Lipid tests are routinely performed on plasma, which is the liquid part of blood without the blood cells. Lipids themselves are a group of organic compounds that are greasy and cannot be dissolved in water, although they can be dissolved in alcohol. Lipid tests include measurements of total cholesterol, triglycerides, high-density lipoprotein (HDL) cholesterol, and low-density lipoprotein (LDL) cholesterol. Lipid tests may also be performed on amniotic fluid, which is the fluid that surrounds the fetus during pregnancy. Prenatal lipid tests include tests for lecithin and other pulmonary (lung) surfactants that cover the air spaces in the lungs with a thin film.
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 (CAD). High blood cholesterol may be inherited or result from such other conditions as biliary obstruction, diabetes mellitus, hypothyroidism, and nephrotic syndrome. In addition, cholesterol levels may be increased in persons who eat foods that are rich in saturated fats and cholesterol, and who lead a sedentary lifestyle.
Low levels of triglyceride are seen in persons with 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 triglyceride levels in some persons. Although triglycerides are not a component of the plaque associated with atherosclerosis, they increase the viscosity (thickness) of the blood and promote obesity, which can contribute to coronary disease. The majority of cholesterol and triglyceride testing is performed to screen persons at increased risk of coronary artery disease.
Amniotic fluid tests
Lipid tests are performed on amniotic fluid to determine the maturity of the fetal lungs. These tests are performed prior to delivery to ensure that there is sufficient pulmonary surfactant to prevent collapse of the lungs when the baby exhales (breathes out).
Cholesterol screening can be performed with or without fasting, but it should include tests of total and HDL cholesterol levels. The frequency of cholesterol testing depends on the patient's risk of developing CAD. Adults over 20 with total cholesterol levels below 200 mg/dL should be tested once every five years. People with higher levels should be tested for LDL cholesterol levels, and tested at least once per year thereafter if their 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
- history of cigarette smoking
- low HDL levels
- family history of CAD
- age over 45 years (men) or 55 years (women)
Measurements of cholesterol and triglyceride levels are routinely performed in all patients.
Measurement of pulmonary surfactants
Lecithin is the principal pulmonary surfactant secreted by the alveolar cells of the lung. Lecithin and the other surfactants prevent collapse of the air sacs when the baby exhales. During the first half of gestation, the levels of lecithin and another lipid known as sphingomyelin in the amniotic fluid are approximately equal. During the second half of pregnancy, however, lecithin production increases while the sphingomyelin level remains constant. Infants born prematurely may suffer from respiratory distress syndrome (RDS) because the levels of pulmonary surfactant in their lungs 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 ratio between lecithin and sphingomyelin in the amniotic fluid, or the L/S ratio.
Tests for triglycerides and LDL cholesterol must be performed following a 12-hour fast. Acute illness, high fever, starvation, or recent surgery lowers the 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 after the 30th week of gestation to evaluate the maturity of the baby's lungs. A miscarriage (spontaneous abortion) may occur as a consequence of this procedure, although its overall incidence following amniocentesis is less than 1%. Possible complications of amniocentesis include premature labor and placental bleeding. The fluid that is withdrawn may be contaminated with blood or meconium (a dark-green material in the intestines of a fetus), which may interfere with some fetal lung maturity tests.
Patients who are scheduled for a lipid profile test should fast (except for water) for 12 to 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 drugs include corticosteroids ; estrogen or androgens; oral contraceptives; some diuretics ; antipsychotic medications, including haloperidol; some antibiotics ; and niacin. Antilipemics are drugs that lower the concentration of fatty substances in the blood. When these medications are taken by the patient, blood testing may be done frequently to evaluate liver function as well as lipid levels.
Aftercare following 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 can resume taking any prescription medications that were discontinued before the test.
Care after amniocentesis requires that the clinician monitor 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 avoid strenuous activity. If the mother appears to be going into labor, she should be given supportive care. She may be given medications known as tocolytic agents to prevent the premature birth of the baby.
The primary risk to the patient from blood tests of lipid levels is a mild stinging or burning sensation during the venipuncture, with minor swelling or bruising afterward.
Although amniocentesis is much safer in the third trimester, and is less risky when it is performed with the guidance of ultrasound technology, does present a risk of miscarriage and fetal injury. The mother should be monitored for any signs of bleeding, infection, or impending labor.
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 and 29 years is approximately 30–63 mg/dL; 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 to 29 years is 45–200 mg/dL; for females of the same age group it is 37–144 mg/dL. As with cholesterol, the normal range rises with age.
Since a person's 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. The 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 considered 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 any value over 190 mg/dL.
- Total cholesterol: HDL ratio: Under 4.0 in males; 3.8 in females.
Fetal lung maturity tests
Low levels of surfactant in amniotic fluid are denoted by an L/S ratio lower than 2.0 or a lecithin level lower than or equal to 0.10 mg/dL. Lung development can be delayed in premature births and in babies whose mothers have diabetes.
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; the more information they are given, the better choices they can make.
Henry, J. B. Clinical Diagnosis and Management by Laboratory Methods , 20th ed. Philadelphia, PA: W. B. Saunders Company, 2001.
"Hyperlipidemia." Section 2, Chapter 15 in The Merck Manual of Diagnosis and Therapy , edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
"Prenatal Diagnostic Techniques: Amniocentesis." Section 18, Chapter 247 in The Merck Manual of Diagnosis and Therapy , edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Wallach, Jacques. Interpretation of Diagnostic Tests , 7th ed. Philadelphia, PA: Lippincott Williams & Wilkens, 2000.
American Dietetic Association. (800) 877-1600. http://www.eatright.org. .
National Cholesterol Education Program. National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health. PO Box 30105, Bethesda, MD, 20824-0105. (301) 251-1222. May 2001 [cited April 4, 2003]. http://www.nhlbi.nih.gov/guidelines/cholesterol/atglance.pdf .
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Jane E. Phillips
Mark A. Best