Lipids are water insoluble molecules and must be transported in the plasma as macromolecular complexes containing protein called lipoproteins. The lipoproteins are large aggregates (micelles) composed of cholesterol, triglycerides, phospholipids, proteins, and small amounts of carbohydrates. Generally, the core of the lipoprotein contains hydrophobic lipids surrounded by a sheath of protein and lipids arranged with the hydrophilic ends directed outward. Carbohydrates and enzymes are present in the outer sheath. The proteins that become part of the lipoprotein are called apoproteins. Lipoproteins are grouped into four main classes depending upon their density. In order from least to greatest density these are: chylomicrons, very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL).
Lipoproteins are measured to classify persons with severe hyperlipoproteinemia or hypolipoproteinemia. The hyperlipoproteinemias result from increased production or decreased clearance of lipoproteins from the blood and may be inherited or secondary to other diseases or conditions. Some common causes of secondary hyperlipoproteinemia include diabetes mellitus, hypothyroidism, biliary cirrhosis, estrogens and pancreatitis. The cholesterol content of the LDL (LDL cholesterol) and HDL (HDL cholesterol) are measured, along with total cholesterol and triglycerides, to evaluate the patient's risk for coronary heart disease. In addition, a subclass of LDL called lipoprotein(a) or Lp(a) may be measured in persons who have a family history of coronary artery disease (CAD) or predisposing risk factors for CAD.
Measurement of lipoproteins may be performed by ultracentrifugation of the plasma. When plasma is subjected to very high centrifugal force, the lipoproteins can be separated in a gradient salt solution on the basis of their density. Since the density is directly related to protein content, the lipoproteins can also be separated by electrophoresis.
Electrophoresis is the separation of charged particles in an electrical field and is dependent on the amount and nature of the apoproteins within the lipoprotein. The electrophoretic positions of the lipoproteins are often used to describe them. Thus, HDL is also called alpha-1 lipoprotein, VLDL is called prebeta lipoprotein, and LDL is called beta lipoprotein. Chylomicrons do not migrate and are not given any designation. When one or more plasma lipid levels are extremely elevated or reduced, either of these methods may be used to determine which lipoproteins are abnormal. On the basis of these findings, abnormal lipoproteins are classified into patterns. Since severe disorders of lipoprotein metabolism are often inherited, the abnormal patterns are called phenotypes. There are five abnormal lipoprotein phenotypes (Type I through Type V), each characterized by the presence of an extremely high quantity of one or two lipoproteins. Persons with severe hyperlipoproteinemia often have skin and tissue infiltration of fat deposits, and persons with Type II and Type III are predisposed to premature atherosclerosis owing to high levels of plasma cholesterol. Lipoprotein phenotyping is not performed as a screening test to evaluate risk of coronary artery disease.
Immunological methods are used to measure the quantity of specific apoproteins present in the plasma. Testing for apoA-I and apoB-100 the principal apoproteins of HDL and LDL, respectively is often performed in persons with elevated lipids who have risk factors associated with coronary artery disease. Measurement of a form of LDL, called Lp(a) is performed on these persons as well as on those who have normal lipid levels, but a family history of CAD.
HDL cholesterol is routinely measured along with total cholesterol and triglycerides as a screening test for coronary artery disease. If the total cholesterol is 200 mg/dL or higher the LDL cholesterol is measured. The measurement of HDL cholesterol is based upon the measurement of cholesterol (see entry on lipid tests ) under conditions that inhibit the reaction with all lipoproteins except HDL. The measurement of LDL cholesterol involves precipitating the HDL, VLDL, and cholymicrons using antibodies to apoproteins A, and E, followed by measurement of the LDL cholesterol in the supernatant. When the triglyceride concentration is below 400 mg/dL, the LDL cholesterol is often estimated using the Friedewald formula [LDL cholesterol = total cholesterol minus (HDL cholesterol + triglyceride/5)]. It should be noted that this formula will underestimate LDL cholesterol when triglycerides are above 400 mg/dL.
