Reactions of direct LDL-cholesterol assays with pure LDL fraction and IDL:comparison of three homogeneous methods

According to the definition of the Lipid Research Clinic's protocol, low-de nsity lipoprotein (LDL) refers to the lipoprotein of density (d)=1.006-1.06 3 g/ml which contains another atherogenic lipoprotein, IDL (d = 1.006-1.019 g/ml). Because metabolic properties are largely different between LDL and IDL, LDL is now defined as the lipoprotein of d = 1.019-1.063 g/ml. Recentl y direct LDL-cholesterol assay kits using novel surfactants (the homogeneou s methods) have become commercially available and widely used in Japan. The aim of this study is to examine how three direct LDL-cholesterol assay kit s, LDL-EX, Choletest-LDL and Determinor-L LDL, react with pure LDL (d = 1.0 19-1.063 g/ml) and IDL (1.006-1.019 g/ml) fractions isolated by ultracentri fugation. Thirty-one healthy subjects and one type III dysbetalipoproteinem ic patient were enrolled in this study. All homogeneous methods highly corr elated with LDL-cholesterol (r=0.95-0.98), although the values for LDL-EX w ere closer to the values for ultracentrifugation than were those of the oth er two methods (95 vs. 86-87%, P < 0.0001). Cross-reactivity with IDL was 3 1, 47 and 64% for LDL-EX, Choletest-LDL, and Determinor-L LDL, respectively . Similar results were obtained in the IDL from a type UI dysbetalipoprotei nemic patient. These results suggest that LDL-cholesterol measured by LDL-E X better reflects pure LDL fraction with weaker cross-reaction with IDL tha n other homogeneous methods. Elsevier Science B.V. (C) 2000 All rights rese rved.