title: Analytical performance of a direct assay for LDL-cholesterol: a comparative assessment versus Friedewald's formula.
abstract: OBJECTIVE
  Because low density lipoprotein-cholesterol (LDL-C) is a modifiable risk factor for coronary artery disease (CAD), its routine measurement is recommended in the evaluation and management of hypercholesterolemia. Concentrations of LDL-C are commonly monitored by means of the Friedewald formula (FF), which provides a relative estimation of LDL-C concentration when the triglyceride (TGs) concentration is <200 mg/dl and there are no abnormal lipids. Because of the limitations of the Friedewald calculation, direct methods for an accurate quantification of LDL-C are needed.
  
  
  METHODS
  We critically examined an immunoseparation method for direct assay of LDL-C in a comparison with FF. 1) We measured intraassay and interassay precision using quality-control sera and patient serum pools. Accuracy was evaluated from total error analyses. Sample stability was examined over 2 months. 2) The LDL-C levels obtained with direct assay were compared with those calculated by the FF in 47 randomly chosen patient samples. The samples were classified as group 1 (patients with TGs 60-308 mg/dl, n=25) and group 2 (patients with TGs 320-695 mg/dl, n=22).
  
  
  RESULTS
  The direct immunoseparation assay displayed an excellent precision (total coefficient of variance (CV)<2.5%, intraassay CV<1.5% and interassay CV<1.5%). Mean total error was 4.34%. The direct assay met the current National Cholesterol Education Program (NCEP) requirements for LDL-C testing for precision and accuracy. The results of direct method (x) and the FF (y) were highly correlated (r=0.9908, y=1.030 x -0.289, n=25) in group 1, but the results of two methods disagreed (r=0.716, y=0.956 x -24.869, n=22) in group 2 (patients with TGs 320-695 mg/dl).
  
  
  CONCLUSIONS
  The direct immunoseparation assay meets the currently established analytical performance goals and may be useful for the diagnosis and management of hyperlipidemic patients.
authors.affiliation: Laboratory of Biochemistry, Diamed Dialysis Center, Istanbul, Turkey. iturkalp@yahoo.com
authors.forename: Işik
authors.initials: I
authors.lastname: Türkalp
authors.forename: Zafer
authors.initials: Z
authors.lastname: Cil
authors.forename: Didem
authors.initials: D
authors.lastname: Ozkazanç
mesh_terms: D015415:Biomarkers / Q000097:blood; D008076:Cholesterol, HDL / Q000097:blood; D008078:Cholesterol, LDL / Q000097:blood*; D003324:Coronary Artery Disease / Q000097:blood / Q000175:diagnosis*; D006801:Humans; D006937:Hypercholesterolemia / Q000097:blood / Q000175:diagnosis*; D008433:Mathematics; D011933:Reagent Kits, Diagnostic / Q000592:standards; D012044:Regression Analysis; D015203:Reproducibility of Results; D014280:Triglycerides / Q000097:blood
chemical_list: D015415:Biomarkers; D008076:Cholesterol, HDL; D008078:Cholesterol, LDL; D011933:Reagent Kits, Diagnostic; D014280:Triglycerides