Friedewald-estimated versus directly measured low-density lipoprotein cholesterol and treatment implications

Objectives: The aim of this study was to compare Friedewald-estimated and directly measured low-density lipoprotein cholesterol (LDL-C) values.

Background: LDL-C is routinely estimated by the Friedewald equation to guide treatment; however, compatibility with direct measurement has received relatively little scrutiny, especially at levels <70 mg/dl now targeted in high-risk patients.

Methods: We examined 1,340,614 U.S. adults who underwent lipid profiling by vertical spin density gradient ultracentrifugation (Atherotech, Birmingham, Alabama) from 2009 to 2011. Following standard practice, Friedewald LDL-C was not estimated if triglyceride levels were ≥ 400 mg/dl (n = 30,174), yielding 1,310,440 total patients and 191,333 patients with Friedewald LDL-C <70 mg/dl.

Results: Patients were 59 ± 15 years of age and 52% were women. Lipid distributions closely matched those in the National Health and Nutrition Examination Survey. A greater difference in the Friedewald-estimated versus directly measured LDL-C occurred at lower LDL-C and higher triglyceride levels. If the Friedewald-estimated LDL-C was <70 mg/dl, the median directly measured LDL-C was 9.0 mg/dl higher (5th to 95th percentiles, 1.8 to 15.4 mg/dl) when triglyceride levels were 150 to 199 mg/dl and 18.4 mg/dl higher (5th to 95th percentiles, 6.6 to 36.0 mg/dl) when triglyceride levels were 200 to 399 mg/dl. Of patients with a Friedewald-estimated LDL-C <70 mg/dl, 23% had a directly measured LDL-C ≥ 70 mg/dl (39% if triglyceride levels were concurrently 150 to 199 mg/dl; 59% if triglyceride levels were concurrently 200 to 399 mg/dl).

Conclusions: The Friedewald equation tends to underestimate LDL-C most when accuracy is most crucial. Especially if triglyceride levels are ≥ 150 mg/dl, Friedewald estimation commonly classifies LDL-C as <70 mg/dl despite directly measured levels ≥ 70 mg/dl, and therefore additional evaluation is warranted in high-risk patients.