The direct measurement of LDL is more expensive and requires specialized testing equipment; therefore, LDL is usually estimated from quantitative measurements of HDL, total cholesterol and plasma triglycerides using the Friedewald equation.

To examine differences in Friedewald-estimated and directly measured LDL values, researchers at Johns Hopkins(Martin SS, J Am Coll Cardiol, 2013;dol:10.1016/jacc.2013.01.079.) examined lipid profiles of more than 1.3 million US adults(mean age, 59 years; 52% women) after cholesterol vertical spin density gradient ultracentrifugation from 2009 to 2011.
Those data were compared with samples using the Friedewald equation.
Adult lipid distribution was comparable to National Health and Nutrition Examination Survey participants.

Overall, greater differences in Friedewald-estimated vs directly measured LDL occurred at lower LDL and higher triglyceride levels.
When Friedewald LDL level was <70 mg/dl, median direct LDL was 9 mg/dl higher(1.8 to 15.4) when triglycerides were 150 to 199 mg/dl and 18.4 higher(6.6 to 36) when triglycerides were 200 to 399.
When the triglyceride level is elevated over 150, the Friedewald estimate is too far off in too many patients.

This study should alert the physician to the fact that they need to get a directly measured LDL for patients with higher triglycerides.