Comparison of friedewald formula and modified friedewald formula with direct homogeneous assay for low density lipoprotein cholesterol estimation

ABSTRACT

To compare the Friedewald and modified Friedewald formulae with direct homogeneous assay for serum lowdensity lipoprotein cholesterol [LDL-C] levels estimation. Cross-sectional study. Armed Forces Institute of Pathology, Rawalpindi, from June to December 2011. Healthy subjects of either gender, from Rawalpindi, aged 18-75 years were included by consecutive sampling. Patients with diabetes mellitus, chronic liver disease, chronic kidney disease, those taking lipid lowering drugs and samples with triglyceride [TG] > 4.52 mmol/l were excluded from the study. Total cholesterol, high-density lipoprotein cholesterol, TG and LDL-C were measured on Hitachi 912 chemistry analyzer [Roche]. LDL-C levels were also calculated by Friedewald formula [FF] and Vujovic modified formula [VMF]. Paired sample t-test and scatter plots were used for statistical analysis. Although both calculated methods showed good correlation with direct assay [r > 0.93] in 300 subjects, but the difference was statistically significant. The ffLDL-C were 0.12 A +/- 31 mmol/l [p < 0.001] lower and vmfLDL-C were 0.11 A +/- 26 mmol/l [p < 0.001] higher than dLDL-C. The difference was not significant between ffLDL-C and dLDL-C at TG levels < 1.70 mmol/l [p = 0.58] and between vmfLDL-C and dLDL-C at TG levels 2.26 - 4.52 mmol/l [p = 0.38]. At all other TG levels, the difference between LDL-C calculated by both formulas and dLDL-C was statistically significant [p < 0.001]. As compared to direct assay, 11% and 14% subjects were classified in wrong National Cholesterol Education Programm's cardiac risk categories by FF and VMF respectively. LDL-C should be measured by direct homogeneous assay in routine clinical laboratories, as the calculated methods did not have a uniform performance for LDL-C estimation at different TG levels