important roles in patients later developing heart disease. The new tests were predicted to help identify important, emerging risk factors for heart disease.
Other, less extensive procedures also are used. For example, if only HDL is to be measured, a chemical is added to the serum that will clump the other classes, leaving HDL free in the serum to be measured by a chemical method. LDL often is not measured directly but its level is calculated based on the measurements of total cholesterol, HDL, and triglycerides. The formula is called the Friedewald formula: LDL = total cholesterol − HDL − (triglycerides/5). The calculated result will be inaccurate in a person with high triglycerides. Results usually are available the same or following day.
The patient must fast for 12 hours before the test, eating nothing and drinking only water. The person should not have alcohol for 24 hours before the test. There should be a stable diet and no illnesses occurring in the preceding two weeks.
A lipoproteins test requires 5 mL (milliliters) of blood. A person's physical position while having blood collected affects the results. Values from blood drawn while a person is sitting may be different from those while the person is standing. If repeated testing is done, the person should be in the same position each time.
Discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Warm packs to the puncture site relieve discomfort.
People with HDL levels between 45 mg/dl and 59 mg/dl carry an average risk for cardiovascular disease. People with HDL levels above 60 mg/dl have a negative risk factor and appear to be protected from cardiovascular disease.
LDL levels below 130 mg/dl are desirable.
Some people have normal variations in their lipoprotein and total cholesterol levels. Repeat testing may be necessary, especially if a value is at a borderline risk category point.
People with HDL levels 36-44 mg/dl have a moderate risk of cardiovascular disease. HDL levels below 35 mg/dl are a major risk.
LDL levels 130-159 mg/dl place a person at a borderline high risk of cardiovascular disease; levels above 160 mg/dl place a person at high risk. Relative proportions between HDL and LDL are important also. Results of a large clinical trial in 2003 showed that the new VAP cholesterol tests increased lipid-lowering therapy by 59% in high-risk patients with diabetes.
Atherosclerosis — Disease of blood vessels caused by deposits of cholesterol on the inside walls of the vessels.
Cardiovascular disease — Disease that affects the heart and blood vessels.
Cholesterol — A fat-like substance called a lipid. It is used to build cell membranes and hormones. The body makes cholesterol and gets it from food.
Lipoproteins — The packages in which cholesterol and triglycerides travel throughout the body.
"Doctors Laboratory to Offer VAP Expanded Cholesterol Test." Heart Diseases Weekly September 7, 2003: 35.
"Results of a Prospective, Multi-center Study Showed that the Availability of Lipoprotein Subclass Testing (Vertical Auto profile û VAP û Cholesterol Test) Increased Use of Lipid-lowering Therapy by 59% in High-risk Patients with Type 2 Diabetes." Diagnostics & Imaging Week June 19, 2003: 6.
American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. http://www.americanheart.org.
National Heart, Lung and Blood Institute. P.O. Box 30105, Bethesda, MD 20824-0105. (301) 251-1222. http://www.nhlbi.nih.gov.