Lipid tests

Definition

Lipid tests routinely performed on plasma include measurement of total cholesterol, triglycerides, high-density lipoprotein (HDL) cholesterol, and low-density lipoprotein (LDL) cholesterol. Lipid tests may also be performed on amniotic fluid and include tests for lecithin and other pulmonary surfactants.

Purpose

The purpose of blood lipid testing is to determine whether abnormally high or low concentrations of a specific lipid are present. Low levels of cholesterol are associated with liver failure and inherited disorders of cholesterol production. Cholesterol is a primary component of the plaques that form in atherosclerosis and is therefore the major risk factor for the rapid progression of coronary artery disease . High blood cholesterol may be inherited, or result from other conditions such as biliary obstruction, diabetes mellitus , hypothyroidism, and nephrotic syndrome. In addition, cholesterol may be increased in persons who have a diet rich in saturated fats and cholesterol and who lead a sedentary lifestyle. Low levels of triglyceride are seen in persons who have malnutrition or malabsorption. Increased levels are associated with diabetes mellitus, hypothyroidism, pancreatitis , glycogen storage diseases, and estrogens. Diets rich in either carbohydrates or fats may cause elevated triglycerides in some persons. Although not a component of the atherosclerotic plaque, triglycerides increase blood viscosity and promote obesity that can contribute to coronary disease. The majority of cholesterol and triglyceride testing is performed to screen persons for increased risk of coronary artery disease.

Lipid tests are performed on amniotic fluid to determine the maturity of the fetal lungs . Tests are performed prior to delivery to ensure that there is sufficient pulmonary surfactant to prevent collapse of the lungs during exhalation.

Description

Cholesterol screening can be performed with or without fasting and should include total and HDL cholesterol tests. The frequency of cholesterol testing depends on the patient's risk for CAD. Adults over 20 with total cholesterol levels below 200 mg/dL need to be tested once every five years. People with higher levels should be tested for LDL cholesterol and tested at least once per year thereafter, if the LDL cholesterol is 130 mg/dL or higher. The National Cholesterol Education Program (NCEP) suggests further evaluation when the patient has any of the symptoms of CAD or if she or he has two or more of the following risk factors for CAD:

• high blood pressure
• cigarette smoking
• diabetes
• low HDL levels
• family history of CAD
• age, men over 45 years and women over 55 years

Measurements of cholesterol and triglycerides are routinely performed using enzymatic methods. For cholesterol, the cholesterol oxidase method is used. Plasma or serum is mixed with a reagent containing cholesterol ester hydrolyase, cholesterol oxidase, peroxidase, and a chromogen. The cholesterol ester hydrolyase converts cholesterol esters (cholesterol coupled to a fatty acid) to free cholesterol. This reacts with cholesterol oxidase forming an oxidation product and hydrogen peroxide. The peroxidase enzyme catalyzes the oxidation of the chromogen by the hydrogen peroxide. This forms a red colored product that can be measured with a spectrophotometer. The amount of light absorbed at 500 nm is directly proportional to cholesterol concentration. HDL cholesterol is usually measured by the same reaction except that the enzymes are coupled to polyethylene glycol (PEG). In the presence of sulfated cyclodextrin, these enzymes will not react with the cholesterol in LDL, VLDL, or chylomicrons. LDL cholesterol is measured by first precipitating the other lipoproteins using a mixture of antibodies to apolipoprotein C and apolipoprotein E. The LDL cholesterol can be separated by centrifugation and then measured using the cholesterol oxidase reaction. Alternatively, LDL cholesterol can be calculated using the Friedewald formula. LDL cholesterol = total cholesterol minus (HDL cholesterol + triglyceride/5). This formula will underestimate LDL cholesterol when triglycerides are above 400 mg/dL.

Triglycerides are routinely measured using the glycerol kinase reaction. The reagent contains the enzymes lipase, glycerol kinase, glycerol phosphate oxidase, and peroxidase. It also contains adenosine triphosphate (ATP) and a chromogen. Triglycerides are composed of glycerol that is bound (esterified) to three long chain fatty acids. The lipase sequentially splits the fatty acids from the molecule forming glycerol and free fatty acids. The glycerol kinase catalyzes the transfer of phosphorus from ATP to the glycerol forming glycerol-phosphate. The glycerol phosphate oxidase is used to oxidize this to dihydroxyacetone phosphate. This reaction generates hydrogen peroxide. In the final step, the peroxidase enzyme catalyzes the oxidation of the chromogen by the hydrogen peroxide. This forms a red-colored product that can be measured with a spectrophotometer. The amount of light absorbed at 500 nm is directly proportional to triglyceride concentration. An important potential interfering substance in this reaction is glycerol, which is a common additive to many medications. If the Friedewald formula is used to calculate LDL cholesterol, the triglyceride measurement must be corrected by subtracting the plasma glycerol concentration from the triglyceride result.

