Comparison of measurement uncertainties in direct plasma low-density lipoprotein cholesterol method of measurement and indirect estimation according to Friedewald equation

Concentration of low-density lipoprotein (LDL) cholesterol in plasma is appropriate to detect coronary heart disease risk. Many laboratories use the Friedewald equation for estimating this quantity instead of direct measurement. Nine clinical laboratories having an Hitachi selected 10 adult patients having results of cholesterol, high-density lipoprotein (HDL) cholesterol, LDL-cholesterol, and triglyceride concentrations in serum or plasma. Additionally, LDL-cholesterol concentrations were estimated using the Friedewald equation. Combined and expanded uncertainties were calculated for all results. The mean relative combined uncertainty corresponding to direct measurement of LDL-cholesterol concentrations is 6.9% and that corresponding to estimation is 19.4%. The expanded uncertainty ( k = 2) of a result equal to 3.40 mmol/L (the discriminant value universally used to detect cardiovascular risk) obtained by direct measurement and by estimation using the Friedewald equation, are 0.22 and 0.55 mmol/L, respectively. The uncertainty of the direct measurement is very much lower than the estimation with the Friedewald equation.