Chylomicrons are made in the intestines mainly from dietary triglycerides. They are approximately 95% triglyceride and only 2% protein by weight. The major apoproteins of chylomicrons are apoC, B, and A. Chylomicrons are degraded in the plasma by the enzyme lipoprotein lipase, which splits the triglycerides into glycerides, and fatty acids, which are mainly absorbed by cells. Smaller chylomicron remnants are returned to the liver, where they are degraded by hepatic lipase. In the blood, some of the apoprotein A and C from chylomicrons are transferred to HDL. The chylomicrons are lighter than water and will float to the top of the plasma when it is stored overnight in the refrigerator. Since plasma from a fasting specimen should not contain chylomicrons, the observation of this floating layer is significant and indicates a deficiency of peripheral lipase activity. Chylomicrons are found in the fasting plasma of persons with Type I and Type V hyperlipoproteinemia.
Very low-density lipoproteins (VLDL)
VLDL are formed in the liver using apoproteins partly recycled from chylomicron remnants. VLDLs are about 10% protein and 60-70% triglycerides by weight; consequently they account for only 10-15% of the plasma cholesterol. The triglycerides carried by the VLDL are derived from carbohydrate metabolism. VLDL is released into the circulation, where it is partly degraded. Excessively elevated VLDL is responsible for Type IV hyperlipoproteinemia and is most often caused by hyperinsulinemia, which promotes triglyceride production. When both chylomicrons and VLDL are greatly increased, the abnormality is defined as Type V hyperlipoproteinemia.
Some free cholesterol, triglycerides, and apoproteins from VLDL are transferred to HDL in the circulation. This forms a lipoprotein of greater density and roughly equal cholesterol and triglyceride content called intermediate density lipoprotein (IDL). The IDL is converted to LDL by enzymatic removal of triglycerides and apoC. IDL is not found in significant amounts in the circulation unless there is a defect in conversion of VLDL to LDL. Such cases are caused by a deficiency of apoE-III or apoC-III activated lipase. This results in the accumulation of IDL in the plasma. This is responsible for Type III hyperlipoporteinemia.
Low-density lipoprotein (LDL)
The LDL is composed of about 25% protein and 45-55% cholesterol by weight. LDL carries cholesterol to the cells and is then degraded by lysosomal hydrolysis. Since LDL contains the majority of the plasma cholesterol and is responsible for cholesterol transport to cells, it is positively correlated with the risk of coronary artery disease. LDL accumulates in the plasma when there is a deficiency of the apoB-100 receptor on cells. This is responsible for the Type II hyperlipoprotienimia. Low levels of LDL occur in two inherited conditions. Abetalipoproteinemia results from a complete deficiency of apoB. This is an autosomal recessive condition associated with severe metabolic problems including intestinal malab-sorption, motor nerve dysfunction, fat soluble vitamin deficiency and anemia. Hypobetalipoproteinemia is an autosomal dominant condition in which LDL levels are about 10% of normal. This condition may be associated with fat soluble vitamin deficiency that is treated by vitamin supplementation and with a very low risk for coronary artery disease.
High-density lipoproteins (HDL)
HDL is approximately 50% protein by weight. Phospholipids account for 25-30% of its mass and cholesterol for 15-20%. HDL is made in the liver partly from VLDL and chylomicrons. It binds to and esterifies cellular cholesterol and transports it to the liver, where it is used to make bile salts and acids. HDL provides the main route for cellular cholesterol clearance and its level is inversely related to coronary artery disease. Absent or nearly absent HDL occurs in an autosomal recessive hypolipoproteinemia called Tangier disease. This is caused by a deficiency of both apoA-I and apoA-II, the principal lipoproteins of HDL. Persons with this disease develop premature CAD.
Lipoprotein a or Lp(a)
Lp(a) contains apoB bound to another apoprotein that is designated apo(a). Like LDL it is about 27% protein and 65% lipid by weight and has prebata mobility on electropohoresis. The amount of Lp(a) in plasma is normally below 150 mg/dL. Elevated levels are considered to be an independent risk factor for developing coronary artery disease. High levels are inherited as an autosomal dominant trait and are not influenced by diet or exercise. It is speculated that the link betweeen Lp(a) and atherosclerosis is related to the similarity between apo(a) and plasminogen. Plasminogen is the precuror of plasmin which initiates the lysis of blood clots.
