Biological variation estimates of direct and indirect LDL cholesterol by Friedewald and Martin-Hopkins formulas in healthy individuals

Understanding biological variation (BV) is crucial for accurate clinical decision-making and for establishing analytical quality standards. This study established the BV of low-density lipoprotein cholesterol (LDL-C), assessed using both direct measurement and calculated values obtained from the Friedewald and Martin-Hopkins formulas in healthy individuals. A total of twenty-six healthy Turkish subjects (15 females and 11 males) underwent fasting LDL-C measurement, along with calculated LDL-C derived from serum cholesterol, triglycerides, and high-density lipoprotein cholesterol, using samples collected concurrently on 10 weekly occasions. All measurements were conducted in duplicate by the enzymatic colorimetric method. Within-subject (CV_I) and between-subject (CV_G) BV estimates, with 95% confidence intervals (CI), were determined by CV-ANOVA following evaluation of trends, homogeneity of variance, and outlier removal. No significant gender-related differences were observed in the BV components for either direct or calculated LDL-C. According to direct LDL-C, Friedewald LDL-C, and Martin Hopkins LDL-C, CV_I values were 8.7%, 9.3% and 9.0%, and the CV_G values were 14.7% for direct LDL-C, 18.5% for Friedewald LDL-C, 18.6% for Martin Hopkins LDL-C. These values supported the use of updated analytical performance specifications and reference change values (RCV). All LDL-C exhibited marked individuality (II < 0.6). By applying a rigorously standardized experimental protocol, the inter-individual variability observed supports the preferred use of RCVs over conventional population-based reference intervals for serial monitoring. These results have important implications for enhancing the clinical utility of LDL-C measurements, regarding cardiovascular risk assessment and individualized therapeutic decision-making.