Lipid Tests Health Article

Definition

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.

Blood tests

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).

Description

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
• diabetes
• 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.

Precautions

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.