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How Do Doctors Measure Serum Cholesterol?

By Kay Paddock
Updated May 17, 2024
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Cholesterol is a fatty compound that is produced by the liver and taken into the body, usually by eating meat and other animal products. Also known as lipids and blood lipids, serum cholesterol can be measured with simple blood tests. The levels are measured in milligrams per deciliter (mg/dL) of blood in the United States, and millimoles per liter (mmol/L) of blood in most other countries. Cholesterol tests measure overall cholesterol as well as the levels of HDL and LDL, which are the two basic types of cholesterol. These tests have become a standard for thorough checkups, as the level of serum cholesterol can indicate the potential for coronary artery disease, heart disease, strokes, diabetes and other largely preventable diseases.

Blood lipid levels are generally broken down to show the amounts of the two main types of cholesterol. High-density lipoprotein (HDL) is commonly known as the "good" cholesterol, while low-density lipoprotein (LDL) is labeled as the "bad" cholesterol. Both are necessary for good health, but higher HDL numbers are preferred while lower LDL numbers are believed to be better for heart health. Triglycerides are a type of blood fat that is also often measured in a blood lipid test.

HDL numbers that are around 40 mg/dL (less than 1 mmol/L) or below are considered poor, while numbers of about 60 mg/L (about 1.5 mmol/L) and higher are considered good. LDL numbers that range from about 100-129 mg/dL (2.6 - 3.3 mmol/L) are about average, but lower numbers are recommended for those with risk factors for heart disease or known heart problems. A level of 130 mg/L (about 3.4 mmol/L) or above is considered high. Both dietary changes and medication are often used to raise HDL and lower LDL. The best overall serum cholesterol level is generally considered to be 200 mg/L (about 5.2 mmol/L) and below, though many experts now believe that the proper levels of both HDL and LDL may be more important than the total overall number.

Serum cholesterol tests are usually most accurate after the patient has fasted for about 12 hours. This keeps digesting food from affecting the test and changing the result. Food, especially choices that contains fat or sugar, can skew the triglyceride results dramatically, so fasting and drinking only water is recommended in the hours leading up to a test. However, a doctor may give different instructions depending on the reasons for the test. Triglycerides are typically considered good if they measure at 150 mg/L (about 1.7 mmol/L) or below, and too high for anything above that.

Poor numbers generally indicate high amounts of blood fats and cholesterol that could contribute to clogged arteries, diabetes, heart disease, obesity and other problems. Doctors typically look at all the numbers in serum cholesterol tests to get an overall picture of health instead of singling out one number or bad result as a definite indication that something is wrong. Serum cholesterol levels can usually be brought into a healthy range by eating a nutritious diet; getting some exercise; and, if necessary, taking the proper medications.

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