Interference in a homogeneous assay for low-density lipoprotein cholesterol by lipoprotein X

Background: Homogeneous assays for cholesterol in low-density lipoprotein (LDL) are currently in wide use for guideline-based diagnosis and monitoring of dyslipaemic or coronary conditions. In some sera from patients with impaired liver function, we measured implausibly low LDL concentrations using a sugar compound-based assay [LDL-cholesterol (LDL-C), Roche Diagnostics]. We investigated whether an interfering factor, possibly associated with cholestasis, is consistently responsible for this disturbance. Methods: We compared results of the LDL-C assay in samples with implausible (n=158) and plausible (n=65) LDL concentrations with those of another assay based on two selective detergents (LDLD, Beckman Coulter) and with sequential density ultracentrifugation. We measured total bilirubin, triglycerides, bile acids and lipoprotein X (Lp X) concentrations in samples with the described disturbance and examined the effect of bile salt addition to normal samples. Results: The LDL-C assay was negatively biased compared to the LDLD assay (bias -0.63 mmol/L) and sequential density ultracentrifugation (bias -0.85 mmol/L) in samples with an implausible lipoprotein profile, but showed good method agreement in all other samples. The bile acid concentration did not correlate with the LDL bias, and addition of bile acids showed no interference with the LDL-C assay. The Lp X concentration correlated with the bias between the LDL-C and LDLID assays (R=0.66, p < 0.0001); there was no interference with the LDLID assay, even at high Lp X concentrations. Conclusions: We conclude that the LDL-C assay is subject to interference by Lp X and can provide grossly negatively biased results in cholestatic conditions. In such patients, LDL measurement with an assay based on a different method should be performed.