title: Calculated low density lipoprotein cholesterol levels frequently underestimate directly measured low density lipoprotein cholesterol determinations in patients with serum triglyceride levels < or =4.52 mmol/l: an analysis comparing the LipiDirect magnetic LDL assay with the Friedewald calculation.
abstract: BACKGROUND
  Increased low density lipoprotein cholesterol (LDL-C) is an established risk factor for the development of coronary artery disease (CAD). Recent guidelines detail specific LDL-C cutpoints for therapeutic goals. In practice, LDL-C is usually derived from the Friedewald formula (FF). This calculation is known to be inaccurate with serum triglyceride (TG) concentrations >4.52 mmol/l, however, its accuracy among relatively healthy patient cohorts with TG concentrations < or =4.52 mmol/l is less well studied.
  
  
  METHODS
  We studied 661 ambulatory adults with TG concentrations < or =4.52 mmol/l and no overt CAD. Fasting venous lipid panels were obtained. LDL-C was calculated from the FF and also directly measured with the LipiDirect Magnetic LDL assay. Linear regression and paired t-test analyses were performed.
  
  
  RESULTS
  Calculated and directly measured LDL-C concentrations were significantly different (4.26+/-0.88 vs. 4.83+/-1.06 mmol/l respectively, p<0.0001). In 93% of measurements directly measured LDL-C exceeded calculated LDL-C. Although calculated and directly measured LDL-C concentrations were related (R=0.90), the discrepancy between them increased linearly with increasing TG concentrations (R=0.67) and clinically important differences existed at normal or slightly increased TG concentrations. Concordant results for NCEP ATP-III risk categories were present for only 48.1% of samples.
  
  
  CONCLUSIONS
  Significant differences between calculated and directly measured LDL-C using the LipiDirect Magnetic LDL assay exist in healthy subjects with TG < or =4.52 mmol/l. These differences are linearly related to TG concentrations and occur frequently at relatively low TG concentrations.
authors.affiliation: Department of Medicine, Division of Cardiovascular Medicine, University of Massachusetts Medical School, 55 Lake Avenue, North, Worcester, MA 01655, USA. tighed@ummhc.org
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Tighe
authors.forename: Ira S
authors.initials: IS
authors.lastname: Ockene
authors.forename: George
authors.initials: G
authors.lastname: Reed
authors.forename: Robert
authors.initials: R
authors.lastname: Nicolosi
mesh_terms: D000328:Adult; D000368:Aged; D008078:Cholesterol, LDL / Q000097:blood*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D014280:Triglycerides / Q000097:blood*
chemical_list: D008078:Cholesterol, LDL; D014280:Triglycerides