Laboratory validation of a low density lipoprotein apolipoprotein-B assay

Objectives: Numerous publications have shown strong association between CHD risk and either apolipoprotein B (Apo-B) or low density lipoprotein (LDL) particle number (LDL-P). It is however unknown if Apo-B or LDL-P has a stronger predictive ability for future CHD. This uncertainty may be due to the inability of current Apo-B assays to separate the contribution of very low-density lipoprotein particles from the total Apo-B concentration. As such we have performed a laboratory validation of the Maine Standards LDL Apo-B assay on the Roche Cobas 6000 analyzer.

Design and methods: Imprecision, linear range, and limit of quantitation studies were performed using quality control materials. Plasma samples collected for lipid profile analysis were analyzed via the LDL Apo-B assay and compared to the LDL cholesterol (LDL-C) concentration determined via direct LDL assay and Friedewald equation.

Results: The LDL Apo-B within-run imprecision was 2.3% at 62 mg/dL and 2.2% at 109 mg/dL. The within-laboratory imprecision was 9.7% at 57 mg/dl and 6.1% at 104 mg/dL. Linear regression analysis of LDL Apo-B versus calculated and measured LDL-c resulted in equations of LDL Apo-B=0.620∗(LDL)+45.4, R=0.9063 and LDL-Apo-B=0.607∗(LDL)+38.8, R=0.9393, respectively. Bias plot analyses revealed that at low LDL-C concentration, there was a tendency for a higher than anticipated LDL Apo-B concentration.

Conclusions: The Maine Standards LDL Apo-B assay is a precise automated assay and comparison of LDL Apo-B to LDL-c concentration demonstrates that low LDL-C concentrations may still carry residual risk of CHD due to increased concentration of small dense LDL particles.