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 which are listed below. 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 40 mg/dl), or other risk factors are present, a complete lipoprotein profile that includes total cholesterol, triglycerides, HDL cholesterol, and LDL cholesterol should be performed.
Initial screening for total cholesterol and HDL cholesterol may be performed on nonfasting persons. The tests require a blood specimen usually collected by venipuncture or fingerstick. The nurse or phlebotomist performing the test should observe universal precautions for the prevention of transmission of bloodborne pathogens. If results require follow-up testing, 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 test for lipo-protein electrophoresis requires a 12-hour fast and a blood sample collected in EDTA.
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.
In April 2001, the NIH released new NCEP guidelines to assist doctors and nurses in identifying who is at risk for CAD.
Total cholesterol guidelines are:
- desirable: <200 mg/dL
- borderline high: 200-239 mg/dL
- high: >240 mg/dL
High density lipoprotein guidelines reflect the fact that this lipoprotein is inversely related to risk for CAD.
LDL cholesterol guidelines are:
- low: <40 mg/dL
- desirable: <130 mg/dL (deciliter)
- optimal: <100 mg/dL
- near optimal: 100-129 mg/dL
- borderline high: 130-159 mg/dL
- high: 160-189 mg/dL
- very high: >190 mg/dL
The NCEP also identified factors that patients may have that make the risk of heart disease higher. Health care professionals are advised to help the patient lower their cholesterol as much as possible, if they have two or more of these risk factors:
- Cigarette smoking.
- High blood pressure, with a measurement of >140/90 mm Hg (millimeters of mercury). In addition, it is considered a risk factor if the patient is on blood pressure lowering medications, even if they have achieved a normal blood pressure.
- Age, over 45 years in men and over 55 years in women. Estrogen, a sex hormone in women protects against heart disease. The levels of estrogen are lower after a woman goes through menopause, roughly after the age of 55.
- Low HDL cholesterol (less than 35 mg/dL). Note that HDL > 60 mg/dL is a negative risk factor for CAD.
- Family history of premature heart disease. Premature heart disease is defined as heart disease seen before age 55 in a male relative or before age 65 in a female relative.
- Diabetes mellitus.
Some people have normal variations in their lipo-protein and total cholesterol levels. Repeat testing may be necessary, especially if a value is at a borderline risk category point.
Health care team roles
Lipoprotein testing is ordered by a physician. A nurse or phlebotomist usually collects the blood sample for the tests. Testing is most often performed by clinical laboratory scientists/medical technologists or clinical laboratory technicians/medical laboratory technicians. All clinicians should be well versed in the NCEP guidelines and treatment recommendations including both dietary and drug interventions. Patient's with high cholesterol levels may be requested to consult a dietician in order to evaluate their meal plans, and learn how to follow the Step 1 or 2 diets that may be needed to lower the LDL cholesterol.
Atherosclerosis— Disease of blood vessels caused by deposits of cholesterol, fats, lipoproteins, cells and calcium 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, hormones and bile acids. The body makes cholesterol and gets it from food.
Lipoproteins— The packages in which cholesterol and TAGs travel throughout the body.
Burtix, Carl A. and Edward R. Ashwood. Tietz Textbook of Clinical Chemistry, Third ed., Philadelphia, W.B. Saunders Company, 1999.
Rifai, Nader, G. Russell Warnick, and Marek H. Dominiczak. Handbook of Lipoprotein Testing. Washington, D.C.: American Association of Clinical Chemistry (AACC) Press, 1997.
American Heart Association. 7272 Greenville Avenue, Dallas, TX, 75231-4596. 214-706-1220. 〈http://www.americanheart.org/〉.
National Cholesterol Education Program. The National Heart, Lung, and Blood Institute. National Institutes of Health. PO Box 30105, Bethesda, MD, 20824-0105. 301-251-1222. 〈http://www.nhlbi.nih.gov/guidelines/cholesterol/atglance.pdf〉, May 2001.