Most annual checkups at a physician's office include blood work, complete with cholesterol measurements. While labs do measure total cholesterol, they also break the total cholesterol count into several subcategories, two of which are called HDL cholesterol and LDL cholesterol. Physicians monitor these two factors because their levels in the blood help doctors to evaluate a person's health status and to determine whether a person is at risk for cardiovascular disease.
Many people think that there are two kinds of cholesterol -- HDL, sometimes called good cholesterol, and LDL, which is sometimes called bad cholesterol. In fact, this is a misconception; there's only one kind of cholesterol. HDL and LDL, which are abbreviations for high-density lipoprotein and low-density lipoprotein, respectively, are actually cholesterol transporters. Both transporters are made from a combination of protein and fat, and that they carry identical molecules of cholesterol. In fact, the only differences between the two are where they originate and where in the body the cholesterol is deposited.
Both HDL and LDL carry cholesterol through the bloodstream, but they have very different effects in the body (and different ramifications for health), which is why both are measured to assess overall heart health. LDL carries cholesterol from the liver toward the body cells, HDL conversely, carries cholesterol away from the body cells and back toward the liver. From there, it is eliminated via the digestive tract. It's important for doctors to measure both HDL and LDL in order to know whether more cholesterol is being carried to or from the cells.
Too much LDL in the bloodstream can result in cholesterol plaques forming inside arteries. This results in atherosclerosis, or hardening of the arteries, and can lead to heart attack and stroke. Low levels of LDL cholesterol are associated with good cardiovascular health and a low risk of heart disease. Since HDL carries cholesterol away from cells and toward the liver, it helps combat the action of LDL. Doctors measure HDL to ensure that levels are high enough to promote good cardiovascular health.
The American Heart Association, or AHA, sets guidelines for healthy blood levels of HDL and LDL. Since these numbers both contribute to total cholesterol, they are independently more meaningful than the total cholesterol count--a high total cholesterol that is high in HDL and low in LDL is much healthier than a high total cholesterol that is low in HDL and high in LDL. According to the AHA, HDL should be at least 40 mg/dL, and optimally, higher than 60 mg/dL. LDL, on the other hand, should be lower than 100 mg/dL for most people, and even lower for individuals at risk for heart disease.
While a healthful diet that is low in cholesterol and saturated (animal) fat contributes to healthy HDL and LDL levels, there are some people for whom simply eating well is not enough--this is another reason that doctors measure both HDL and LDL. Individuals with a genetic disease called familial hypercholesterolemia tend to have very high LDL even if they eat optimally. For these individuals, physicians often recommend pharmaceutical intervention in order to reduce the risk of cardiovascular disease.