Evaluation of equations for calculating LDL-cholesterol in patients with concomitant hypertriglyceridemia and low LDL-cholesterol: is there a preferred equation?

It is critical to reliably estimate Low Density Lipoprotein Cholesterol (LDL-C) in patients with concomitant hypertriglyceridemia and low LDL-C. We retrospectively compared the performances of the Friedewald (LDL-F), Martin-Hopkins (LDL-MH) and Sampson (LDL-SA) equations against a direct homogeneous LDL-C assay (dLDL-C) on observations presenting mild hypertriglyceridemia (triglycerides between 1.69 and 3.9 mmol/L) and low LDL-C (< 2.58 mmol/L). Observations were stratified according to their LDL-C. Agreement of the equations with dLDL-C was assessed using Intraclass Correlation Coefficients (ICC) with an agreement cut-off of 0.9, and analysis of Bland-Altman plots. Independently of the LDL-C stratum evaluated, the three equations failed to meet the 0.9 ICC cut-off, although their agreement with dLDL-C improves as LDL-C increases. Analysis of Bland-Altman plots shows a downwards discordance of LDL-F with dLDL-C, and an upwards discordance of LDL-MH and LDL-SA with direct LDL-C. LDL-MH resulted in the least observations outside the Bland-Altman limits of agreement. While no equation can be deemed satisfactory enough to replace direct assays in patients with low LDL-C and concomitant hypertriglyceridemia, LDL-MH seems to perform better than the other equations in estimating LDL-C in these patients.