Measurement of pulmonary surfactants

During the first half of gestation, lecithin and sphingomyelin levels in amniotic fluid are approximately equal. During the second half of pregnancy , lecithin production increases, but the sphingomyelin remains constant. Lecithin is the principal pulmonary surfactant secreted by the alveolar cells (type II granular pneumocytes) of the lung. Lecithin and the other surfactants prevent collapse of the air sacs during expiration. Infants born prematurely may suffer from respiratory distress syndrome (RDS) because levels of pulmonary surfactant are insufficient to prevent collapse of the air sacs. Tests for RDS are called fetal lung maturity (FLM) tests. The reference method for determining fetal lung maturity is the amniotic fluid L/S ratio. This is measured by thin layer chromatography in which lecithin and sphingomyelin in the amniotic fluid are separated and stained to determine their relative concentrations. An L/S ratio of 2:1 or higher is consistent with fetal lung maturity. Amniotic fluid levels of other surfactants such as phosphatidyl glycerol (PG), phosphatidyl glycerol, phosphatidyl inositol, and phosphatidyl ethanolamine may be measured by high performance liquid chromatography (HPLC). PG in amniotic fluid can be detected by a latex-coated antibody (latex agglutination) test. PG is an important marker for fetal lung maturity because a falsely positive test for lecithin may occur when the fluid is contaminated with blood or meconium. Since PG is not present in blood or meconium, and is only present when lecithin is adequate, a positive test is conclusive evidence of lung maturity. Measurement of lecithin which comprises about three quarters of the total surfactant composition at birth is most often measured by fluorescence polarization. This assay has replaced the L/S ratio as the FLM test in most labs. Most of the pulmonary surfactants are present in the form of lamellar bodies. These can be counted in the amniotic fluid using an electronic cell counter at the platelet threshold. The number of lamellar bodies is proportional to the quantity of surfactant.

KEY TERMS

Amniocentesis —A procedure to remove amniotic fluid from the womb using a fine needle.

Atherosclerosis —A disease of the coronary arteries in which cholesterol is deposited in plaques on the arterial walls. The plaque narrows or blocks blood flow to the heart. Atherosclerosis is sometimes called coronary artery disease, or CAD.

High-density lipoprotein (HDL) —A type of lipoprotein that protects against CAD by removing cholesterol deposits from arteries or preventing their formation.

Hypercholesterolemia —The presence of excessively high levels of cholesterol in the blood.

Hypertriglyceridemia —The presence of excessively high levels of TAG in the blood.

Lecithin —A phospholipid found in high concentrations in surfactant.

Lipid —Any organic compound that is greasy, insoluble in water, but soluble in alcohol. Fats, waxes, and oils are examples of lipids.

Lipoprotein —A complex molecule that consists of a protein membrane surrounding a core of lipids. Lipoproteins carry cholesterol and other lipids from the digestive tract to the liver and other body tissues. There are five major types of lipoproteins.

Low-density lipoprotein (LDL) —A type of lipoprotein that consists of about 50% cholesterol and is associated with an increased risk of CAD.

Plaque —An abnormal deposit of cholesterol, TAG, dead cells, lipoproteins and calcium on the wall of an artery.

Surfactant —A compound made of fats and proteins that is found in a thin film along the walls of the air sacs of the lungs. Surfactant keeps the surface pressure low so that the sacs can inflate easily and not collapse.

Tocolytic drug —A compound given to women to stop the progression of labor.

Triacylglyceride —A chemical compound that forms about 95% of the fats and oils stored in animal and vegetable cells. TAG levels are sometimes measured as well as cholesterol when a patient is screened for heart disease.

Precautions

Tests for triglycerides and LDL cholesterol must be performed following a 12-hour fast. The nurse or phlebotomist collecting the blood sample should observe universal precautions for the prevention of transmission of bloodborne pathogens. Acute illness, high fever , starvation, or recent surgery lowers blood cholesterol and triglyceride levels. If possible, patients should also stop taking any medications that may affect the accuracy of the test.

Amniotic fluid is collected by a process called amniocentesis . This procedure is usually performed between the after the 33rd week of gestation to evaluate lung maturity. Spontaneous abortion can occur as a consequence of this procedure. Its overall incidence following amniocentesis is approximately 1%. Complications include premature labor and placental bleeding. The fluid may be contaminated with blood or meconium (intestinal contents of the fetus), which can interfere with some fetal lung maturity tests.

