Comparison of Four Formulas for Calculating LDL-Cholesterol with the Direct Homogeneous Method

Background/Objectives: Low-density lipoprotein cholesterol (LDL-C) is a primary therapeutic target in cardiovascular risk assessment. While direct assays are available, LDL-C is frequently estimated using various formulas, whose performance can vary by population and lipid concentration. This study evaluates the analytical agreement and clinical classification performance of four formulas (Spasić-Kotur-Vujović, Friedewald, De Long, and Martin-Hopkins) against directly measured LDL-C. Methods: Lipid profiles from 3935 fasting adult patients were analyzed. LDL-C was directly measured via a homogeneous assay and compared with calculated values using linear regression and Bland-Altman analyses. Analytical bias and classification accuracy were assessed across stratified lipid levels. Results: Among the evaluated equations, the Spasić-Kotur-Vujović formula showed the closest agreement with directly measured LDL-C. It had the lowest mean absolute bias and mean percentage differences across most lipid strata, as well as narrower limits of agreement compared with Friedewald, De Long, and Martin-Hopkins formulas. Although all formulas showed strong correlations with directly measured LDL-C (Pearson's r correlation coefficient = 0.956-0.958; intraclass correlation coefficient > 0.974), systematic underestimation of LDL-C was observed for all equations and became more pronounced with increasing total cholesterol, LDL-C, triglycerides, and non-high-density lipoprotein cholesterol concentrations. The Spasić-Kotur-Vujović formula achieved the highest overall classification accuracy (70%), followed by De Long (68%), Martin-Hopkins (61%), and Friedewald (58%). Conclusions: Despite strong correlations, clinically relevant differences in bias and classification exist among LDL-C formulas. The Spasić-Kotur-Vujović formula demonstrated superior agreement and accuracy in this population, suggesting it is a more reliable tool for routine laboratory practice. It should be noted that these findings reflect agreement with the specific homogeneous LDL-C assay used on the Alinity c platform (Abbott Laboratories,, Chicago, IL, USA) and may not be directly generalizable to other analytical systems.