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LDL Cholesterol

A lipid panel is commonly ordered to assess the risk of coronary artery disease. This panel consists of measured total cholesterol, HDL cholesterol, and triglycerides and calculated LDL cholesterol. The latter is calculated using the Friedewald equation: LDL cholesterol = Total cholesterol - (HDL cholesterol + triglycerides/5).This equation assumes that the amount of cholesterol in very low density lipoproteins (VLDL) can be accurately estimated by dividing triglyceride concentration by a factor of five. In the majority of cases, this assumption is valid and calculated LDL cholesterol levels are accurate. However, the Friedewald equation does have limitations and should not be used in three situations: [1] when chylomicrons are present (e.g. nonfasting state and Type I &V hyperlipoproteinemias) , [2] when triglyceride concentrations exceed 400 mg/dL, and [3] when patients have dysbetalipoproteinemia (type III hyperlipoproteinemia, incidence of 1 in 1000 individuals).In the first two situations, LDL cholesterol will be underestimated and in the third situation it will be overestimated. LDL cholesterol levels calculated on nonfasting specimens may be 10 to 20% lower than fasting results.

Another option is to directly measure LDL cholesterol by the beta quant method, which involves ultracentrifugation and polyanion precipitation. Because of the increased expense of this and the limited reimbursement for LDL cholesterol, it is not feasible to screen all sera with this method. Initial screening should be done with calculated LDL cholesterol. Sera will be retained in the laboratory for two weeks. If triglycerides are greater than 400 mg/dL or Type I, III or Type V hyperlipoproteinemia is suspected, Direct LDL cholesterol can be ordered and performed on the original specimen.

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