Comparison between Values of Low Density Lipoprotein Cholesterol as Estimated by Direct Enzymatic Method with Calculated Methods Applying Friedewald’s Equation and Novel’s Equation: A Cross-sectional Study

Abstract

Introduction: Lipid profile is routinely used as a screening test to identify the risk of Cardiovascular Diseases (CVD). Elevated Low Density Lipoprotein cholesterol (LDL-c) is an important modifiable risk factor of atherosclerotic CVD. The LDL-c lowering strategy is a known recommendation for the prevention and treatment of CVD. The gold standard method of LDL-c estimation is β-quantification by ultra centrifugation. Other methods include Direct LDL-c measurement (D-LDL-c) using enzymatic assay which is tedious, time consuming and expensive. Hence, calculated method using Friedewald’s equation (F-LDL-c) is routinely used in clinical laboratories in India. Aim: To compare LDL-c values as estimated by direct enzymatic method with LDL-c values obtained by Friedewald’s equation and Novel’s equation, and, also to assess the effects of LDL-c values obtained by both the methods towards the risk stratification of CVD. Materials and Methods: A cross-sectional study, was conducted in the Central Diagnostic Laboratory of Mandya Institute of Medical Sciences, Mandya, Karnataka, India, for a duration of three months from July to September 2020, where, 600 subjects, aged 20-75 years, visiting for routine lipid profile estimation were included. LDL-c was estimated by direct enzymatic method (D-LDL-c) and calculated methods using Friedewald’s {F-LDL-c=TC-HDL-c- (TG/5)} and Novel’s equation {N-LDL-c=TC-HDL-c-(TG/Adjustable factor)}. Values obtained by calculated methods were compared with D-LDL-c values. The LDL-c values obtained were compared at different ranges of Total Cholesterol (TC), Triglycerides (TG) and High-Density Lipoprotein cholesterol (HDL-c). The association between direct and calculated LDL-c values were analysed by Pearson’s correlation. Receiver Operator Characteristic Curve (ROC) analysis was done to predict the better diagnostic method among the calculated methods of LDL-c. Results: The mean±SD of D-LDL-c (115.68±36.94 mg/dL) was high compared to F-LDL-c (106.95±33.48 mg/dL) and N-LDL-c (110.78±32.58 mg/dL). The mean difference between D-LDL-c and N-LDL-c (4.9±4.36 mg/dL) was low compared to F-LDL-c (8.75±3.46 mg/dL). Significant positive correlation was observed between D-LDL-c vs F-LDL-c (r=0.96; p=<0.001) and D-LDL-c vs N-LDLc (r=0.97; p=<0.001). The ROC showed maximum AUC value for N-LDL-c than F-LDL-c at a cut-off value of 100 mg/dL. LDL-c as estimated by Novel’s and Friedewald’s equation led to approximately 5% and 10% less patients being subjects for lipid lowering therapy respectively as compared to D-LDL-c. Conclusion: In conclusion, the use of Novel’s equation for LDL-c estimation instead of Friedewald’s equation could be associated with the small net increase in lipid lowering agent eligible population for primary prevention of atherosclerotic CVD. Replacement of Friedewald’s equation by Novel’s equation would enable for the improved accuracy of LDL-c estimation especially at higher levels of TC, TG and lower levels of HDL-c.