High lipoprotein(a) results in overestimation of BQ-based low-density lipoprotein-cholesterol measurement

BACKGROUND: Most conventional low-density lipoprotein cholesterol (LDL-C) assays are standardized by the CDC reference procedure of beta-quantification (BQ) method, which involves density-based ultracentrifugation to separate very low-density lipoprotein (VLDL) and LDL. Apolipoprotein (apo) (a) covalently binds to apoB, altering lipoprotein density thereby affecting this separation. We evaluated the impact of lipoprotein(a) [Lp(a)] on LDL-C assay accuracy. METHODS: The distribution of apo(a) between VLDL and LDL was examined by ultracentrifugation and gel permeation high performance liquid chromatography (GP-HPLC) analysis in three subjects. LDL-C values were compared between the conventional assays calculated by Sampson's formula (S-LDL-C) and direct homogeneous assay (D-LDL-C) and the GP-HPLC analysis (GP-LDL-C) for 1560 human samples, and the influence of Lp(a) concentration was analyzed. RESULTS: Apo(a) was detected in the LDL fraction by ultracentrifugation and in both VLDL and LDL fractions by HPLC. Strong correlations were observed between S-LDL-C, D-LDL-C and GP-LDL-C, but significant differences existed among the methods (P < 0.001). The differences between S-LDL-C or D-LDL-C and GP-LDL-C positively correlated with Lp(a) (r = 0.302 and 0.321, respectively, P < 0.001). When Lp(a) exceeded 30 mg/dL, these differences were significantly larger (n = 139). The difference between S-LDL-C and D-LDL-C was not significantly correlated with Lp(a) (P = 0.774). Conversely, VLDL-C measured by Sampson's method showed a negative correlation with Lp(a) (r = -0.262, P < 0.001). CONCLUSIONS: Apo(a) increases VLDL density, causing it to be recovered in the LDL fraction during density-based separation. Consequently, Lp(a) causes concentration-dependent overestimation of BQ-based LDL-C, and underestimation of VLDL-C.