Friedewald equation

Indirect Methods. Indirect methods for measuring LDL cholesterol are based on measuring a number of lipid-related analytes followed by their use in calculating the LDL cholesterol content of a specimen. This includes the use of the Friedewald equation and the beta-quantification method. 

The Friedewald Equation, In the most widely used indirect method (Box 26-6), cholesterol, triglyceride, and HDL cholesterol are measured and LDL cholesterol is calculated from the primary measurements using the empirical equation of Friedewald and colleagues ... 

The opposite error can occur if the Friedewald equation is used in patients with type III hyperlipoproteinemia. Type III hyperlipoproteinemia is characterized in part by the presence of p-VLDL, not normally present in the blood. Biochemically, p-VLDL occurs in the VLDL, density range, but is much richer in cholesterol than normal VLDL with a ratio of triglyceride/cholesterol on the order of 3 1. Application of the factor [triglyceride]/5 in type Ills would underestimate VLDL cholesterol and in turn overestimate LDL cholesterol. Thus a patient with type III hyperlipoproteinemia may appear to have an artifactually high LDL cholesterol concentration. 

Density gradient ultracentrifugation is used to measure hpoprotein subclasses it is performed in a vertical rotor with measurement of cholesterol continuously in the fractions eluted from the gradient. Mathematical curve resolution derives the component lipoprotein profiles and allows calculation of their cholesterol concentrations. The method can determine concentrations of VLDL, IDL, LDL, Lp(a), and HDL cholesterol. LDL cholesterol subclasses can be expressed separately or combined to give a measurement similar to that provided by the Friedewald equation or beta-quantification. A disadvantage is that the procedure is technically demanding and requires instrumentation not usually available in clinical laboratories. 

Friedewald equation using [triglyceride/5] as an estimate of very low-density lipoprotein cholesterol concentration. 

Warnick GR, Knopp RH, Fitzpatrick V, Branson L. Estimating low-density lipoprotein cholesterol by the Friedewald equation is adequate for classifying patients on the basis of nationally recommended cut-points. Clin Chem 1990 36 15-19. 

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