The cholesterol test is a way of measuring the cholesterol levels in a sampleof the patient's blood. Total serum cholesterol (TC) is the measurement routinely taken. Doctors sometimes order a complete lipoprotein profile to betterevaluate the risk for atherosclerosis (coronary artery disease, or CAD). Thefull lipoprotein profile also includes measurements of triglyceride levels (a chemical compound that forms 95% of the fats and oils stored in animal or vegetable cells) and lipoproteins (high density and low density). Blood fats are also called lipids.
The type of cholesterol in the blood is as important as the total quantity. Cholesterol is a fatty substance and cannot be dissolved in water. It must combine with a protein molecule called a lipoprotein in order to be transportedin the blood. There are five major types of lipoproteins in the human body; they differ in the amount of cholesterol that they carry in comparison to other fats and fatty acids, and in their functions in the body. Lipoproteins areclassified according to their density. Chylomicrons are normally found in theblood only after a person has eaten foods containing fats. They contain about 7% cholesterol. Very low-density lipoproteins (VLDLs) carry mostly triglycerides, but they also contain 16-22% cholesterol. Intermediate-density lipoproteins (IDLs) are short-lived lipoproteins containing about 30% cholesterol that are converted in the liver to low-density lipoproteins (LDLs).
LDL molecules carry cholesterol from the liver to other body tissues. They contain about 50% cholesterol. LDL particles are involved in the formation of plaques (abnormal deposits of cholesterol) in the walls of the coronary arteries. LDL is known as "bad cholesterol." High-density lipoproteins (HDLs) are made in the intestines and the liver. HDLs are about 50% protein and 19% cholesterol. They help to remove cholesterol from artery walls. Lifestyle changes,including exercising, keeping weight within recommended limits, and giving up smoking can increase the body's levels of HDL cholesterol. HDL is known as"good cholesterol."
Because of the difference in density and cholesterol content of lipoproteins,two patients with the same total cholesterol level can have very different lipid profiles and different risk for CAD. The critical factor is the level ofHDL cholesterol in the blood serum. Some doctors use the ratio of the totalcholesterol level to HDL cholesterol when assessing the patient's degree of risk. A low TC/HDL ratio is associated with a lower degree of risk.
The purpose of the TC test is to measure the levels of cholesterol in the patient's blood. The patient's cholesterol can also be fractionated (separated into different portions) in order to determine the TC/HDL ratio. The results help the doctor to assess the patient's risk for CAD. High LDL levels are associated with increased risk of CAD, whereas high HDL levels are associated with relatively lower risk.
In addition, the results of the cholesterol test can assist the doctor in evaluating the patient's metabolism of fat, or in diagnosing inflammation of thepancreas, liver disease, or disorders of the thyroid gland.
The frequency of cholesterol testing depends on the patient's degree of riskfor CAD. People with low cholesterol levels may need to be tested once everyfive years. People with high levels of blood cholesterol should be tested more frequently, according to their doctor's advice. The National Cholesterol Education Program offers guidelines for testing based on a person's risk factors.
The cholesterol test requires a sample of the patient's blood. Fasting beforethe test is required to get an accurate triglyceride and LDL level. The blood is withdrawn from one of the patient's veins. The blood test takes betweenthree and five minutes.
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 (milligrams per deciliter) in adults, although as many as 5% of the population has TC higher than 300 mg/dL. Among Asians, the figures are about 20% lower. As a rule, both TC and LDL levels rise as people get older.
Some doctors prefer to speak of "desired" rather than "normal" cholesterol values, on the grounds that "normal" refers to statistically average levels that may still be too high for good health.
It is possible for blood cholesterol levels to be too low as well as too high. TC levels less than 160 mg/dL are associated with higher mortality rates from cancer, liver disease, respiratory disorders, and injuries. The connectionbetween unusually low cholesterol and increased mortality is not clear, although some researchers think that the low level is a secondary sign of the underlying disease and not the cause of disease or death.
Low levels of serum cholesterol are also associated with malnutrition or hyperthyroidism. Further diagnostic testing may be necessary in order to locate the cause.
Prior to 1980, hypercholesterolemia (an abnormally high TC level) was definedas any value above the 95th percentile for the population. These figures ranged from 210 mg/dL in persons younger than 20 to more than 280 mg/dL in persons older than 60. It is now known, however, that TC levels over 200 mg/dL areassociated with significantly higher risk of CAD. Levels of 280 mg/dL or more are considered elevated. Treatment with diet and medication has proven to successfully lower risk of heart attack and stroke.
Elevated cholesterol levels may also result from hepatitis, blockage of the bile ducts, disorders of lipid metabolism, nephrotic syndrome, inflammation ofthe pancreas, or hypothyroidism.