the ultracentrifugal measurement of LDL is time consuming and expensive and requires specialist equipment. For this reason, LDL-cholesterol is most commonly estimated from quantitative measurements of total and HDL-cholesterol and plasma triglycerides (TG) using the empirical relationship of Friedewald et al.(1972).
[LDL-chol] = [Total chol] - [HDL-chol] - ([TG]/2.2) where all concentrations are given in mmol/L (note that if calculated using all concentrations in mg/dL then the equation is [LDL-chol] = [Total chol] - [HDL-chol] - ([TG]/5))
the quotient ([TG]/5) is used as an estimate of VLDL-cholesterol concentration. It assumes, first, that virtually all of the plasma TG is carried on VLDL, and second, that the TG:cholesterol ratio of VLDL is constant at about 5:1 (Friedewald et al. 1972). Neither assumption is strictly true.
Limitations of the Friedewald equation
The Friedewald equation should not be used under the following circumstances:
when chylomicrons are present
when plasma triglyceride concentration exceeds 400 mg/dL (4.52 mmol/L)
in patients with dysbetalipoproteinemia (type III hyperlipoproteinemia