Preparation

Patients who are scheduled for a lipid profile test should fast (except for water) for 12-14 hours before the blood sample is drawn. If the patient's LDL cholesterol is to be measured, he or she should also avoid alcohol for 24 hours before the test. When possible, patients should also stop taking any medications that may affect the accuracy of the test results. These include corticosteroids , estrogen or androgens, oral contraceptives, some diuretics, haloperidol, some antibiotics , and niacin . Antilipemics are drugs that lower the concentration of fatty substances in the blood. When these are taken by the patient, blood testing may be done frequently to evaluate liver function as well as lipid levels.

Aftercare

Aftercare with the blood lipid tests includes routine care of the skin around the needle puncture. Most patients have no aftereffects, but some may have a small bruise or swelling. A washcloth soaked in warm water usually relieves any discomfort. In addition, the patient should resume taking any prescription medications that were discontinued before the test.

Care after amniocentesis requires that the clinician watch the patient for any signs of infection or possible injury to the fetus. Some things to look for are fever, vaginal bleeding, or vaginal discharge. The patient may feel sick and there may be some cramping. She should be advised to rest and to avoid strenuous activity. If labor is impending, supportive care should be provided to the patient and tocolytic agents may be necessary to prevent the premature birth of the baby.

Complications

The primary risk to the patient with the lipid blood tests is a mild stinging or burning sensation during the venipuncture, with minor swelling or bruising afterward.

Amniocentesis, while much safer in the third trimester, and much safer now that it is done with the guidance of ultrasound technology does present a risk of miscarriage and fetal injury. The patient should be watched for any signs of bleeding, infection, or impending labor.

Results

The normal values for serum lipids depend on the patient's age, sex, and race. Normal values for people in Western countries are usually given as 140-220 mg/dL for total cholesterol in adults, although as many as 5% of the population have a total cholesterol higher than 300 mg/dL. Among Asians, the figures are about 20% lower. As a rule, both total and LDL cholesterol levels rise as people get older. Normal values for HDL cholesterol are also age and sex dependent. The range for males between 20-29 years is approximately 30-63 mg/dL and for females of the same age group it is 33-83 mg/dL. Normal values for fasting triglycerides are also age and sex dependent. The reference range for adult males 20-29 years is 45-200 mg/dL and for females of the same age group it is 37-144 mg/dL. As with cholesterol, the normal range increases with age.

Since diet and lifestyle affect normal values, which are determined by the interval between the 5th and 95th percentile of the group, it is more helpful to evaluate cholesterol and triglycerides from the perspective of desirable plasma levels. Desirable values defined by the Nation Cholesterol Education Program (NCEP) in 2001 are as follows:

• Total cholesterol: Less than 200 mg/dL; 200-239 mg/dL is borderline high and greater than 240 mg/dL is high.
• HDL cholesterol: Less than 40mg/dL is low.
• LDL cholesterol: Less than 100 mg/dL is optimal; near optimal is 100-129 mg/dL; borderline high is 130-159 mg/dL; high is 160-189 mg/dL; and very high is anything over 190 mg/dL.
• Total cholesterol: HDL ratio: Under 4.0 in males; 3.8 in females.

FLM tests

Low levels of surfactant in amniotic fluid are denoted by an L/S ratio less than 2.0 or a lecithin level less than or equal to 0.10 mg/dL. Lung development can be delayed in premature births and in babies whose mothers have diabetes.

Health care team roles

Physicians will order the blood lipid tests on patients who have risk factors for heart disease or who have not been tested within the past five years. A dietician may be consulted if test results are abnormal. FLM tests are ordered by a physician, usually an obstetrician. Lipid tests are performed by clinical laboratory scientists/medical technologists or clinical laboratory technicians/medical laboratory technicians.

Patient education

Nurses should explain the results of abnormal blood lipid tests to patients and advise them on lifestyle changes. Patient education is important in fetal lung maturity testing. The situation faced by the expectant parents may be very critical, and the more information they can be given, the better choices they can make.

Resources

BOOKS

Baron, Robert B. and Warren S. Browner. "Lipid Abnormalities." In Current Medical Diagnosis & Treatment 1998. edited by Lawrence M. Tierney et al. Stamford, CT: Appleton & Lange, 1997.

Kaplan, Lawrence A. and Amadeo J. Pesce. Clinical Chemistry, Theory, Analysis and Correlation. St. Louis: Mosby Publishers, 1996.

OTHER

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).

Jane E. Phillips