The guidelines for healthy cholesterol levels are set by the National Heart, Lung and Blood Institute (NHLBI). Recommended cholesterol levels depend on a person’s 10-year risk of a heart attack, according to his or her coronary risk profile. This risk level is determined by the presence of risk factors such as a family history of heart attack, smoking, obesity, lack of exercise and other measures. People at elevated risk should strive for lower cholesterol levels than people who are at low or moderate risk. However, the ultimate therapeutic goal depends on each physician’s judgment.

The recommended cholesterol levels for patients at low to moderate risk of a heart attack are as follows (in milligrams per deciliter [mg/dL]):

Labs outside of the United States may use different units of measure for cholesterol levels. To convert a cholesterol level from milligrams per deciliter (mg/dL) to international units (IU), multiply the mg/dL cholesterol level by 0.0259 millimoles per liter (mmol/L). For example, a cholesterol level of 200 mg/dL is equal to a cholesterol level of 5.18 IU.

Hypercholesterolemia, or high cholesterol, is diagnosed by measuring total cholesterol levels, as well as by separate HDL and LDL levels. The total cholesterol equals HDLs + LDLs + 1/5 triglycerides.

Directly measuring LDL cholesterol is difficult and expensive. However, LDL can be reliably calculated by directly subtracting HDL and triglycerides (which are easier to measure) from the total cholesterol, as long as triglycerides are under 400. However, this formula becomes unreliable when triglyceride levels are over 400, in which case LDL must be directly measured.

Another calculation gives the cholesterol ratio, which is the total cholesterol divided by the HDL level. According to the American Heart Association, the level of total cholesterol should not be more than five times the level of good cholesterol. This may be expressed as the ratio 5:1.

A ratio of 3.5:1 is considered optimal. The goal is to keep the ratio below 5:1. Anything over 5:1 indicates a possibly unhealthy balance of LDL cholesterol in the blood. Regardless of total cholesterol, experts generally recommend that the level of HDL cholesterol should be at least 40 mg/dL in men and at least 45 mg/dL in women.

Low total cholesterol (below 160) is not directly harmful to the human body but could indicate the presence of other medical conditions that may require attention. These medical conditions include: